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THE ECOLOGY OF INSECT PEST POPULATIONS
IN MAIZE STORAGE CRIBS IN NIGERIA
by
Richard Hugh Markham, B.A. (Nat. Sci.)
A thesis submitted
Doctor of Philosophy of
and the Diploma of
for the degree of
the University of London
Imperial College.
Tropical Stored Products Centre, (Overseas Development Administration), London Road, Slough, Berkshire. March 1981
THE ECOLOGY OF INSECT PEST POPULATIONS
IN MAIZE STORAGE CRIBS IN NIGERIA
Richard Hugh Markham
Abstract
This study considered the insect populations infesting white dent maize stored in well-ventilated cribs at two localities in South West Nigeria. The pest complex was dominated by Sitophilus zeamais (Col.: Curculionidae) but included a great diversity of other pest species and natural enemies. The incidence of individual species was studied from pre-harvest infestation through six to ten months of storage and was shown to follow a consistent succession. The spatial distribution of insects within a crib was not uniform and individual species showed consistent patterns of distribution at a particular time. The seasonal incidence and distribution patterns of major species are discussed in terms of observed changes in grain moisture content, temperature and grain damage. The roles of intra- and interspecific relationships in limiting populations are considered. Sitophilus populations rapidly reach a 'plateau' and it is concluded that further significant increase is prevented by this insect's responses to its own high population density. The relationship between the field infestation and subsequent pest population increase in store is considered with particular reference to the effects of time of harvest, removal or retention of the husks and of damage* caused in the field by Lepidoptera larvae. Colonisation was found to be mainly by active migration of insects to the newly-loaded crib. Storage of maize in the husk provided no protection against insect damage although it did affect the distribution of insects between cobs. Techniques for sampling of insects from cribs are considered and the results of the study are discussed in terms of their implications for pest control strategies.
iii.
TABLE OF CONTENTS
ABSTRACT ii TABLE OF CONTENTS iii LIST OF TABLES v LIST OF FIGURES viii
CHAPTER 1 Introduction 1
CHAPTER 2 THE PHYSICAL ENVIRONMENT 2.1 The Crib as a Drying Structure 6 2.2 Macroclimate at Ibadan 11 2.3 Microclimate and Physical Conditions within the Crib 16
2.3.1 Grain Moisture Content 17 2.3.2 Temperatures within the grain bulk 30
2.4 Maize as a substrate for Insect Development 36
CHAPTER 3 SAMPLING TECHNIQUES 3.1 Introduction 39 3.2 Insect Sampling:Constraints and Considerations 41
3.2.1 General Objectives 41 3.2.2 Choice of Sampling Universe 42 3.2.3 Insect Mobility . 42 3.2.4 Sampling Units and Variability 44 3.2.5 Sample Size in Relation to the Size of Cribs 52 3.2.6 Sample Size and Handling Time 53
3.3 Assessment of Insect Sampling Techniques 55 3.3.1 Iowa Corn Probe 56 3.3.2 Destructive Sampling of Whole Cribs 62 3.3.3 Replacement Sampling 63 3.3.4 Sampling to Estimate Recruitment 65
3.4 Extraction of Insects from Grain Samples 66 3.5 Damage Assessment 70
CHAPTER 4 INSECT DISTRIBUTION WITHIN THE CRIB 4.1 Introduction 73 4.2 Sampling and Analysis for Insect Distribution 75 4.3 Preliminary Distribution Trial 82 4.4 Long-Term Changes in Insect Distribution 85 4.5 Short-Term Changes in Insect Distribution 96 4.6 Distribution of Losses within the Cribs 115 4.7 Species Interaction and Habitat Selection 121
iv.
CHAPTER 5 THE INITIATION OF INFESTATION 5.1 Introduction 131 5.2 Pre-Harvest Infestation 135 5.3 The Effects of Harvesting Practice on Infestation 145 5.4 Persistent Effects of Pre-Harvest Damage 162 5.5 Sources of Storage Infestation 171
CHAPTER 6 THE INSECT COMMUNITY:COMPOSITION AND SUCCESSIONAL CHANGES 6.1 Introduction 175 6.2 Treatments and Sampling Techniques 177 6.3 The Abundance of Major Insect Groups and Changes in
the Physical Environment 182 6.4 Incidence and Role of Individual Insect Species 189
6.4.1 Primary Pest Species 189 6.4.2 Secondary Pest Species - Coleoptera 196 6.4.3 Predatory Coleoptera 204 6.4.4 Heteroptera 204 6.4.5 Hymenoptera 207 6.4.6 Diptera 213 6.4.7 Psocoptera 213 6.4.8 Other Insect Groups 214
6.5 Other Arthropods 214 6.6 Vertebrates 216 6.7 Grain Weight Losses 217 6.8 Conclusions 220
CHAPTER 7 DISCUSSION: CHARACTERISTICS OF THE MAIZE CRIB SYSTEM AND
• IMPLICATIONS FOR CONTROL STRATEGIES 223
APPENDIX .1 An annotated list of species of insects and 235 mites recorded from maize cribs at Ibadan and Ilora
APPENDIX II Collated data: Distribution Studies 250
APPENDIX III Collated data: Succession Studies 259
APPENDIX IV Methods for estimation of moisture content of ' 288 grain and cores
APPENDIX V Methods for analysis of grain weight loss 295
APPENDIX VI Collated analysis of variance tables 298
ACKNOWLEDGEMENTS 305
BIBLIOGRAPHY 306
LIST OF TABLES
CHAPTER 2 2.1 Probability levels from 3-factor analysis of variance of
moisture contents from different positions in a crib, at four different stages during the storage season
2.2 Summary results of analysis: effects of time of day and position in crib on grain moisture content (Short-Term Distribution Trial) (a) probability levels from analysis of variance (b) treatment means
2.3 Summary results of analysis: effects of position in crib on grain moisture content (Termination of Short-Term Distribution Trial) (a) probability levels from analysis of variance (b) treatment means
2.4 Summary results of analysis: effects of time of day and position in crib on grain temperature (a) outline design (b) probability levels from analysis of variance (c) treatment means
CHAPTER 3 3.1 Difference in adult insect abundance in probe samples from
different positions in 3 cribs (Carpophilus) of 4 cribs (Sitophilus) (a) probability levels from a single factor analysis of
variance (b) treatment means
'3.2 Insect abundance in probe samples from different parts of a single crib (a) probability levels from a 3-factor anovar (b) treatment means
3.3 Effect of time after collection on numbers of insects emerging from grain samples
CHAPTER 4 4.1 Effect of sampling position within crib on insect numbers for
(a) Sitophilus zeamais (b) Carpophilus dimidiatus
4.2 Results of Factorial Analysis of Variance for dispersion of Sitophilus zeamais in Long-Term Distribution Trial for: (a) adults (b) emergences
4.3 Results of Factorial Analysis of Variance for dispersion in Long-Term Distribution Trial of: (a) Carpophilus dimidiatus (b) Cathartus quadricollis
vi.
4.4 Results of Factorial Analysis of Variance for dispersion of insects in Sample 4 (Long-Term Distribution Trial) 95
4.5 Analysis of crib totals from Short-Term Distribution Trial 100 4.6 Dispersion of adult insects within cribs: effect of time of
day and sampling position on insect abundance (Short-Term Distribution Trial) 106 (a) probability levels from 3-factor analysis of variance (b) treatment means from factorial analysis of variance
4.7 Summary of adult insect distribution pattern (Short-Term Distribution Trial) 107
4.8 Distribution pattern of emergences (Short-Term Distribution Trial) based on 2-factor analysis of variance 112 (a) probability levels (b) treatment means
4.9 Summary results of analysis of variance: effect of position within crib on grain weight loss 119 (a) summary anovar table (b) treatment means
4.10 Correlation matrices showing associations between insect species and with two environmental parameters 124
CHAPTER 5 5.1 Summary results of field samples: infestation by major pest
groups, grain damage and husk cover 138 5.2 Percentage of cobs infested by each species (or group) 139 5.3 Summary of sampling regime for Harvesting-Practice Trial 146 5.4 Selection of damaged and sound cobs for storage - Harvesting-
Practice Trial 149 5.5 Effects of harvesting on adult insect populations comparison
of adult insect numbers before harvest (field samples) and 24 hours after harvest (snapped and husked) ' 152
5.6 Effects of early harvesting on insect.populations 153 (a) .comparison of mean moisture contents (b) comparison of emergent insects (c) comparison of adult insects from early and late
harvested maize 5.7 Effect of time of harvest and retention of husks on grain
moisture content 155 5.8 Probability levels from analysis of variance: effects of
time of harvest, removal of husks and position in crib on insect numbers 157 (a) adult insects - first storage sample (b) emergences - first storage sample (c) emergences - second storage sample
5.9 Effects of time of harvest and removal or retention of husks on adult insect infestation (first storage sample) 158 (a) Cathartus quadricollis (b) Oryzaephilus mercator (c) Gnatocerus maxillosus -(d) Palorus subdepressus
5.10 Effects ori emergences of time of harvest and removal/retention of husks at the time of: 159 (a) the first storage sample (b) the second storage sample
5.11 Effects of time of harvest on losses 160 5.12 Effects of field damage by Lepidoptera on initial infestation
of maize by storage pests (Pre-Harvest Damage Trial) 167 (a) probability levels from 3-factor analysis of variance (b) mean number of insects emerging from samples
5.13 Effects of field damage on subsequent infestation in store 168 5.14 Effects of field damage on subsequent infestation in store 169 5.15 Progressive changes in mean moisture content and mean dry
weight over storage period 170 5.16 Final weight loss (4 months in store) for maize damaged in
the field by Lepidoptera (Pre-Harvest Damage Trial) 170
CHAPTER 6 6.1 Summary of crib 'treatments1 used for Succession Studies 181 6.2 Mean loss in dry weight at the end of the storage period 218 6.3 Weight loss in dry season cribs at termination: effect of
position/sampling: 219 (a) mean .weight loss for different positions (b) probability levels from a 2-factor analysis of variance
viii.
LIST OF FIGURES
CHAPTER 2 2.1 Examples of three traditional and one 'improved1 designs
of maize cribs from S.W. Nigeria. 9 2.2 Examples of moisture content/relative humidity isotherms
for rice, maize and sorghum 10 2.3 Climate at the study site, Ibadan: (a) weekly total rainfall
(b) mean daily solar radiation 12 2.4 Climate at the study site: typical daily cycles of temperature
and relative humidity: (a) during the wet season (b) during 'harmattan' conditions
2.5 Climate at the study site: minimum, maximum and mean daily temperature and relative humidity 14
2.6 Comparison of moisture content observed in maize from cribs at Ibadan with 'predicted' equilibrium moisture content 15
2.7 Variability in moisture content of grain and cores: (a) intercob variation in grain moisture content 18 (b) relationship between grain and core moisture content 19
2.8 Grain moisture content in samples from the centre of cribs at Ibadan and Ilora 21
2.9 Grain moisture content in different parts of a single crib 23 2.10 Design of factorial analysis of variance for moisture content
data (long-term trial) . 24 2.11 Design of factorial analyses of variance for moisture content
data (short-term trials) - 2 7 2.12 Changes in temperature at different points (7) in. a single
crib during a 24-hour period: (a) harmattan. conditions 31 (b) wet season conditions 32
2.13 Changes in temperature at different points (32) in a crib during a 24-hour period 33
CHAPTER 3 3.1 Distribution of Sitophilus zeamais (adults) on maize cobs
stored Tin the husk' 45 3.2 Distribution of Sitophilus zeamais (adults) on maize cobs
stored without husks. 46 3.3 Dependence of variance on mean for samples of Sitophilus adults 47 3.4 Distribution of emergences of major primary pests from cobs
stored without husks. 48 3.5 Distribution of Cathartus quadricollis adults on maize cobs
stored: 51 (a)1 in the husk1. (b) without husks
3 . 6 Scatter diagram showing lack of dependence of insect numbers on grain sample size for Iowa corn probe 57
3.7 Design for analysis of the effect of position in the crib on the number of insects collected in probe samples 58
3.8 Arrangements used for the rapid extraction of insects from samples of shelled grain and cobs 68
3.9 Weight loss of grain for cobs from sampled and (adjacent) unsampled parts of a crib under more and less intensive sampling regimes 71
CHAPTER 4 4.1 Modification of cribs for Distribution studies 76 (a) general view of crib
(b) stacking of cobs in sampling tunnels 4.2 Designs for analysis of effects of position in crib on insect
numbers (Distribution Trials) 81 (a) Preliminary and Long-Term Distribution Trials (b) Long-Term Distribution Trial (emergences) (c) Short-Term Distribution Trial
4.3 Distribution pattern of Sitophilus zeamais at different stages of the storage season (Long-Term Distribution Trial) 86 (a) Adults - no. insects/500g shelled grain @ 17% m.c. (b) Emergences- no'.s insects emerging during one week from
100 g. samples of shelled grain (fresh weight) 4.4 Distribution patterns of secondary pest species and natural
enemies at different stages of the storage season (Long-Term Distribution Trial) 90
4.5 Experimental design and analysis for the Short-Term Distribution Trial: 97 (a) allocation of sampling times and sampling occasions in the
3 replicate cribs (b) analysis for effect of time of day and position within crib
on insect numbers 4.6 Distribution of adult insects within cribs (Short-Term
Distribution Trial) 101 4.7 Distribution of Sitophilus (Short-Term Distribution Trial):
effect of East-West position and 'vertical' position 105 4.8 Distribution of emergences (Short-Term Distribution Trial).
Numbers of insects emerging in one week from lOOg. samples of shelled grain 109
4.9 Progress of weight loss in different parts of a single crib (Long-Term Distribution Trial) 117
4.10 Distribution of weight loss within cribs (Short-Term ' Distribution Trial) 118
4.11 Summary of trends in environmental factprs 123 (a) weight loss of grain (dry weight basis) (b) grain moisture content (fresh weight basis) (c) grain temperatures at different times of day
X .
CHAPTER 5 5.1 Summary of sampling programme from cribs (Harvesting-
Practice Trial) 147
5.2 Arrangement of samples in crib (Pre-Harvest Damage Trial) 164
CHAPTER 6 6.1 Modification of cribs for succession trials 173
(a) structural modifications (b) arrangement of cobs for sampling
6.2 Incidence of major insect groups through the storage season 183 6.3 Seasonal incidence of major Coleoptera families 185 6.4 Changes in grain moisture content through the storage season 187 6.5 Increase in grain damage over the storage period 188 6.6 Seasonal incidence of Sitophilus zeamais (adults) 190 6.7 Seasonal incidence of Sitophilus zeamais and Sitotroga
cerealella (emergences) 191 6.8 Seasonal incidence of Carpophilus spp. (Nitidulidae) 197 6.9 Seasonal incidence of main species of Silvanidae 198 6.10 Seasonal incidence of main species of Tenebrionidae 201 6.11 Seasonal incidence of main species of Heteroptera 206 6.12 Seasonal incidence of main species of Pteromalidae 208 6.13 Seasonal incidence of various parasitoids 209
APPENDIX IV.1 Drying curves for samples of whole grains APPENDIX IV.1 Drying curves for samples of ground grain APPENDIX IV.3 Drying curves for cores
CHAPTER 1
THE ECOLOGY OF INSECT PEST POPULATIONS
IN MAIZE STORAGE CRIBS IN NIGERIA
INTRODUCTION:
Staple food grains in many areas of Tropical Africa, Latin
America, and South-East Asia are still largely stored using trad-
itional techniques. These include the use of small granaries holding,
at most, only a few tons of grain. In dry areas such granaries are
often closed structures with solid mud walls, but in more humid
regions, where maize is the main cereal crop, the grain is characteristically
stored 'on the cob1 in a ventilated granary, known as a 'crib1. The
walls of the crib are of basketwork, matting or slats, or in some
cases consist only of the cobs themselves, regularly stacked. These
arrangements allow air to flow through the grain, drying it and, to some
extent, inhibiting mould development. However, it also gives ready
access to insect pests. While it is difficult to estimate .the losses
caused by insects in such stores they are certainly potentially severe
(Parkin, 1956 and 1959; Hall, 1970; Pingale, 1970; Adams and Harman,
1977).
Grain losses can be markedly reduced by the use of sealed silos
in which insecticide-treated, artificially dried grain is stored, (e.g.
Pingale, 1968); however, these techniques are, for a variety of reasons,
impracticable in many situations (see Discussion, Chapter 7). Acc-
ordingly, some effort has been devoted to the attempt to develop im-
proved storage techniques which retain the basic concept of the crib:
a ventilated structure, of low cost, which serves both for the drying
and storage of grain on the farm. Both structural modifications and
the use of insecticides of various kinds have been considered (Kockum,
2.
1953 and 1958; Cornes and Riley, 1962; Pointel, 1969; Schulten, 1972;
de Lima, 1978; Boshoff, 1978; F.A.O., 1980); unfortunately, most of
these studies have not given serious attention to the insect pest «
populations which were supposed to be the cause of the storage problem.
After more than twenty years of field trials, no consistently reliable
method exists for the control of insect pests in maize cribs (Hind-
marsh, Tyler and Webley, 1979) and the ecology of the insects them-
selves has been scarcely considered.
In contrast to this dearth of field data, many of the insect
species which occur in cribs are very well known from laboratory
studies and, to a much lesser extent, from studies of the bulk storage
of grain in silos and warehouses. Various insects that are normally
associated with stored products have proved ideal subjects for the
work of physiologists and geneticists (see for instance, the monograph
on Tribolium by Sokoloff, 1972, 1974 and 1977) and theoretical ecol-
ogists (from the early works of Utida, 1941 and 1942, Crombie, 1944
and 1945, Park, 1948 and Birch, 1948, reviewed by Solomon, 1953 to
more recent studies such as those of Bellows, 1979), while their
physiology, behaviour and dynamics have been widely studied by applied
biologists with specific regard to the storage environment (see, for
instance, the numerous papers by Howe and Surtees which will be quoted
later). Studies of storage pest ecology under realistic conditions
have been relatively few and have considered mainly temperate conditions
(e.g. Coombs and Woodroffe, 1963, 1968 and 1973; Sinha, 1973) but some
aspects have been investigated in tropical stores (Smith, 1963; Prevett, 1964;
Graham, 1970).
Against this background the aim of the current study is to
provide some basic understanding of the ecology of a maize storage crib -
3.
to identify and describe the main attributes both of the physical
environment and of the insect community that develops within it.
Such a study can only be a beginning. One type of crib in a single
locality was chosen for consideration and priority was given to
observing as many features of that system as possible rather than
trying to analyse particular aspects in detail experimentally. The
intention was not to evaluate the particular storage technique used,
nor to try to provide solutions to the 'problems of crib storage*.
It is hoped, however, that the insights provided by this study can
contribute to the improvement of research and assessment methods in
the applied field of crop preservation and so to the more rational
evaluation of small-scale storage technology.
The selection of the physical and biological parameters to be
considered largely reflects the practical context of the work. The
progress of storage infestation was monitored by periodic sampling
of infested grain to give estimates of the adult insect population
levels and net recruitment, grain moisture content and grain weight
loss. Different initial conditions were used to investigate the
initial source of infestation, the effects of grain damage in the
field prior to harvest and the impact of different harvesting practices.
Particular attention was given to the evaluation of sampling methods
and features of the insect populations such as distribution and move-
ment which bear directly on them. Insect distribution was also con-
sidered in relation to temperature and moisture content gradients
within the crib.
A maize crib must be considered as an integral part of a particular
farming system. Ibadan in S.W. Nigeria was chosen as the base for
this study, largely because in this area two crops of maize can be
grown each year, with correspondingly flexible possibilities for
storage trials. Maize is the main cereal crop and widely grown in
the Ibadan area but, under current economic conditions, is not gen-
erally stored in farmers' cribs for subsistence use. In the absence
of a strong local tradition it was decided to base trials on specially
constructed cribs using an 'improved' design developed by the F.A.O.
African Rural Storage Centre at Ibadan (F.A.O., 1980). The principles
on which this system is based are considered in more detail later
(see Chapter 2), but essentially it depends on the use of an unusually
narrow crib (80cm wide) whose slatted walls allow maximal airflow
through the grain; the crop can be harvested soon after physiological
maturity, at a much higher moisture content than is possible with most
traditional designs. Such cribs are currently being introduced by
some national extension services in West Africa.
Similarly, the maize varieties for these trials (TZPB and TZB)
were chosen as. being typical of 'improved', high-yielding varieties
which are now widely grown. It has often been reported that such
varieties are more susceptible to storage insect attack than the trad-
itional varieties which they displace (Dobie, 1974) and this appears
to be true for these two varieties (Olusanya, pers. comm).
The picture of the maize crib that emerges from this study is
a complex one. The community supported by the crib is a large and
varied one in which insect species, including primary grain feeders,
detritus feeders, predators and parasitoids, are prominent but in which
mites and moulds also play an important role. At any particular time
there are slight differences in the physical conditions in different
parts of the crib and marked differences in the distribution of various
insect species. With time, climatic conditions (and so conditions within
the crib) vary according to daily and seasonal cycles, and on these
changes are imposed the progressive change in the substrate as it
dries and becomes increasingly damaged. The occurrence of the insect
species, too, follows a consistent succession through the storage
season, but within this succession individual species may be strongly
constrained by biotic factors, such as competition and predation.
This thesis seeks to develop the theme of the maize crib as
an ecosystem. The evidence is considered for the importance of various
kinds of interaction within the insect community and between that
community and its environment. The performance of the more important
species is discussed in the light of their known physiological tol-
erances, behaviour and dynamics. Finally, the conclusions of the
biological studies are briefly considered in terms of their implications
for insect control strategies and for the effort to develop improved
crib storage techniques.
6 .
CHAPTER 2
THE PHYSICAL ENVIRONMENT
2.1 The Crib as a Drying Structure
When maize reaches physiological maturity the grain has a
moisture content of 30 - 35% (F.A.O. 1975). If such grain were to
be harvested immediately and placed in a closed store it would rapidly be
destroyed by a combination of mould and bacterial activity (and, to a
lesser extent, its own respiration). The limiting conditions for
damaging mould growth are determined primarily by the moisture content
of the grain and the rate of airflow over it. The precise relationship
between these factors is complex but, for practical purposes, in humid
tropical areas such as Ibadan, shelled grain must be dried to a moisture
content below 15% if it is to be safely stored in jute sacks, or to
below 13% for storage in silos (Boshoff, pers. comm.). The more gen-
eral statement is sometimes made that, for safe bulk storage, grain
must have a moisture content below that which is in equilibrium with
an air relative humidity of 70%; this is equivalent to a moisture content
of 13.5% for maize at 27°C (Hall, 1970). In a climate where air i
humidity is predominantly low, as in the northern savannah areas of
West Africa, maize left standing in the field will dry naturally to
such a moisture content within a matter of days. However, in the more
humid Guinea savannah and forest zones, this process may take several
weeks and during this period losses to birds, rodents, insects and
moulds may be severe (F.A.O. 1975).
Small quantities of grain harvested at high moisture content are
traditionally hung up to dry in dwelling houses, outdoor 'kitchens', or trees,
or they may be spread out on the ground during periods of sunshine. Larger
quantities of grain may be dried artificially, using solar energy or in
batch-driers using wood or hydrocarbon fuels; however, such methods
are often prohibitively expensive in fuel costs or in the materials
and/or labour required. In such situations a maize crib, acting as
a combined drying and storage structure, may constitute a satis-
factory alternative. The crib may be cheaply constructed, using
locally available materials, during periods of low labour demand.
If suitably designed the crib can afford some protection against birds
and rodents while the grain dries in the natural airflow through the
structure. Drying of the maize can be hastened, and possibly some
protection against insect attack achieved, by lighting a slow-burning
fire under the crib - as is done in rural areas to the South of Ibadan
(Jambawai pers. comm.).
The use of cribs to dry and store maize has been investigated
in several studies in Southern Nigeria. Cornes and Riley (1961) found
that maize harvested at 20 - 25% m.c. dried in traditional cribs to
9 - 11% m.c. over six months of storage. 'Improved1 cribs with
increased ventilation were found in one study to achieve higher rates
of drying than a traditional crib over the first 20 days in store
(2% per 10 days and 1.5% per 10 days, respectively), drying thereafter
being uniform (Cornes and Riley, 1962). In a later trial, however,
under more humid conditions, no difference was found between the drying
rates in cribs of three different designs, leading Cornes (1963) to
conclude that there was no scope for improvement of drying rates in
small cribs. Noting that the minimum daily relative humidity was higher
than 60% for most of the year, he also concluded that the use of maize
cribs was not satisfactory in this area.
Workers at the African Rural Storage Centre, Ibadan, studying
the critical factors determining the rate of drying and the degree of
mould development in cribs, concluded that the most important factor
was the width of the crib (F.A.O., 1980). It was shown that, even in
the most humid areas, if a crib with a maximum width of 60 cm. is used,
maize could safely be harvested and placed in it at a moisture content
of 30%; in less humid areas the 'safe' width could be progressively
increased (to approximately 80 cm under Ibadan conditions and to as much
as 150 cm. in drier northern areas). If the husks are retained on the
cob, as is the traditional practice in many areas, maize can be harvested
and stored in such a crib at a moisture content of 20% or below (Thorshaug,
1975). Traditional cribs may be 3m. or more in diameter but drying
rates in such structures do not seem to have been critically studied.
Typical designs of cribs, traditional and improved, are illustrated
in Figure 2.1.
The drying of grain depends on the equilibrium relationship between
grain moisture content and the relative humidity of the air; the
relationship (Figure 2.2) is characteristic for a particular grain
species, though it may vary slightly between varieties. Artificial drying
depends on the principle that heating air reduces its relative humidity
and so increases its tendency to absorb water from the grain. The
theory of moisture and relative humidity relationships in stored grains
and their measurement have been discussed by Mackay (1967), Pixton
(1967), Gough (1974) and Haward Hunt and Pixton (1974). As Boshoff
(1978) has pointed out, even during the wet season at a rather humid
location such as Ibadan, the mean daily relative humidity is 70 - 80%;
the corresponding equilibrium grain moisture content is 13.5 - 16.5%
and so maize harvested at a higher moisture content and stored in a
crib will dry towards this figure without the need for artificial drying.
9.
a) b )
d )
FIGURE 2.1 Examples of traditional low-ventilation maize cribs from S.W. Nigeria and an 'improved' design : large-diameter crib from Shawunju, near Abeokuta. 'inverted-cone' crib from Ofiki, Oyo State, 'smoking crib' in which the drying rate is increase by lighting a slow-burning fire beneath the platform; a traditional design from Shawunju. 'improved', highly ventilated crib from F.A.O. African Rural Storage Season, Ibadan.
a) b) c)
d)
10.
3 0
\
o
£
C
2 5
20
15
(0 c 1 0 U)
m a i z e ( d e s o r p t 1 o n ) * *
m a l z o ( a d s o r p t i o n ) * *
r f c © ( d o s o r p t f o n ) ^ • I
s o r g h u m ( d e s o r p t l o n )
5 -
0 0 10 2 0 3 0 4 0 5 0 6 0
R . H . / *
7 0 8 0 9 0 1 0 0
FIGURE 2.2 Examples of moisture content/relative humidity isotherms
for three cereal crops : drying curves only for rice and
sorghum, drying and rehydration curves for maize (a yellow
dent variety). Isotherms for rice and maize determined
at 25°C, that for sorghum at 27-28°C. (Data from Gough
and Bateman, 1977).
* »
2.2 Macroclimate at Ibadan.
The climate of coastal West Africa depends on the annual move-
ment of the Inter-Tropical Convergence Zone (I.T.C.Z.) - the meeting
point of the dry continental (Saharan) air mass and the moister oceanic
one. In the more southerly regions the northward movement of the
I.T.C.Z. and its return are marked by two distinct periods of heavy
rain, in May - July and September - November, separated by a humid,
cloudy period of rather lower rainfall, in August/September (Figure 2.3).
Throughout this period temperatures vary daily between about 25 and 35°C
and relative humidity from 60 to 100% (mean 70 - 80%). During the
months of December to March, when the I.T.C.Z. lies to the South,
the climate is dominated by the movement of dry air-streams from
the Sahara (the'Harmattan' ) which bring stronger daily cycles of temp-
erature (20 - 40°C) and relative humidity (30 - 100% - mean 50 - 60%).
The typical daily cycles are illustrated in Figures 2.4a) and b). In
more northern areas of West Africa the two wet seasons converge until
at about latitude 8° and above in Nigeria there is only a single wet
season.
In areas such as Ibadan, with a bi-modal pattern of rainfall,
two crops of grain can be grown each year. The first planted in April,
benefits from the more prolonged and consistent rainfall of the first
part of the wet season but must be harvested in August (during the 'short dry
season1) under conditions of high humidity and low insolation. The
second crop, planted in September and harvested in December or January,
may suffer from drought in some seasons but ripens under conditions
much more favourable to rapid drying, with lower daytime humidity and more
sunshine. These differences are exemplified by the climatic data for
the research site, covering the period of the current study (Figures
2.3 to 2.5). The contrast in the environmental conditions at the
times of the two harvests, and therefore in the background to the storage
12.
FIGURE 2.3 Climate at the study site, Ibadan : a) weekly total rainfall and b) mean daily solar radiation on a weekly basis for the period of the storage trials (August 1978 to July 1979). Maize was harvested in mid August ('Wet Season Harvest') and late December/early January ('Dry Season Harvest'). (Data courtesy of T.L. Lawson, Agroclimatologist, I.I.T.A.).
FIGURE 2.4 Climate at the study site: daily cycles of air
temperature and relative humidity typical of a) Wet
Season and b) 'harmattan' conditions. (Data for 17-20th
October and 21-24th November 1978, respectively; trans-
cribed from F.A.O. A.R.S.C. recorder).
13. a) WET SEASON
relative humidity s %
b> DRY SERSON
relative humidity / %
14
40
35 -O O \
o c. 3 +> a c_ ID Q. £ O
30
25 *
20
15 •
dally temperature
i i i i t
100
* 80 \ X t 6 0 "O
E J 40
o c 20
0
daily relative humidity
t i,
Rug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul
FIGURE 2.5 Climate at the study site: minimum, maximum and mean daily
temperature and relative humidity on a weekly basis for the
study period. (Data courtesy of T.L. Lawson, Agroclimatologist,
I.I.T.A.).
15.
• x
\
25 r
2 0
15 o
£
C 10
(0 i-U)
5 •
0
f j. observed m.c.. ....... predicted equilibrium tn.c.
j i i 1 1 1 1 1 » i 1 1
Rug Sep Oct Nov Dec Jan Feb Mar Rpr May Jun Ju 1
FIGURE 2.6 Comparison of moisture content observed in maize from cribs at Ibadan with 'predicted' equilibrium moisture content. Equilibrium values derived from daily mean relative humidity using an isotherm for white dent maize (from Gough and Bateman, 1977). 'Bars' on observed data represent the actual mean values for two replicate cribs (i.e. range).
trials discussed, is of great importance to the development of the
insect populations.
The data on mean relative humidities through the year (Figure
2.5b,) may be used, with a standard equilibrium moisture curve for
maize (Gough and Bateman, 1978), to plot the expected equilibrium
grain moisture content through the year. This may be compared with
the actual moisture content data obtained from the cribs (Figure 2.6).
It may be seen that the newly harvested grain rapidly approaches and
indeed falls below the 'predicted' equilibrium moisture content. Both
histeresis and the tendency for higher wind speeds to occur during
the afternoon(when relative humidity is at its lowest) may have contri-
buted to the apparent discrepancy between 'expected1 and observed
figures during the dry season.
2.3 Microclimate and Physical Conditions within the crib
The collection of data on the physical conditions within the cribs
was felt to be most important for interpretation of the biological data
and for any experimental investigations which might be carried out
later under controlled conditions. Particular instances will be dis-
cussed later (see §2.4 & §4.5), but at this stage it may simply be
noted that temperature and grain moisture content strongly affect the
reproductive success of storage pests (eg. Birch, 1945; Howe, 1956,
1960, 1962; Utida, 1971) and their behaviour (eg. Surtees, 1963a and 1965;
Amos et al.1967 & 1968). In addition to these two major parameters
(i.e. grain moisture content and temperature) it would seem likely that the
rate of air flow and the level of air relative humidity might be im-
portant variables in the crib environment. However, both are technically
difficult to measure and no attempt was made to do so in this study.
Considerable attention has been given to temperature and moisture
distribution in bulk grain stores and to their practical implications
(e.g. review by Muir, 1973), but little information is available for
maize cribs.
2.3.1 Grain Moisture Content
Insect sampling was based on the collection of samples of in-
fested grain and in all trials sub-samples of grain were retained for
determination of moisture content. An oven method, based on the
International Standards Organisation routine method (I.S.O., 1979),
was used. The method is described fully in Appendix IV but, briefly,
it depends on heating a small sample of ground grain in an oven at
130°C and determining the resultant weight loss (which is assumed
to be equal to the water lost). Samples of higher moisture content
(above 17%) require a two-stage process in which some of the water is
driven off (and the weight loss determined) at a lower temperature.
Electrical resistance methods for measuring moisture content, using
both the Marconi meter pressure cell and Reethorpe sensors (for
measurements within cribs), were investigated but both proved unsat-
isfactory in the relatively moist conditions encountered in these trials
consistent results could not be obtained with the Marconi meter while
grain inside some of the Reethorpe 'probes* became mouldy.
Determinations of moisture content were made either on grain from
single cobs or on 'pooled' grain from several.. At harvest time the
variation in m.c. between cobs (and, indeed, in grain from different
parts of single cobs) was wide (Figure Z. 7a).Moreover the grain often
appeared to have a markedly different moisture content from the core
(Figure 2.7b), however techniques for determining the latter variable
are not well established and the method used here may not be entirely
reliable. As the grain moisture content approached equilibrium the
18 .
8
\
> « 4
u 4> 0
e ta
samples from newly-harvested cobs
15 20 n o m m.o.
25 • * 30 35
2.0 r
\
1.5
> ® 1.0 TJ
• o +> id 0.5
0.0 10
samples -from cobs In store
* *
15 20 no an w • o • / %
25
FIGURE 2.7a) Variability in moisture content of grain and cores: intercob variation in grain moisture content showing greater variability in freshly harvested material (Data collected from several trials; storage samples were from material that had been in store for periods from 2 weeks to 10 months).
c o r e m.c- / %
FIGURE 2.7b) Variability in moisture content of grain and cores: b) relationship between grain and core moisture content; each •
point represents determinations for an individual cob. Straight line is for grain moisture content = core moisture content.
2 0 .
inter-cob variation decreased, but then increased again in the later
stages of storage due to uneven moulding and insect infestation of
the cobs.
This variation in the moisture content means that, at a particular
time, a variety of microhabitats are available to the insects, and
indeed there is evidence that moister cobs are preferentially in-
fested by some species (see §4.6). In practical terms, this variation
leads to particular problems in the accurate estimation of weight losses
(see §3.5 iand Appendix V), which must be assessed on a dry-weight basis.
Long-term changes in moisture content were monitored by deter-
minations at short intervals (2 to 4 weeks) on samples from the
centre of each of eight cribs. In addition, samples were taken more
rarely (1 to 2 month intervals), from 24 points in a single crib to
investigate the possibility that drying might be uneven in different
parts of the crib. Finally, uneven moisture distribution was more
intensively studied over a short period, using 32 samples removed on
each of 3 occasions from 3 cribs. The results of these trials are
described briefly below but discussion of their biological implications
will be left until the results of the associated insect sampling have
been considered (§4.6 and 6.6).
Data from the long-term trials are presented in Figure 2.8. Details
of the methods used are given in Appendix IV and of the overall crib
treatments in Chapter 6. Plotted points represent means from each
pair of cribs receiving the same treatment, based on determinations
on subsamples from 3 - 5 cobs from each crib; 'bars' indicate the
actual values obtained (i.e. range not standard deviation).
As already mentioned, the grain apparently dries to its equil-
ibrium moisture content very rapidly, although in the case of the wet-
season harvested material, this equilbrium level itself remains high
FIGURE 2.8 Grain moisture content in samples from the centre
of cribs at Ibadan and Ilora (as indicated). Plotted
points are mean values for each pair of replicate cribs
with bars indicating means for each crib individually.
Data based on three oven determinations for individual
cobs, from each crib.
2 1 .
25
N 2 0
o
E C "5 15 c. a
10
UNRERTED CRIBS - Ibadan
25 r
\ 20
o
e c « 15 Y CD
10
FUMIGRTED CRIBS - Ibadan
25 r FUMIGATED CRIBS - Ilora
\ 20
o
£
C « 15 c. O)
10 j 1 u J 1 L
Rug Sep Oct Nov Doc Jan Feb Mar flpr May Jun Jul
for several weeks (until the onset of 'harmattan* conditions). Cobs
used for all six 'wet-season' cribs were filled with maize from the
same field and harvested over a period of only three days. Comparison
of Figures 2.8) and b) indicates, however, that the fumigated grain
was loaded at a higher moisture content; this was due to a slight
build-up of moisture during the fumigation procedure (the material was
fumigated with phosphine for four and a half days in hermetically
sealed drums). The maize stored in cribs at Ilora Farm Settlement
appears to have dried more rapidly than that at Ibadan and also remained
at or near the minimum moisture content for a longer period. Ilora
is only 35 - 40km north of IITA, however it lies in the 'derived savannah
zone (while Ibadan lies within the current limits of the forest) and
seems to have a distinctly drier climate.
The data from the (multi-point) long-term distribution samples
are presented schematically in Figure 2.9, indicating the positions in
the crib from which samples were drawn; the figures given are the .
means of three determinations from each sampling point (carried out on
samples from three randomly-selected cobs). Differences between one
part of the crib and another are small, at most only 1 - 2 % , but
analysis of variance indicates that these differences are consistent
at a particular time. A preliminary analysis (not presented), in-
cluding the data from .all four sampling occasions and taking the 24
sampling points as separate 'treatments', confirmed the impression
that the moisture distribution was markedly different on each occasion
and, accordingly, the four samples were analysed separately. The
sampling points were grouped factorially, as indicated in Figure 2.10,
to allow different elements of the 'Position' effect to be separated;
the results of the analysis of variance are summarised in Table 2.1.
2 3 .
sample 1 (8/9/78) sample 2 (6/10/78)
y k . y y?.~y\ ^ y i s . y y U y ^ y y ? . y y i s . y y y'iG.y yxs.y
y / i s . y E / | ^ y s . y ySa.
y i i ,-y' y i ? . y y'\ 1 ^ " y * \
I y ^ - y y i y * y / s , / x
X | y ^ . y y s y
y A ? . y y ? . y y . y t s . y yis.^ y \ ? . y y \ ? . y y I y~\g.y y i 5 .y" y
y u . y y ? . y y y ™ - v y ^ - y y
sample 3 (22/11/78) sample 4 (11/4/79)
y y^.qy- y j . y y y * y y * y
' y * . y y - . y \ y y * - y .-- '"14 . S .s
S y * y y \ y ^ y y [ 5 - y -
i y ^ - y y ^ y y ' i y * y y * \ y y \ y I . ? y y ^ > y y i \ y s . y y s . y y
I y y i y y ^ y | ^ y s y y * - y y y 2 . y AI.AS | x y u . y y s . y '
| y y i i y y \ i y y * y y * - y
y ^ . y y * . y y y * y y u . y y
y i d . y y u . y y i y * y y * y y y i s . y y ' • y ^ . y y i s . y y
FIGURE 2.9 Grain moisture content in different parts of a single crib on four occasions during the storage period (dates as indicated on figure). Each figure is the mean of three oven determinations from individual cobs. Samples 1 and 2 were taken during the humid wet season (after 2 and 6 weeks in store) sample 3 under harmattan conditions (after 3 months in store) and sample 4, after the onset of the following wet season (after seven months in store).
24.
upper
I outer
_ middle 3 / 7East
Rrtalysls of Variance
F a c t o r 1 a l
R level 1n ortb C2)
B East-Heat (3)
C % e x p o s u r e ' (4)
replications (3)
FIGURE 2.10 Design of factorial analysis of variance for moisture
content data from long-term trial: data from various
sampling points in the crib were grouped factorially, as
shown in diagram; data for each sampling occasion were
analysed separately.
TABLE 2 . 1
Probabilities:
Source of variation df.
Total 71
A top-Bottom 1
B East-West 2
C exposure 3
AB 2
AC 3
BC 6
ABC 6
Sampling error 48
Sample 1 Sample 2 Sample 3 Sample 4
(8/9/78) (6/10/78) (22/11/78) (11/4/7$)
<0.001
<0.001
0.15
0.66
<0.001
0.31
0.55
0.15
<0.001
<0.001
0.33
<0.001
0.06
0.27
<0.001
<0.001
<0.001
0.07
<0.001
0.03
<0.001
0.21
0.001
<0.001
0.08
0.01
0.01
<0.001
TABLE 2.1 Probability levels from 3-factor analysis of variance of
moisture contents from different positions in a crib, at
four different stages during the storage season.
(3 'factors' are different components of a 'position'
effect : design as in Figure 2.10).
Samples were taken after 2, 6, 12^ and 30^ weeks in store.
Different factors seem to be operating on the four occasions.
Comparison of the analysis of variance with the raw data (Figure 2.9)
suggests that on the first three occasions there was a strong East-
West gradient (though not always in the same direction), possibly
imposed by the direction of the prevailing wind; on the the fourth
occasion, at the beginning of- the following wet season, the dominant
effect was a more uniform rehydration from all surfaces of the crib.
It should be noted in the former case that the prevailing wind may
serve either to increase the apparent moisture content, if it carries
rain, or to reduce it if the air relative humidity is low.
The short-term investigation was carried out over a six week
period during the main wet season (of 1979) using material harvested
and stored during the previous dry season. Data on grain moisture contents
was obtained from nine 32-point samples (i.e. on three occasions from
each of three cribs) and an additional 64-point sample from each crib
as it was unloaded. The design of the sampling schedule will be
discussed more fully in connection with the insect distribution studies
which were based on the same samples (see §4.2). At this.stage it
may be noted that in the analysis of the former series of samples the
sampling occasions appear as major blocks but that the inter-crib
variation cannot be assessed (each 'sampling occasion' and each 'Time
of day' including one sample from each crib); this should not be a serious
problem so long as the 'crib' and 'time of day* interaction is not
significant. The sampling points were grouped differently for the two
analyses, as shown in Figure 2.11a) and b); the groups were chosen so
that further comparisons could be made to separate different elements
of the 'position' effect (as in the previous analysis). However,
this did not, in the event, provide significant additional information.
27.
a)
Rnalysla of Variance Factor1 a! R time of day (3) B position In orlb (16)
(as shown) sampling occasions (3)
as blocks minor reps. (2)
top
uppsr-m1d
Touts r-mld
bottom
Rnalysls of Varlanes Faotorlal fl Ieve1 In orlb (4) B Nsxposurs' (8)
cribs (3) as blooks minor reps. (2)
FIGURE 2.11 Design of factorial analyses of variance for moisture
content data from short-term trials:
a) pattern for successive determinations during the
course of the trial.
b) pattern for analysis of final determinations at com-
pletion of trial.
2 8 .
TABLE 2 . 2
a) Anovar
Source of variation
Total
Blocks (sampling occasion)
A Time of d
B Positions
AB
Block error
Sampling error
df,
287
F ratio
B Positions:
probability
2 8 .37 <0.001
15 2 .44 0.004
30 0 .34 0.999
94
144
;: moisture content / %
r - a.m. : 17.1
mid-day : 17.5
p.m. : 16.8
East corners Upper 1. 18 .0 Lower 9. 17 .3
West corners 2. 17 .8 10. 17 .4
East faces 3. 17 .2 11. 16 .6
West faces 4. 17 .5 12. 17 .0
N/S faces (East) 5. 17 .5 13. 17 .1
N/S faces (West) 6. .17 .5 14. 17 .2
Interior (East) 7. 16 .9 15. 16 .5
Interior (West) 8. 17 .0 16. 16 .6
TABLE 2.2 Summary results of analysis: effects of time of day
and position in crib on grain moisture content
(Short-term distribution trial).
a) probability levels from analysis of variance
(design as in Figure 2.11a)).
b) treatment means.
29.
TABLE 2.4
a) Anovar
Source of variation df. F ratio probability
Total 191
Blocks (cribs) 2
A. Level in crib. 3 41.95 <0.001
B. Exposure 7 15.02 <0.001
AB 21 0.67 0.84
Block error 62
Sampling error 96
b) Treatment means (grain m.c./%)
A. Levels: Top 18.1
upper middle 17.2
lower middle 17.1
bottom 17.0
B. Exposure: East corners 17.4 West corners 17.8 East face 16.8 West face 17.6 South face 17.9 North face 17.0 Interior (East) 16.9 Interior (West) 17.0
TABLE 2.3 Summary results of analysis: effects of position in crib on grain
moisture content (termination of Short-Term Distribution trial).
a) probability levels from analysis of variance (design as in
Figure 2.11b)
b) treatment means.
Each moisture content determination was carried out on a subsample
of pooled grain from three cobs drawn from each sampling point.
The data on which these analyses were based are given in Appendix
IV and a summary of the results of the analysis of variance in Tables
2.2 and 2.3. Both analyses indicate consistent differences in moisture
content at different levels within the crib and at different points
within each level. The time of day at which the samples were taken
also appears to have a significant effect (Table 2.2). This is, ini-
tially, surprising, given the slow rate at which maize grains lose
and gain moisture, however, it may be that the differences are at
least partly due to superficially absorbed moisture, from rain and/or
dew.
2.3.2 Temperatures within the grain bulk.'
Temperature measurements were taken from the cribs during both
long-term and short-term distribution trials. A set of readings was
taken at the time of each insect sampling and additional readings were
taken to assess temperature changes through the day. Thermistors
were placed at different points within the grain bulk and readings
taken using a Telemax electrical thermometer. For the long-term
trial only seven thermistors were available and so these were arranged
to indicate the extreme conditions within the crib (i.e. near the
six surfaces and at the centre). In the short-term trials 33 thermistors
were used - 32 spaced regularly through the bulk and the last measuring
air temperature in the free space above the grain. Thermistors were
placed in contact with the cobs, to measure the grain surface temp-
erature, and even those in 'surface1 sampling positions were placed
within the grain bulk so that they could not be directly warmed by
the sun.
2 1 . Q 31.
21 .0
20. 0
2 6 . 5
20.5 s
20.5 •21 .0 2 1 . 0 -
19. 0 - 2 1 . 0
2 1 . 0 2 2 . 8
2 0 . 0
26. 0 27.0 y
y y :5.0
• 2 6 . 0
20. 0 1 3 . 0 0 3 . 0 0 h r s ( 2 2 / 1 1 / 7 5 )
0 7 . 3 © h r s ( 2 2 / 1 1 / 7 8 3
y 2 7 . 0
2 6 . 5
1 1 . 0 0 h r s C £ 2 / 1 1 / 7 8
3 0 . 0
2 9 . 5
3 0 . 0 3 0 . 0 -
2 5 . 0
^ 2 1.0-
29. 0
30.0
30.0
/ 3 0 . 5
3 0 . 0
3 1 . 0
29 .5
3 0 . 5
3 0 . 5
30.0- • 2 9 . 0
3 0 . 5
3 0 . 0
2 9 . 5 1 5 . 3 0 h r s ( 2 2 / 1 1 / 7 3 ) 3 0 - 0
1 3 . 4 5 h r s ( 2 2 / I 1 / 7 8 ) 1 7 . 3 0 h r s C £ 2 / 1 1 / 7 3
2 3 . 5
2 9 . 0
2 7 . 0
2 1.5
20.0
26.5 • 2 9 .0
22 . 0
26. 0 - 2 4 . 0 -
22.
s 2 5 . 0
22 . 0
2 0 . 5 2 1.0
20.0 /
19.0
2 1 . 0
22. 0 2 4 . 0 0 1 . 0 0 h r s ( 2 3 / 1 1 / 7 8 ) 1 8 , 5
£ 2 . 0 0 h r s C 2 2 / 1 1 / 7 3 ) 0 7 . 4 5 h r s ( £ 3 / 1 1 / 7 3
FIGURE 2.12 Changes m temperature at different points in a single crib
during a 24-hour period: Data for period
a) immediately preceding 3rd sample (long-term trial) under
harmattan conditions.
25. 3 29.4 32.
2 5 . 5 2 3 . 2
25.8 •28 .0 s
23.8
2 9 . 0
26. 2
08.08hrs (11/4/73)
30. 1
33. 5
•29.8 30.0-
33. 0
3 1.0
29.5 30.0 •32.0
3 1 . 0
29. 1 12.00hrs CI 1/4/73)
2 9 . 5
27.8
30.5
• 2 9 . 0
30.5
29 .5 29. 2
23 . 1
29. 3
2 8 . 0 - • 2 ? . a
30.8 3 1 - 5 20.00hrs (11/4/75) 2 9 - 2
16.00hr s Cll/4/79) 24.09hrs CI 1/4/79)
25. 0
6 . 2
26.4 25. 0 25 .5
23 .5
27. 8
04.00hrs (12/4/73]
25.2
26 . 4 •24.8 s
25.2 y
25.8
/ 2 B . 2
27.3 03.00hrs C12/4/79)
FIGURE 2.12 Changes in temperature at different points in a single crib
during a 24-hour period: Data for period
b) immediately following fourth sample (long-term trial)
under Wet Season conditions.
3 3 .
m \
\
k OJ
\
\
Tf \ \ OJ \ «-* \
» \ OJ * >
•
CJ \ V PJ \
PI
s: CD o CO
W in cj \
SI s V
ID
s
V s
Is
s s s
IS
•3 P7 m o
IS
N
k oc-Vi \
\ \
\
N
fs
\
x a 'V
rr> \
s.
r*
[ \
13 ©
OJ
IS
\ \
\ SI
IV s IS, N s
OJ
\
\
\
o CD
CO OJ
K K
VJ
\
\
\ \
\ s
s Q ©
'35 CD
\
\
\ X
k
N - ^ S
s
Is
\ \
IS \
•n ID 13 CU
FIGURE 2.13 Changes in temperature at different points in a crib
during a 24-hour period: data for 23rd February 1980,
during the dry season, but without 'harmattan1 conditions.
The temperature data corresponding to samples three and four of
the long-term trials are given in Figures 2.12a) and b) . In both
cases the grain seems to be providing considerable insulation against
the extremes in the ambient temperature: in the former case against
the low night temperature characteristic of harmattan conditions and in
the latter against the high afternoon temperature. It should also
be noted that the actual surface temperature" of cobs exposed to direct
sunlight will be considerably higher than the maxima recorded here
(from positions approximately 10cm from the surface of the crib). The
observations presented in Figure 2.13, from 32 points in a crib re-
corded in February, indicate a similar pattern to that shown in Figure
2.12b); again the largest temperature differential, of approximately
4°C, was recorded in the late afternoon.
The temperature gradients indicated here are low compared with
those encountered in shelled bulk grain stores, but, given the small
size and open construction of the cribs used in this study, it is
perhaps surprising that there are appreciable gradients at all. In
biological terras the insulating effect provided by the grain bulk
even in small cribs may well be significant in keeping the temperature
of most of the grain within a narrow range of the optimum for insect
development (c.27°C for several major pest species).
The temperature data from the short-term trials are set-out
schematically inEigura 4.11c. The data were analysed using the same
design as that used for the moisture contents. The summary results of the
analysis of variance and the basic design are given in Table 2.4, however,
it should be noted in this case that analysis of variance is not
strictly appropriate as the readings were, inevitably, collected from
single points in a systematic grid and so were not random samples from
3 5 .
TABLE 2 . 4
a) Design of analysis (as for moisture contents Figure 2.11a))
2 Factors : A Time of day (3)
B Positions (16 - as before).
3 Blocks : Sampling occasions
2 Replicates
b) Anovar,
Source of variation df Total 287 Blocks 2 A - Time of day 2 B - Positions 15 AB 30 Block error 94 Sampling error 144
probability
<0.001 0.06
<0.001
c) Treatment means (A x B interaction means) time of day:
Positions: a.m. mid-day p .m. 1 upper East corners 23.8 26.1 27.8 2 West corners 23.0 25.9 28.0 3 East faces 24.5 26.2 27.4 4 West faces 23.2 25.9 27.7 5 N/S faces (East) 23.7 26.1 27.7 6 N/S faces (West) 23.7 26.0 27.7 7 Interior (East) 24.3 25.8 27.1 8 Interior (West) 23.9 25.8 27.3 9 lower East corners 24.3 26.4 27.8 10 West corners 23.1 26.1 28.3 11 East faces 24.9 26.2 • 27.8 12 West faces 23.3 26.0 28.7 13 N/S faces (East) 24.2 26.3 27.8 14 N/S faces (West) 23.9 26.1 27.7 15 Interior (East) 25.1 26.2 27.5 16 Interior (West) 24.7 26.1 27.4
TABLE 2.4 Summary results of analysis: effects of time of day and position in crib on grain (surface) temperature. a) outline design. b) probability levels from analysis of variance c) treatment means (each figure is mean of 6 determinations).
3 6 .
the 'population' of possible readings within each sampling position.
This proviso apart, the results tend to support the obvious prediction
that the distribution of temperatures within the crib, as well as
their level, is markedly affected by the time of day.
It is of interest that comparable differences in both surface
temperatures and grain moisture content between East and West faces
of cribs were noted in an earlier study at Ibadan (F.A.O. 1975), but
not critically investigated.
2.4 Maize as a substrate for Insect development.
To complete the description of the environment in which the
pest populations develop, the characteristics of the substrate need
to be briefly outlined. The grain moisture content and temperature
have already been considered; in addition, the physical state of the
grain and its nutritional characteristics are of great importance
to the insects.
A particular batch of grain will have certain inherent character-
istics which may be partly or wholly determined by heredity (i.e.
varietal characters) and others resulting from the conditions under
which the particular crop was grown (for instance, periods of drought
stress or nutritional deficiencies). Such characteristics in the
case of maize include the nature of the endosperm (e.g. hard and
vitelline in the case of 'flint' types, soft and floury in 'dents'),
and its chemical composition (e.g. its lysine content), the nature of
the seed coat, and the number, size and form of the 'spathes' that
cover the cob. These characteristics are known to have a very marked
effect on the susceptibility of the maize to insect attack, (see,for
instance, the summary by Dobie, 1977).
These inherent features will be modified by any damage caused
to the grain by the harvester (human or machine), by rodents and
birds, by moulds and by the insects themselves. The intact grain is,
to some extent, protected by the intact seed coat and cob sheaths (if
present), and its hardness when dry confers resistance to some pests.
A limited range of agents (including some insect pests) will be able
to overcome these barriers and their activity will in turn make the
grain a suitable substrate for a much wider range of species. Damage
begins in the field (§5.2) and will be continued in store at first
mainly by the primary pest species (Sitophilus zeamais and Sitotroga
cerealella in the cribs studied) and by rodents. The progressive
increase in damage which occurs in store represents a significant change
in the nature of the substrate. No attempt was made in this study
to quantify these changes, except in as much as they are implied by
the weight loss figures; a great increase in the proportion of 'holedf
grains and in the quantity of 'frass' was however obvious in all cribs
over the storage period.
Moulds are of particular importance to the progress of substrate
degradation. Apart from the direct damage they cause to the grain,
moulds may promote insect damage either by favourably altering the
structure and nutritional status of the Substrate or by providing a
supplementary source of food (Sinha, 1971). Fungal infection occurs
in the field, involving both pathogenic species (e.g. Diplodia macrospord
and Ustilago maydis) and others which usually accompany insect damage
(e.g. Fusarium moniliforme). In the well-ventilated cribs used for
this study there was little visible spread of moulds to undamaged
grains and with the onset of dry-season conditions even damaged grains
did not become affected. When more humid conditions returned at the
beginning of the next wet season, extensive sporulation and dis-
colouration of grains was again observed on damaged cobs, especially
on those exposed on the surface of the cribs. Wallace (1973), con-
sidering mainly temperate conditions, describes a succession in the
community of fungal decomposers of grain: in the field, species of
Alternaria, Cladosporium and Helminthosporium are the most common but
in storage these are displaced by Aspergillus spp. and Penicillium
spp..
In assessing the importance of the various changes that occur
in the grain, it is important to recognise that only a small minority
of the insect species found in cribs are primarily adapted to a stored
grain environment, or even to seed-feeding. (The former group will
include, for instance, species that have become adapted to the ex-
ploitation of the 'natural' stores of rodents). The majority of species
have 'moved' to grain stores from rather different habitats such as
wood (Bostrichidae and Lyctidae), under-bark (Tenebrionidae and Silvanidae),
leaf litter and various other types of decaying vegetable matter
(Nitidulidae, Mycetophagidae). The various natural sources of 'storage
insects' have been reviewed by Linsley (1944). As will be discussed
later (Chapters 4 and 6), different features of a particular environment
will be limiting for each of these diverse groups of insects.
CHAPTER 3
3 9 .
SAMPLING TECHNIQUES
3.1 Introduction
The sampling of storage insects presents many of the same problems
that are encountered in other branches of insect ecology: the insects
are active, they may be unevenly distributed in space or time, their
numbers need to be related to an appropriate unit of their environment
and so on. The most significant problem particular to the crib en-
vironment is that of access : material can only easily be removed from
the top surface, while the insects infest the whole grain bulk.
Stratified samples may be collected relatively easily from stores
of shelled grain by the use of hollow probes, of which various designs
are available. In most, the grain and infesting insects are retained
in a chamber (or chambers) within the probe while in others they are
removed continuously via an aspirator tube running down its centre.
(Burges, 196o). A probe of similar design, the Iowa Corn Probe, is
available for use in cob-maize stores, the outer sheath in this case
bearing stout teeth which rasp the grain off the cores fin situ';
unfortunately this instrument has a number of drawbacks which make it unsui
for quantitative sampling of adult insects (§3.3.1)'. The alternative
is to remove whole cobs. However, in a maize crib the cobs are packed
so tightly together that it is virtually impossible to withdraw even
single cobs from the interior of the store. Cobs can be removed from
up to c.30cm from the top surface, but many of the more active insects
will be lost in the process.
Considerable attention is given in the existing literature to
appropriate methods for selecting stores for sampling (for instance,
de Lima (1975), Adams and Harman (1977), Drew.(1978))but relatively
little to the problem of obtaining representative samples within a
store. Most authors (e.g. Adams and Harman (1977), Pointel (1969))
have been content to take samples which mimic, or accompany, the
consumption of grain as it occurs in a subsistence store (i.e. grain
is removed at frequent intervals from the top of the store so that
the grain is entirely 'consumed' by the end of the storage season).
De lima (1973), sampling from rather small, shallow stores, found that
by reaching down 'as far as possible' into the cobs it was possible
to remove cobs from as deep as the bottom of the store after only
a few weeks of'consumption'. While such surface-sampling techniques
may be adequate for many practical purposes (for instance for loss
assessment surveys), they depend on the assumption that insect in-
festation is essentially uniform at different levels within the crib.
There seems to be little evidence on which to base this assumption.
Samples can be taken from all levels of a crib at the time of
unloading - and, clearly, cribs could be broken down after different
periods of time to obtain information on the progress of infestation
(although this method does not seem to have been widely used, pre-
sumably because of the quantity of materials required). Evidence from
such studies on the uniformity of infestation is conflicting. Schulten
(1972) found'little stratification of damage' while Kockum (1953 and
1958) did find significant differences between damage at different levels -
though the pattern was not uniform for the various treatments (in-
secticides) he used. Both Kockum (1953) and Pointel (1969) found differences
in the levels of damage near the surface and in the interior of cribs.
The variety of crib designs and environments in which they are
used is such that one might expect quite different patterns of insect
41.
infestation in different localities. It is, however, clear that
much more work is required to improve sampling techniques in general.
The assumptions on which particular methods are based need to be
defined and then tested in a variety of locations to investigate the
extent to which they are justified. In the remainder of this chapter,
the main objectives of, and constraints on, insect sampling in cribs
will be set out and the particular techniques considered for use in
this study will be discussed.
3.2 Insect Sampling : Constraints and Considerations
3.2.1. General Objectives
An acceptable storage method must, clearly,meet a variety of
social and economic requirements, but its technical success will be
assessed primarily on the extent to which it is able to prevent damage
to the stored commodity. In some instances it may be sufficient to
base the choice of storage method on loss assessment alone, but a more
rational strategy will involve the quantitative investigation of
particular components of the loss, including that due to insects.
Work is urgently needed to relate the losses due to insects
to their abundance. This relationship is complex, depending as it
does on the biology, behaviour and dynamics of each species involved;
this question was only superficially considered in this study. The
main concern here was to look at insect abundance itself and therefore
to assess the ability of various sampling techniques to estimate par-
ticular measures of abundance. Only a few of the trials went beyond
this to the application of sampling techniques to investigate questions
of practical importance - for instance, to study the effect of harvesting
practices on insect numbers and damage in store (Chapter 5).
42.
Particular attention was given to the study of dispersion
patterns and variability at the habitat and microhabitat level and
to changes in abundance related to short-term, daily cycles and
long-term, seasonal cycles in the environment (Chapters 4 and 6).
The criteria to be used for judging sampling methods must be those
applied in any branch of ecology (including repeatability, objectivity,
quantifiable accuracy etc) however it was often not possible here
to obtain independent estimates of particular population parameters
and so, in- many cases, discussion of the merits and drawbacks of the
various methods must be largely qualitative.
3.2.2 Choice of Sampling Universe.
The majority of insects found in maize cribs appear to remain
closely associated with the grain throughout their life cycles. For
many of the species the population level-in the crib may represent
a dynamic equilibrium, involving the exchange of individuals with
populations in the surrounding environment, but, on the whole, one
is dealing with species which are able to feed, reproduce and find
shelter within the sampling universe of the crib. The most obvious
exceptions to this rule are some of the larger and more active ants,
both phytophagous and predatory, which form colonies outside the
crib, but which forage within it. Probably several of the larger
species of Hymenoptera recorded from the cribs are only transitory vis-
itors from outside populations but these were never present in app-
reciable numbers. In general, one may reasonably hope to obtain a
useful estimate of the effective population levels of both adults and
iirmatures by collecting samples only from the grain in the crib.
3.2.3 Insect Mobility
In collecting samples of infested grain one has, at first sight,
43.
a direct estimate of the number of insects per unit of habitat or
substrate. The accuracy of this estimate will, however, be severely
affected by the level of activity of the insects. The pest complex
in maize cribs includes species, at one extreme, which are highly
vagile and easily disturbed (e.g. various Lygaeidae) and, at the
other, those which are developing within individual grains (e.g.
the immatures of Sitophilus spp. and Sitotroga cerealella) which
cannot escape at all. The level of activity of most species is affected
by the ambient temperature and many will respond to physical shocks
by flight or thanatosis.
A sampling programme based entirely on the collection of the
immatures of the primary pest species, combined perhaps with loss
assessment, could therefore be relatively easily carried out (using
virtually ainy method that allowed retrieval of grain sample's). It
is arguable that such a programme would be adequate for many practical
purposes. However, for this study it was clearly desirable to sample
from as much of the pest complex as possible. In an undisturbed crib
the majority of the insects feed and take refuge within damaged grains
or in the interstices between them. In practice it was found that, if
a cob is carefully picked-up and placed rapidly within a closed con-
tainer, the majority of insects associated with it may be collected;
if, however, the cob is 'jolted' or exposed to direct sunlight for more
than a.few seconds, insects will leave the cob in large numbers.
Methods had, therefore, to be evolved which allowed ready
access to the interior of the crib and with a minimum of physical
disturbance. Careful stacking of the cobs at the time of loading
was found to be necessary so that cobs could be removed singly (without setting-
44.
off 1 avalanches'of cobs) and collection of insects was easier in the
early morning when the low temperature made the insects less active.
The effectiveness of retrieval could not be quantified but it must
be recognised that, despite all precautions, collection of insects
probably was not equally effective for the adults of all species.
Moreover the retrieval of insects is markedly sensitive to practical
details in sampling and to the care and dexterity of the sampler.
Standardisation would present a serious problem if the methods des-
cribed here were to be used, for instance, as part of a more extensive
survey, involving several collectors.
3.2.4 Sampling Units and Variability
Individual cobs represent the smallest sampling subunit that
can be conveniently collected, but it is apparent that under normal
circumstances inter-cob variation in the level of insect infestation
is large. Single-cob samples were not taken on a regular basis, but
examples taken at various times over the study period give an indi-
cation of the distribution of most of the more abundant species from
the initial infestation in the field through the first half of the
storage period. In all cases for which sufficient data were available
for analysis, chi-squared tests for over-dispersion showed variances
to be significantly greater than means - i.e. distributions were
clumped rather than random, normal or regular. Negative binomial
distributions were found to give a good fit for most samples. Examples
of the observed distributions with fitted negative binomials (using
Fisher's maximum likelihood method) are given in Figures 3.1, 3.2, 3.4
and 3.5.
The major pest species, Sitophilus zeamais, was found to become
less strongly clumped with time in store (compare a) to c) in Figures
3.1 and 3.2) and, at a particular time, to be more ciumped when cobs
FIGURE 3.1 Distribution of Sitophilus zeamais (adults) on maize
cobs stored 'in the husk' after a) 6 weeks, b) 10 weeks
and c) 14 weeks in store. Data are the numbers of cobs
with the degree of infestation shown, based on samples
of 30 cobs on each sampling occasion (10 from each of
three cribs) collected c. 30cm below the upper surface
of the grain bulk. 'Expected* distributions are negative
binomials, fitted using Fisher's maximum likelihood
method.
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number of Insects • cob FIGURE 3.2 Distribut ion of Sitophilus zeamais (adults) on maize cobs stored
without husks after a) 6 weeks, b) 10 weeks and c) 14 weeks.
. Data presentation as in Figure 3.1.
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FIGURE 3.3 Dependence of variance on mean for samples of Sitophilus
adults from maize stored without husks. Data collated
from various trials: sample size 1 - 10 cobs, taken at
different stages in the storage season.
Correlation coefficient for the regression : 0.95.
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FIGURE 3.4 Distribution of emergences of major primary pests from.cobs
stored without husks after five months in store : a) Sitophilus
zeamais and b) Sitotroga cerealella. Data based in each case
on forty samples of lOOg (one sample per cob); emergences
scored after six days.
were stored in their husks (compare Figures 3.1 and 3.2 at each level)
The data from samples of different sizes and for different
levels of abundance may be collated (as described by Southwood,
1978) to provide a plot of log (variance) against log (mean), as
shown in Figure 3.3. The gradient of the regression is 1.38, corr-
2 — 1 38
esponding to the 'power' in Taylor's Power Law (i.e. S = 0.48 x * ),
indicating that the underlying distribution of the insect is moderately
aggregated and that an appropriate transformation of sample counts (to 0 3
normalize the distribution) would be of the form i Z = x . Southwood
notes that, for most purposes, data from 'slightly contagious' pop-
ulations may be transformed satisfactorily by using square roots and .
those from 'distinctly aggregated' populations by using logarithms.
The distribution of immatures (as judged by emergences) was also
clumped (Figure 3.4a) but in a number of examples (not shown) the
negative binomial did not show a close fit. Insufficient data were
collected to allow assessment of the distribution of the other major
primary pest, Sitotroga cerealella, but immatures of this species
showed a similar distribution (Figure 3.4b) to those of Sitophilus.
Among the secondary pest species Cathartus quadricollis was like
Sitophilus in being generally more strongly clumped in maize stored
in the husk (Figure 3.5), but differed in becoming more clumped with
increasing time in store. This aggregation reflects the concentration
of Cathartus in the diminishing patches of sufficiently moist (and
usually mouldy) maize grains. The commonest Tenebrionids, Palorus
subdepressus and Gnatocerus maxillosus,were not sufficiently abundant
in these samples for satisfactory assessment but their distribution
appeared to be more clumped on maize without husks. Carpophilus spp.
showed a similar (high) degree of clumping in both situations. All
species were highly aggregated in field samples (k values for the
negative binomial being less than one in most cases), both in
samples taken immediately before harvest and a month earlier.
Although the insect distributions have been described as 'clumped
this term is not intended to imply the existence of a behavioural
response to the presence of other insects. It is much more likely
that the clumping reflects variation in the cobs (for instance in
their moisture content) and results from their aggregation in favour-
able microhabitats (as mentioned for Cathartus quadricollis, see also
Section 4.7). This aspect is most important from the point of view of
sampling : in some situations it may be desirable to sample from
populations showing the full 'natural' range of variation. However,
where the effect of only one particular factor (for instance, position
in the crib) is of interest, it may be possible to reduce some of this
extraneous variability by prior selection of more uniform cobs.
Part of the variation is simply due to the fact that some cobs
are larger than others. Accordingly, in reporting data from all trials
a simple correction has been applied so that insect numbers are related
to standard quantities of shelled grain at a specified moisture content
Some further variation is attributable to genetic variability in the
inherent susceptibility of the grain to insect attack. For example
the material used for the initial trials here included a wide range
of endosperm types from typical 'flints' to 'dents'. While such
variability is inherent in some 'composite' varieties, such as those
used here, more uniform selections within the variety can be chosen
in advance. Finally, some variation will be attributable to difference
in the physical state of the cobs, for instance, relating to their
physiological stage at the time of harvest or to damage in the field.
Where such conditions are visible at the time of loading the affected
5 1 .
a)
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FIGURE 3.5 Distribution of Cathartus quadricollis adults on maize
cobs after three months in store: a) cobs stored 'in the husk'
and b) without husks. Samples collected as for Figure 3.1.
cobs can be removed and the subsequent reduction in basic variability
will make it easier to detect any 'treatment' effects.
3.2.5 Sample size in relation to the size of cribs.
The variability shown by the populations to be sampled, even
when the above measures have been taken to reduce it, is such that
large samples are desirable in order to obtain population estimates
with an acceptably low standard error. However, the number of cobs
that may be collected for a particular sample is constrained by various
considerations arising from the small scale of cribs.
The cribs used for these trials each held about a half tonne
of maize cobs. Larger cribs might have been desirable but their size
was effectively determined by the availability of suitable material.
Trials carried out at the study site (by other workers) indicated that
large differences in insect infestation could be expected in cribs
loaded only a few days apart (Boshoff, unpublished data). It was
clearly desirable that the cribs to be used for this study should be
loaded with maize of uniform origin and that loading should be com-
pleted over as short a period as possible.
In studying small grain cribs, there is a risk that the sampling
programme itself may significantly deplete the pest populations - both
directly, by removing insects and indirectly in that the 'disturbance'
involved may cause them actively to leave. An half tonne of maize
contains only about 2,000 cobs. Clearly, if say, 20 to 30 cobs are
required for an adequate sample, repeated sampling may represent an
appreciable drain on the population. The cobs removed may be replaced
with infested material from elsewhere but there are problems in ensuring
that the cobs brought in are truly comparable. To minimise active
emigration it may be helpful to ensure that as little of the g r a m bulk
as possible has to be disturbed in removing the sample and to leave
parts of the bulk as 'refuges* which are not used for sampling and
from which reinfestation of depleted parts may occur.
There is also a problem with small grain stores that 'edge effects'
may in fact affect the whole grain bulk. In the experimental cribs here,
the maize cobs occupied a volume of approximately 90 x 90 x 140cms.
Accepting for the moment the likely existence of some form of gradient
in insect abundance from surface to interior, it is clear that any
changes must occur over rather short distances and accordingly, if
sampling is to detect such changes, the cobs must be collected from a
comparably small sector of the crib. This too tends to conflict with
the desire to collect large samples..
3.2.6 Sample Size and Handling Time.
The 'handling time' for a particular set of samples includes
three main'components : the time required to collect the samples from
the cribs, that needed to remove the insects from the grain samples
(and to carry out any tests, such as moisture content determinations,
that require the grain samples to be 'fresh') and finally the time
taken subsequently to identify and count the insects collected. Of
these, the last is the least critical. Although it is often desirable
to have the results from a particular sample immediately (and, moreover,
fresh samples are often easier to score than preserved ones), the insects
can, in principle, be preserved in fluid for later attention. The
time taken to collect the live insects is, however, critical to the
success of sampling, and imposes severe constraints on the number and
size of samples that may be taken.
Firstly, it is important that samples should be collected from
all comparable 'treatments' over a very short period of time. It
has been noted that the retrieval of insects will be markedly affected
by different environmental conditions,- such as one might encounter
if samples were to be collected on different days or at different
times on one day. If sufficient replicates are available this will
not be important, however, if replication is limited (for instance by
the supply of materials or time), it will be important to minimise
such 'non-treatment' variation.
Having collected the samples from the cribs, it is important to
'process' them as quickly as possible and, in practice, this should be
completed within a few hours. Various undesirable effects occur in
infested grain samples stored in closed containers : in moist or
heavily infested grain, the build-up of carbon-dioxide., condensation
and sometimes temperature can kill the insects or at least make them
difficult to collect; the moisture content of the grain may be appreciably
raised - which will be important if this is to be determined or if the
material is to be retained to assess emergences; there may be inter-
actions between the adult populations and immatures within the grain -
for instance there may be a significant number of emergences over a
period, or parasitism and predation may be intensified; many storage
insects are capable of perforating and escaping from plastic, paper or
cardboard containers. Finally, if material is to be returned to the
crib, it is desirable to do this as soon as possible.
The various possible methods for the extraction of insects from
infested grain will be discussed in a separate section (§3.4). Whatever
the method employed, the time required for this is considerable and
represents the major practical limitation on the quantity of material
that can be used in sampling.
All the issues discussed in this chapter will arise again in
considering the methods used for particular trials and in the imm-
ediately following sections relating to basic sampling techniques.
It must be recognised that, in all cases, the choice of sampling regime,
and in particular the number and size of samples, represents a com-
promise between the desire to estimate accurately population parameters
and the constraints imposed by the habitat under study and the limited
time and materials available. In a number of instances this compromise
cannot be regarded as entirely satisfactory.
3.3 Assessment of Insect Sampling Techniques
The field studies commenced in April (1978), approximately three
months after the dry-season harvest, at which time six half tonne cribs
of heavily infested maize were available for study. Initially a large
number of samples were taken using the Iowa Corn Probe. A number of
cribs were then emptied, in the course of which samples were taken for
comparison with those obtained using the probe and to assess more
generally the potential of such 'destructive* sampling.
These preliminary investigations indicated serious short-comings
in both methods. Accordingly a technique was developed which will be
referred to as 'partially destructive' or 'replacement' sampling. This
involved structural modification of cribs to allow cobs to be removed
from, and replaced within, the grain bulk without unloading the whole
crib. The major part of the material removed on a particular occasion
would be immediately replaced in position in the crib while small sub-
samples were retained. This method was employed, with progressive
modifications, for most subsequent trials. Details will therefore be
considered later, in the context of particular trials, but the general
.5 6 .
principles will be set out here for comparison with the other two
options considered.
3.3.1 Iowa Corn Probe
The Iowa Corn Probe has, at first sight, a number of advantages
as a sampling tool. i) The probe could be used to collect grain from
any part of the crib; ii) no structural modification to the crib or
changes in the wall material were required (the probe could be pushed
through wire mesh or between palm slats); iii) grain could be collected
from a crib filled in the normal 'haphazard' way (as compared with the
ordered stacking required for replacement sampling - see §4.2); iv)
all species, including active Hymenoptera and Heteroptera could be
collected (though not necessarily quantitatively); and v) a large number
of samples could be collected rapidly and with relatively little effort.
In practice, however, the probe proved unsatisfactory. The
probe collects too small a quantity of grain at each insertion while
causing excessive disturbance to the insect population and damage to
the grain. Each insertion of the prqbe collects only 20 - 30g of
shelled grain and, although the wide 'catchment-area' of the sample
should tend to reduce the effects of inter-cob variation, this is in-
adequate to provide a quantitative estimate of population density.
Figure 3.6, based on samples collected from a single crib indicates that
the numbers of adult insects are not related to the quantity of grain
collected by the probe.
There is evidence that the number of insects collected by the
probe is in some way related to insect abundance in as much as reasonable
consistent results can be obtained. Table 3.1 shows the results of
the analysis of variance of some of the data collected from four different
cribs, sampling from sixteen different positions as shown in Figure 3.7a).
.5 7 .
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FIGURE 3.6 Scatter diagram for numbers of Sitophilus adults collected
in each sample against the weight of grain collected in
that sample, using the Iowa Corn Probe. Probe samples
collected randomly from different parts of a single
crib.
.5 8 .
a) Rnalyata of vtrltno*
Single Factor position* (16) as treatment® or Ibe (3 or 4) me blooke
- top
- middle
- bottom
R n« 1 ye 1 e of varleiiea
Fmotorlal
R North-South (2) B Eiet-Weet (3) C bottom-top (3)
treatment mean* teeted ve • 3-y»ey Interaction
mid
FIGURE 3.7 Design for analysis of the effect of position in the crib
on the number of insects collected in probe samples. Diagrams
indicate approximate positions in the cribs from which
samples were drawn.
a) samples collected from 16 points in 3 (or 4) cribs.
b) samples collected from 18 points in one crib.
.5 9 .
TABLE 3.1 Differences in adult insect abundance in probe samples
from different positions in 3 cribs (Carpophilus) or
4 cribs (Sitophilus). Sampling positions as shown in
Figure 3.7a).
a) Probability levels from a single factor analysis of variance with
positions as treatments (16) and cribs as blocks on data transformed
log e
Sitophilus Carpophilus Source of variation df. zeamais dimidiatus
Total 63 0.01-0.025 <0.01
Positions 15 <0.0l <0.01
blocks (cribs) 3
error. 45
b) Treatment means - number of adult insects/50g fresh weight
(arithmetically corrected from sample weight).
Position Sitophilus Carpophilus
1 73 197
2 71 460
3 74 61
4 74 113
5 96 40
6 91 75
7 78 36
8 53 33
9 124 36
10 117 42
11 96 56
12 105 52
13 195 104
14 113 140
15 160 60
16 105 97
6 0 .
TABLE 3 . 2
a) Probabilities:
Source of variation df. Sitophilus Carpophilus Sitophilus Sitotroga
(adults) (adults) (emergences) (emergences)
Total 17
A North-South 1 0.05-0.1
B East-West 2 <0.01 <0.01
C Bottom-Top 2 0.05-0.1 0.025-0.05
AB 2 0.01-0.025
AC 2
BC 4
ABC 4
b) Treatment means
A North 82 43 2.1 . 3.4 South 89 . 35 1.7 2.3
B East 93 47 0.8. 3.0 Mid 61 19 2.7 2.7 West 103 52 2.2 3.5
C Bottom 80 45 • 0.7 1.0 Middle 87 36 1.3 2.3 Top 89 37 3.7 5.8
TABLE 3.2 Insect abundance in probe samples from different parts of
a single crib (sampling positions as in Figure 3.7b).
a) Probability levels from a 3-factor anovar, using the 3 major axes
of the crib as factors (probabilities greater than 0.1 are omitted).
Data were insects/sample (arithmetically corrected for sample size)
for adults or number of insects emerging/sample over ten days; data
transformed logarithmically for analysis.
b) Treatment means (actual values): adult counts are -numbers of insects/50g.
of grain, fresh weight (arithmetically corrected); emergences are
numbers retrieved from 25g samples over the period 10 to 20 days after
collection (figures for 0-10 days were severely affected by parasitism).
Some sampling positions yielded higher numbers of insects than others,
both for Sitophilus and Carpophilus spp. (although the pattern was
different for the two species), but the figures were not consistent
for all the cribs (and, indeed, the anovar indicated no significant
'position 1 effect when all possible figures were included). Similar
'significant' differences between insect numbers in groups of samples
(Table 3.2a) were found when figures were analysed for several samples
within one crib (Figure 3.7b) or when subsequent emergences from the
samples, rather than adult counts were analysed (Table 3.2b). While
these results provided an interesting indication that insect distribution
was non-uniform within the cribs and suggested that the probe might be
useful in some circumstances where only a comparative index of in-
festation was required, they clearly showed that the probe could not
give sufficiently precise estimates of adult insect populations for
use in this study.
Initially it seemed possible that the probe might, nevertheless,
be used to collect grain samples for 'breeding-out* of primary pests
and for damage assessment, however, in these applications too it proved
unsatisfactory. Considerable force is required both to insert the
probe into the crib and then to rasp the grain off the cobs. This
leads to four major problems : i) samples cannot be taken near the
top of the crib as the weight of the cobs provides insufficient resistance
against which to shell the grain; ii) large quantities of grain are
shelled but not collected and so accumulate at the bottom of the crib;
iii) both moist grains early in the season and badly damaged grain
later on, are broken by the probe rather than simply shelled off the
cob; and iv) the disturbance caused by the probe is considerable: The
sensitivity of the insects to mechanical shocks has already been men-
tioned; during probe sampling insects could be seen to leave the crib
6 2 .
in considerable numbers.
Finally, although not of importance to this particular study,
it was clear that the results obtained with the probe were markedly
affected by the person taking the sample. An inexperienced operator
tends to collect large numbers of insects (and quantities of frass)
but little grain, and will also tend to obtain markedly inconsistent
results. 1
3.3.2 Destructive Sampling of Whole Cribs
The collection of samples at the time of unloading the crib
(i.e. 'destructive sampling') has three obvious advantages: i) the
size and origin of the sample can be precisely controlled and an
appropriate selection procedure applied (for instance, it is easy to
choose cobs in a precisely defined stratified random manner); ii)
larger samples can be taken as 'depletion' is not a consideration; and
iii) samples will be truly representative in that they will have re-
mained undisturbed until the time of sampling.
For the purposes of this study, however, destructive sampling
presented three problems i) Adult insect counts could not be obtained
from such samples. Even when cobs were removed a few at a time, with
great care, appreciable numbers of insects were clearly falling from the
cobs as they were removed thus increasing the apparent infestation in
the lower cobs still to be sampled; ii) frequent sampling was required
to monitor pest population increase and, if whole cribs had to be un-
loaded on each occasion, this would have required prohibitively large
quantities of material (it would be effectively meaningless to reload
a crib and use it for a later sample); iii) the unloading of an infested
crib causes a massive exodus of insects, inevitably affecting nearby
cribs. Even when rapid-knockdown pyrethroid insecticides were used to
.6 3 .
try to minimise cross-infestation, a marked increase in insect counts
in nearby cribs was apparent after unloading.
Destructive sampling remains the obvious choice for trials
where the assessment of damage and hidden infestation will yield
sufficient information. It may also be practicable to obtain adult
counts in this way where cobs are stored in their sheaths (which markedly
reduces the speed with which insects escape from the cobs).
3.3.3 Replacement Sampling
The cribs to be sampled in this way required extensive structural
modification. The crib was divided with partitions (horizontal and
vertical) of galvanised 'chicken wire' ( 5 " mesh) supported on light
wooden lathes - (see Chapter 4.2 for details). Any of the sections
so formed could be emptied of cobs, the remainder of the grain being
supported by the wire partitions. Access was gained-via 'trap-doors'
in one (or two) of the vertical faces of the crib, which had itself
to be of wire (rather than the normal wooden 'slats' or palm-frond
petioles). The cobs were marked, with a band of indellible ink, and
those from separate sections retained separately so that all could
be replaced in approximately the position from which they had been
removed. Cobs retained on each occasion (for instance those used to
estimate emergences and moisture content) were replaced with cobs from
another crib that had been in storage for the same period.
In the preliminary trial of this system, samples of five cobs
were collected from each of 24 points. Initially adult insects were
removed from intact cobs and the cobs replaced in their former positions;
however, as infestation increased, this system became too slow and the
grain had instead, to be shelled^nd sieved to remove the insects. This
.64.
in turn, meant that large quantities of cobs had to be brought in from
another crib on each occasion but these cobs, though of similar age
in store, would not have any physical attributes or the level of
infestation characteristic of the positions into which they were
placed. The method required considerable time and effort and so could
not be very frequently repeated. Moreover the samples, though large
enough to indicate that distribution was not uniform, did not give
a satisfactory estimate of population density in absolute terms. The
number of cobs to be collected from each position could not be in-
creased without losing positional 'resolution 1 (see 3.2.5), increasing
the depletion effect and impossibly increasingly the handling time.
It was concluded that separate trials should be conducted to
monitor changes over time and to investigate distribution within the
cribs. For the former, a small number of large samples could be taken
on each occasion from one part of the crib; this could be repeated
frequently and oh several cribs and would give an accurate estimation
of one index of the population changes (i.e. only from one point).
These results could be compared with those from less-frequent samples
taken from many points in a crib which would indicate any major changes t
in population distribution. For the 'sequential* samples the adult
insects could be removed from the cobs without shelling and the cobs
replaced; reinfestation should be rapid from the remainder of the
undisturbed grain. Sampling in the distribution trials would be
'destructive' (i.e. samples would have to be shelled) but, because of
the wide sampling interval, cobs introduced from the reserve crib would
have a considerable period to 'equilibriate' to their new position
before being used for sampling. The methods and modifications used
in the various trials will be detailed in the subsequent chapters relating
to them (i.e. "Succession Studies" in Chapter 6 and "Distribution
.6 5 .
Studies" in Chapter 4).
3.3.4 Sampling to Estimate Recruitment
It was decided at an early stage that the recruitment of the
main pest species, Sitophilus zeamais and Sitotroga - cerealella, would
be assessed only by scoring the emergence of adults and that no attempt
would be made to sample the immatures of secondary pest species. It
was felt that the extra information which could be gained would not
justify the time required, in the former case to dissect the grains
and in the latter simply to collect and identify the larvae.
Two methods were considered for the sampling of the primary pests.
One possibility was to plant uninfested (fumigated) cobs into the crib
for a short period (say, one week) to allow eggs to be layed on them;
the cobs could then be retrieved, all adults removed, and the cobs
retained for at least fifty days to allow all the developing insects
to emerge. The developing larvae would suffer competition only from other
larvae of the same age and would not be subject to parasitism.
The alternative was to collect cobs that had remained undisturbed
in the crib for a long period and to retain these for a rather short
period, say five days. The adults emerging over that period would
represent an estimate of net recruitment. In this case the larvae would
be subject to the 'normal' pressure of parasitism and competition except
for the short period afte.r their removal from the crib.
The former method was rejected mainly on the grounds that ovi-
position on the newly introduced, uninfested cobs might well be very
different from that experienced on the surrounding infested ones; the
rate of oviposition of Sitophilus spp, at least, is known to be markedly
affected by adult density. In practice, moreover, it proved difficult
to obtain a satisfactory independent es timate of the rate of parasitism
6 6 .
in the crib and difficulties were encountered in retaining the
samples for so long an emergence period: mould development on damaged
grain was severe, plastic or card containers were readily perforated by
emerging Coleoptera (especially Bostrychidae), and parasitism was
often severe (presumably the parasitoids were introduced with the
late instars and pupae of secondary pest species that had entered
the sample during its exposure in the crib) .
The method adopted was not without problems. Parasitism,
especially of the late instars of Sitotroga, was at times severe and
so the estimate of recruitment was very much dependent on the period
over which emergences were scored (Table 3.3). Parasitism is also
likely to occur during the assessment period; this could be largely
avoided if emerging insects were removed each day, but this was precluded
in this study by the time required. Despite these shortcomings, the
method was felt to provide at least a satisfactory indication of the
rate of recruitment for comparative purposes.
3.4 Extraction of Insects from Grain Samples.
The method used to remove the insects from the maize varied
somewhat, depending on whether it was to be replaced in the crib (in
which case it could not be shelled) or, if not, whether the sample
was to be- subsequently used for estimation of recruitment and/or
moisture content.
For samples which were to be replaced in the crib, the cobs
were transferred, a few at a time, to a large plastic bag, inside which
they could be knocked gently together, and the insects so dislodged
collected in an aspirator. The main limitation on the speed with
6 7 .
TABLE 3.2
emergence period (days after collection). p (no diffs.)
0 - 1 0 1 0 - 2 0 20 - 30
Sitophilus zeamais
5.0 6.4 6.0 >0.5
Sitotroga cerealella
2.8 13.4 12.0 <0.001
parasitoids 6.9 2.8 0.7 <0.001
TABLE 3.3 Effect of time after collection -on numbers of insects emerging.
Mean no. of insects emerging from ten lOOg samples (from
individual cobs) in successive ten day periods after sampling,
with probability levels from a single factor analysis of
variance of the data transformed logarithmically,
(cobs were collected from approximately 30cm below the surface
of the cobs in Crib 25 - that used for the probe sample <
investigation, Table 3.2).
68 .
9UNLXGHT
FIGURE 3.8 Arrangements used for the rapid extraction of insects from
samples of a) shelled grain and b) cobs. Insects were
retained in the collecting tube either by painting a band
of 'Fluon' around the top (inside) or by adding alcohol.
Funnels used were approximately 25cm diameter. When using
solar heating (case b)), evaporation of alcohol (and con-
densation on the perspex lid) could be reduced by standing the
extraction funnel in a large jar of cold water.
which the insects can be collected is their tendency to take refuge
within damaged grains, where they may remain undetected for long
periods. If the grain was warmed slightly, for instance by leaving
the bagged sample in the sun for a few minutes, the insects became
more active and so more easily collected; this had, however, to be
carried out very carefully to avoid killing the insects or driving
moisture out of the grain. If a light source was placed at the closed
end of the bag during insect collection this both served to increase
the general level of insect activity and to attract the actively
flying species, in both cases making them easier to collect. The
proportion' of insects which could be retrieved by this method was
limited largely by the time available; it became increasingly difficult
to collect the insects as the grain became heavily damaged.
When the grain could be shelled,collection of insects was made
much faster by the use of nested sieves (2.5mm and 0.8mm mesh). This
afforded the added advantage of providing insect samples less con-
taminated with frass, but a number of the smaller species (especially
Cucujidae, Corylophidae and Scelionidae) were able to pass through
the finer mesh and were then very difficult to separate from the
debris. Sieving had to be used cautiously on samples to be used
for estimation of recruitment as violent sieving can cause considerable
mortality of immatures.
Collection of insects from shelled grain was more efficient if
the insects were sieved directly into a tube of alcohol via a large
funnel (Fig. 3.8a)). This method could, however, only be used in
samples showing little damage. In samples from late in the storage
season the large numbers of dead insects in the grain would also be
collected (and could not always be distinguished in the preserved
.7 0 .
samples) together with quantities of frass; a second separation was
then required, usitig wet-sieving or a kerosene flotation method.
Tullgren futinels, suitably modified, have been recommended for
the extraction of storage insects (Golob, Ashman and Evans, 1975).
In practice, this method proved too slow to deal with the large
quantities of grain that had to be processed for this study. A more
convenient alternative, using sunlight as the heat source,was developed
(Fig.. 3.8b)); this method proved much faster than the conventional
Tullgren funnel but, of course, suffers from the same drawback of heating
the grain that the sample cannot subsequently be used (except perhaps
for damage assessment).
3.5 Damage Assessment
Estimates of loss in quality and/or weight (in the stored grain)
which can be related to economic loss y are vital to the development
of rational pest control strategies. As previously mentioned, little
attention has been given to the relationship between the level of insect
populations and the damage they cause. In this study weight loss was
monitored in parallel with all insect population studies to show how
damage progresses over the storage period and how losses are distri-
buted in the crib. In the absence of information on the contribution of
individual species to the overall loss, these figures can be only
imprecisely related to the observed insect populations; the data required
for a more complete interpretation of the figures could, however, now
be collected in laboratory studies.
A variety of established methods are available for the estimation
of losses in stored grain (Harris and Lindblad, 1978). However, all
were rejected for the purposes of this trial on the basis of the time
required to carry them out or their lack of precision. The method
.71.
v—ni
• * » * » '
4
» ' / / « 1 — 1 < f J
• - e 'L / ' • » 'i i / / e
moan final weight loos
unaampTod
sampled
unaampled
ProlImlnary Trial
31.1 o
25.3 a
28.1 b
analysis of variance
Succession Trial 34.0 o
30.1 b
25.0 a
source. . df P P total 59 R cribs I 0.51 0.14 B pos'n 2 <0.001 <0.001 R x B 2 0.53 0.10 error 54
FIGURE 3.9 Weight loss of grain for cobs from sampled and (adjacent)
unsampled parts of a crib under more and less intensive
sampling regimes: cribs in the Preliminary Trial were
sampled six times in three months and those in. the succession
trial six times in six months.
Means (percentages, dry weight basis) are based on estimates
for ten cobs in each sampling position in each of two cribs.
For details of methods see Chapter 6.
developed here is described in detail in Appendix V . It depends
on the identification of individual cobs (numbered and*banded*with
indellible ink) which are weighed at the beginning and end of the
storage season and, in some cases, at intermediate times. Deter-
minations of grain and core moisture contents are made in parallel,
on comparable cobs, or on sections cut from the loss assessment cobs
to allow corrections to be made for changes in weight due to loss or
gain of water. The precision of the method depends largely on the
inter-cob variation in moisture content : trials commenced at high
moisture contents, soon after maturity, when variation is greatest,
are less accurate than those begun when the grain moisture content
is approaching equilibrium*. There o.s-also the possibility that some
of the inaccuracy when grain moisture content is high is due to phy-
siological processes in the grains producing real dry weight losses.
In addition to information oh damage per se, loss assessment
was also found to be useful as at least a crude check on the effects
of the insect sampling procedures. In most trials, cobs for weight
loss estimation were included in both sections of cribs that were to
be used for insect sampling and in adjacent sections that were not.
If the cobs in the sampled sections proved, at the end of the season,
to have been damaged significantly less than those in the undisturbed
sections, this would be strong evidence that the sampling had depleted the
insect populations, or at least affected their pattern of distribution.
This was found to have occurred in some trials (Figure 3.9).
.73.
CHAPTER 4
INSECT DISTRIBUTION WITHIN THE CRIB
4.1 Introduction
The factors which influence the distribution of insects within
bulks of grain have been intensively studied in the laboratory. Surtees
(1964b) showed that, even in uniform darkened cubes of grain, each of
the five species studied took up a characteristic three-dimensional
distribution, which he describes as a 'dispersion pattern 1. The re-
sponses of several species to temperature and moisture content gradients
and surfaces in small grain bulks have also been analysed (Surtees,
1963, 1965c; Amos, et al., 1968) and the effect on these responses of
the condition of the insects and the presence or absence of light have
been noted (Amos, 1968 and 1969). Surtees (1964c, 1965 a-c ). and others
have shown how these reactions together will tend to result in aggre-
gation of pest species in conditions favourable to them and particularly
in patches of damaged grain.
It is generally accepted that, in stores of bulk grain, insect
infestations will be patchily distributed, and the implications of
this for insect control and general store management have been widely
discussed (see, for instance, Cotton and Wilbur, 1974). Insect movement
in response to changing physical conditions in large-scale stores in
the tropics have been described by Smith (1963), Graham (1970) and
Prevett (1964) . Grain stored in small cribs will be exposed to a
variety of changing environmental conditions and it may reasonably be
assumed that the infesting insect populations will react similarly to
the stimuli provided. This possibility, however, does not seem to have
been critically investigated and has rarely been considered in the design
.74 .
of sampling programmes for studies of crib stores.
Uneven distribution of insects in small-scale maize stores has
been implied in a number of observations. As mentioned previously
(Section 3.2), Kockum (1953) and Pointel (1969) studying quite different
types of cribs, both noted higher damage levels near the surface, while
the former author also noted some vertical stratification. A *sumpf
effect in stores of shelled grain (i.e. a limited area of very highly
damaged grain at the bottom of the store) has been noted by various
authors (e.g. Adams and Harman 1977). Direct evidence in terms of
actual insect numbers is less common in the literature although it
has often been reported that Sitotroga cerealella is only abundant on
the surface of stores (e.g. Kockum, 1953, in the study noted above;
Coyne, 1945; Salmond, 1957).
The need for more quantitative data on insect distribution
patterns has already been discussed (3.2.3) in relation to sampling
methods and loss assessment. The methods used in this study to in-
vestigate insect distribution are derived from those used by Surtees
(1964a) in the laboratory: the grain bulk (in this case a complete
half-tonne crib of maize cobs rather than a small cube of wheat) was
subdivided so that it could be rapidly broken down and the insects from
various parts collected separately; differences between the numbers
from the different parts could then be investigated by analysis of
variance (though this poses certain difficulties here which are dis-
cussed in the following section). In addition, some effort was invested
in estimation of the naturally existing gradients of temperature and
moisture content (as described in Chapter 2) and grain damage.
In the field it is likely that changes in distribution will
occur not only in space but also in time. Successional changes in
insect abundance over a period of several months were noted by de Lima
(1978) in small maize stores in Kenya while daily cycles in the activity
of several storage species have been noted in the laboratory (Barnes
and Kaloostian, 1940; Amos et al., 1968), in fields of the growing
crop and in stores (Riley, 1965; Ajibola-Taylor, 1971; Giles and Ashman,
1971). The possibility that such temporal effects might be reflected
in changes in spatial distribution patterns was investigated both over
the period of the storage season and over a matter of days at one time
in the season.
4.2 Sampling and Analysis for Insect Distribution.
Three trials were carried out using a similar basic technique.
This involved partitioning the crib into a number of sections from
which many samples could be removed, rapidly and with a minimum of
disturbance to the grain bulk. The first trial was concerned prim-
arily with developing the sampling technique and will not be described
in detail. In the second trial samples were taken at widely spaced
intervals to investigate any successional changes in the insect pop-
ulation over the storage period. In the third trial samples were
collected from the cribs at different times of day over a short period
of time to investigate the possibility of daily cycles of movement
within the crib. These trials will be referred to respectively as
the 'Preliminary', 'Long-Term* and 'Short-Term Distribution Trials'.
The. methods used for this series of investigations have already been
outlined (Section 3.3.3) and the main problems that were encountered
already discussed (Section 3.2).
The cribs were structurally modified as shown in Figure 4.1 by
the introduction of light partitions (horizontal and vertical) of J"
galvanised mesh stretched on light wooden laths. Access was, in the
a) b)
ftno nitre trays
vortical wtro partitions
lath'uitre floors
arrangement for long term trial
Host mid East
arrangement for short term trial
West mH mE
O
East
oobs for Inseot sampling
oobs for lot rment
FIGURE 4.1 Hod1ftoatton of orlbs for Distribution Studies
a) general view of ortb b) stacking of oobs 1n sampling tunnels
unsampled oobs
trap bags
first two trials, through only one (vertical) face of the crib; this
was covered with wire mesh in which 'trap-doors' were cut so that cobs
could be removed. In the third ('Short-Term') trial tunnels were
accessible from two faces so that samples could be removed more easily
and quickly. The palm frond slats that form the sides of a normal,
unmodified crib were fitted to removable panels so that a consistent
degree of shading and shelter from rain was maintained.
It was clearly desirable, both from the point of view of the time
and effort involved and of minimising disturbance to the insects, to
unload as little of the grain bulk as possible when removing samples.
Accordingly it was decided that the cobs in-half of the tunnels should
remain totally undisturbed throughout the trial and would form a
reservoir of insects from which the sampled tunnels would be rapidly
reinfected; weighed cobs were included in all tunnels at loading so
that at the end of the trial any general reduction in infestation pressure
due to sampling could be assessed from the weight loss (Appendix V
for methods). The tunnels to be sampled and to be- left undistrubed
were allocated alternately"in successive layers, as indicated in Figure
4.1. This allowed variation in all three dimensions to be assessed, but
resolution in the direction of the tunnels would be clearest. The
cribs were orientated with the tunnels running East-West because it
was assumed, a priore, that the most consistent directional influence
in the environment would be provided by the sun moving in that direction.
In the 'preliminary' and 'long-term* trials cobs in the tunnels
to be used for sampling were loaded in shallow trays of 2mm galvanised
mesh ('mosquito wire'). When sampling, after careful removal of a few
surface cobs, the whole tray could be removed from the crib allowing
immediate access to the interior sections; most of the insect pests were
able to crawl through the mesh when moving normally but insects
falling onto the tray when disturbed during sampling were retained
at least briefly and could be collected in an aspirator. The main
drawbacks of this system were that the mesh may well itself have
affected insect distribution (because it impeded the movement of larger
species while others tended to cling to it) and the tray collected 'extra'
insects falling from cobs not intended for inclusion in the sample. In
the short-term trial the improved access provided by the two removable
sides made it possible to dispense with the trays.
The stacking of cobs within a crib is important because it is
likely to affect the rate of airflow through the crib and, thus, the
microclimate at different points and the distribution of insects. In
the storage system on which this study is based the cobs would normally
be tipped into the crib at random, leaving considerable interstitial
space. In the preliminary trial an attempt was made to simulate this
in the loading of each tunnel but in practice this disorderly arrangement
proved too unstable, the removal of one or two cobs causing 'subsidence'
and immediate disturbance of the infesting insects. Subsequently the
cobs were arranged regularly as shown in Figure 4.1; the cobs forming the
'stack' at each sampling position could be safely removed one at a time
and, moreover, each cob in a particular position had a comparable degree
of exposure to the surface of the crib. At loading particular attention
was given to stacking cobs closely against the partitions so that the
grain bulk was effectively continuous. The regular stacking of cobs
may have reduced airflow and so accentuated differences between parts of
the crib.
The cobs removed on each sampling occasion were shelled imm-
ediately and the adult insects collected for later determination. Grain
.79.
from all the cobs from a particular position was shelled together,
mixed thoroughly and then subsampled (by 'coning and quartering')
for estimation of moisture content (3 x lOg samples) and insect re-
cruitment (3 x lOOg samples). Cobs taken as samples were replaced
with ones which had been stored for the same length of time in a similar
crib nearby; these would have been similarly damaged but the infesting
population somewhat reduced by insects leaving the cobs during the
transfer. Cobs so introduced were marked with indelible ink so that
on subsequent occasions they would not be included in samples.
In the preliminary and long-term trials samples of approximately
lkg were taken (from each of 24 positions in the crib) during the
early stages of the trial when insect numbers were low, falling to
c.500g as insect numbers built up. In the short-term trial, for which
samples were taken at a late stage of succession when insect numbers
were high, four cobs were collected from each of 32 positions, pro-
viding 300-400g of shelled grain. The size of samples taken, particu-
larly when the insects were most abundant, was rather smaller than the
ideal but this was dictated by practical considerations as discussed
in Sections 3.2 and 3.4. The time required to handle larger samples 1
would have been prohibitive.
The method for selecting cobs to be sampled within each position
was slightly different in the two main trials. In both trials the
top and bottom layers of cobs within each 'stack' (i.e. sampling
position) were not used for sampling: many storage insects show thig-
motactic responses which it was felt.might produce aggregations of
insects adjacent to the partitions. The second layer of cobs (from the bottom)
consisted of the marked and weighed cobs to be used for loss assessment
and these also could not be used for insect counts. In the long term
.8 0 .
trials the loss assessment cobs were weighed on each sampling occasion
(to obtain a time-course of damage) so all the cobs above had to be
removed. The cobs for the insect counts were selected at random from
among these as they were removed (excluding any that had been intro-
duced on the previous sampling occasions). The samples, then were
effectively 'stratified random* samples (though considerably restricted).
In the short-term trial it was felt that priority must be given to
minimising disturbance to the crib. Accordingly on the first sampling
occasion the top layer of cobs was carefully removed and those in the
second layer collected; on the second occasion the top two layers were
removed and third collected and on the third occasion three layers
removed and the fourth collected. The cobs were also packed in 'envelopes'
of flexible nylon netting (2|cm mesh), each containing four cobs, which
could be removed and bagged quickly and with minimal loss of insects.
The sampling was thus, in the latter case, systematic rather than random.
Statistical analysis of the data from these trials presents a
number of problems. As already mentioned, although the selection of
samples in the long-term distribution trial did not differ seriously
from a stratified random pattern, sampling in the short term trial
(and for loss assessment in both trials) was systematic. For practical
purposes this is perhaps not too serious: the main interest is in com-
parisons between similarly selected samples (rather than an absolute
estimate of insect numbers) and there is no reason to suspect a particular
bias in this selection. (Milne,(1959) has pointed out that in most
circumstances systematically collected data may safely be analysed as
if it had been randomly collected).
The distribution of insect numbers between samples was found to
differ from normal in many instances (all groups tested showing over-
FIGURE 4.2,
Designs for analysis of effects of position in crib on insect
numbers (Distribution Trials).
a) Preliminary and Long-Term Distribution Trials - based on
one crib in each case; in the Long-Term. Trial each sampling
occasion was analysed separately; one sample from each position.
b) Long-Term Distribution Trials (emergences) - three replicate
samples from each position; (this pattern was also used in
some cases for analysis of adult counts (unreplicated),
testing treatment mean squares against the 3-way interaction
M.S.)
c) Short-Term Distribution Trial -
based on 3 cribs, each sampled on three occasions at different
times of day; one sample from each position.
Figure 4.2 81.
a) Preliminary 8» Long Term Distribution Trials (adults)
ZL Rnalysis of Variance
Factor1al
R "exposure'(2)
(interior-exposed)
B East-West(3)
minor replicattons(4)
b ) Long Term Distribution Trial (emergences)
zp^l upper
lower
Rnalysis of Variance
Factorial
R vertical pos'n (2)
B East-West (3)
C "exposure'(4)
minor repl1cattons(3)
c ) Short Term Distribution Trial
« / l y ^ m l d H * X » X - X » X m 1 d E
Rnalysis of Variance
Factor1al
R (time of day - 3 )
B East-West pos'n (4)
C vertical pos'n (8)
Blocks (3) sampling occasions
Note: * Indicates unsampled sections
.8 2 .
dispersion) but equally did not fit simple negative binomial dis-
tributions (indeed one might expect, a priore, compound distributions).
Simple logarithmic or inverse transformations did not consistently
reduce deviations from normality so the data has been analysed as
collected. Routine tests for homogeneity of variances, kurtosis
and skewness were made to exclude from analysis populations showing
extreme deviations, but given that the populations are in general
non-normal, undue weight should not be given to actual probabilities
and separations produced by the analyses.
Factorial analysis of variance was used in both trials to in-
vestigate components of the 'position 1 effect and in the short-term
trial to test for any effect due to the time of day. An alternative
approach to the investigation of the effect of position was tried:
this involved grouping the sampling positions into categories chosen on the
basis of a prior knowledge of the insect behaviour (for instance grouping
*top corners,''interiors' etc., following Surtees, 1964b) followed by
a single factor analysis of variance, but this proved less informative
in most cases. The basic designs used are set out in Figure 4.2;
these had to be modified in each case, depending on the number of cribs •
in the trial, the presence or absence of replication and so on, and
that information will be set out alongside each set of results. In
conclusion, it should be stressed that the statistical analyses are
only to be regarded as a useful indicator of the various postulated
effects, and that the formal problems in applying the techniques to
this data are recognised.
4.3 Preliminary Distribution Trial
The methods to be used for subsequent investigation of insect
.83 .
TABLE 4.1 Effect of sampling position within crib on insect numbers
for a) Sitophilus zeamais and b) Carpophilus dimidiatus.
Probabilities are for the null hypothesis and figures in
brackets are main effects means (no. insects/500g maize)
Design is 2-way factorial as specified in Figure 4.2a).
Note: i) No data on adult insect numbers were collected for sample
ii) 1 C . dimidiatus' includes all 'dimidiatus group' species -
in this case mainly C. dimidiatus (s.s.) with a few
C. pilosellus.
a) Sitophilus 'zeamais'
Effects
Sample 1
P (x)
'Exposure' <0.01 (Exterior/Interior) (133-62)
East/mid/West
Interaction
<0 .01 (42-91-158)
0.29
Sample 3
p . GO
0.02 (317-222)
0.06 (203-294-309)
0.48
Sample 4
P (x)
0.05 (264-208)
0.02 (182-241-285)
0.73
b) Carpophilus 'dimidiatus'
Sample 1
Exposure <0.01 (Exterior-Interior) (142-62)
East/Mid/West
Interaction
0.09 (80-87-139)
0.37
Sample 3
<0.01 (153-89)
0.85 (116-129-118)
0.50
Sample 4
0.04 (262-197)
0.07 (217-193-279)
0.68
distribution were tested using a single crib of heavily infested
maize. The maize was initially harvested in January (i.e. during
the dry season). The crib was emptied in May to allow it to be
modified (as in Figure 4.1), the maize fumigated with phosphine to
kill all infesting insects and then reloaded. The cobs were mixed
before reloading, destroying any stratification that might already
have developed. Samples were taken on four occasions in June and
July.
Only two species Sitophilus zeamais and Carpophilus dimidiatus
were sufficiently abundant for their distribution to be assessed and
the data for them is presented schematically in Appendix II. Both
species tended to be more abundant in samples from near the surface
of the crib than in those from the interior and the highest numbers
were recorded in the corners. There was an increasing.^gradient in
Sitophilus abundance from East to West while for Carpophilus, although
numbers were also generally higher on the West, the pattern was. less
regular. Factorial analysis of variance, taking the degree of exposure
and East-to-West position of each sample point as the two factors
(see Figure 4.2), also indicated this pattern (Table 4.1).
The situation considered here was to a large extent artificial,
involving recolonisation of already heavily damaged grain. However,
the results indicated that the insects were adopting reasonably con-
sistent patterns of distribution and that these could develop over
a short period of time (i.e. after reloading in this case). The
loss assessment figures, already mentioned in Section 3.5, also showed
that the sampling regime was reducing insect numbers sufficiently to
be reflected in reduced damage in the sampled sections.
.85 .
4.4 Long-Term Changes in Insect Distribution
The long-term distribution trial followed the development of
pest populations on maize harvested in August (i.e. during the humid
'small dry' season) and was carried out in parallel with the studies
on succession described in Chapter 6. Practical constraints meant
that only a single crib could be used for this study and the results
of the preliminary trial indicated that samples should be widely
spread to avoid depletion of the infesting populations. In the event,
four samples were taken: the first and second during the second wet
season, respectively two and six weeks after harvest, the third in
late November soon after the onset of dry 'harm^ttan' conditions,
and the fourth in April at the beginning of the main wet season.
The ambient conditions on each sampling occasion were clearly quite
different and were reflected by changing physical conditions within
the crib (as described in-Chapter 2). The accompanying seasonal
succession in the insect populations will be considered in detail
later (Chapter 6), but at this stage it should be pointed out that
very marked changes occur in the composition of the pest complex over
the storage season, in addition to the changes in distribution which
are the main concern here.
Initial colonisation of the cribs is mainly by species that
are primarily adapted to moist conditions such as Cathartus quadricollis
and Monanus concinnulus (Silvanidae), and various species of Carpophilus
(Nitidulidae). As the grain dries these species become less numerous
while Sitophilus zeamais, the major primary pest species, rapidly increases;
Sitophilus then remains abundant, and usually the dominant species,
throughout the remainder of the practicable storage period (c. 6-9
months). Sitotroga cerealella, which is also potentially a highly
damaging primary pest, occurs in large numbers for only a short period
FIGURE, 4.3
Distribution pattern of Sitophilus zeamais at different stages
of the storage season. (Long-Term Distribution Trial).
a) Adults - no. insects/500g shelled grain @ 17% m.c.
(Arithmetically corrected from actual sample size and grain
moisture contents)
b) Emergences - no. insects emerging during one week from lOOg
samples of shelled grain (fresh weight) - data are means for
3 samples from each position.
Samples were taken after the following times in store:
sample 1 - 2 weeks
sample 2 - 6 weeks
sample 3 - 3 months
sample 4 - 7 months
8 6 .
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FIGURE 4.3 « ) Sftophilus zoamals
(adults)
b ) S . zoamala
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.8 7 .
TABLE 4.2 Results of factorial Analysis of variance for dispersion of
Sitophilus zeamais in long-term distribution trial,
a) adults and b) emergences.
(Designs for analysis are given in Figure 4.2 a) and b),
respectively). Figures are probabilities for the null
hypothesis with main effects means in brackets.
a)
'Effect1
Exposure (Outside/inside)
East/Mid/West.
Interaction
Sample 1
P .95
.29
.87
Sample 2
P .23
.23
.62
Sample 3
P x .04 (448/329)
.50
.51
Sample 4
P .52 •
.26
.11
b)
A Top/Bottom
B East/Mid/West
C Exposure
AB
AC
BC
ABC
.05 .23 .73 (4.0/2.3)-
.95 .17 .36
.64 .01 .15 (8.9/5.9/5.6/4.8)
.80 .19 .31
.43 .02 <.001
.78 .04 .84
.49 .81 .04
,44
.07 (17/16/12)
.21
<.01
.17
.14
.23
at the end of the dry season and into the wet. Over the second half of the
storage period, and particularly stimulated by the onset of the rainy
season, there is a great increase in the variety and ^abundance of
secondary pests and detritus feeders attacking the already damaged grain.
Most abundant among these are Gnatocerus maxillosus, Palorus subdepressus
and other Tenebrionidae, Cryptolestes spp. (Cucujidae) and Carpophilus spp.
(Nitidulidae). The succession in the pest species is accompanied by
changes in the parasitoid and predator species that depend on them.
The data on all species recorded in the individual samples is given in
Appendix II; only the distributions of the most abundant species can be
discussed here.
The distribution data for Sitophilus (adults and emergences) are
given in Figure 4.3 and summary results of analyses of variance in
Table 4.2. (The complete anovar tables for all significant species
are given in Appendix VI). The adult counts suggest that colonisation
and subsequent infestation by Sitophilus (samples 1 and 2 in Figure
4.3) are essentially uniform. There is some indication of aggregation
near the surfaces of the crib on the second and third sampling occasion,
but no sign of the East-West gradients noted in the preliminary trial
(and subsequently in the short-term trial- §. 4.5), The emergences,
although essentially uniform on the first sampling occasion, subsequently
indicate some preference for positions on the surface (in some cases
appearing as an 'interaction' effect in the analysis). On the fourth
sampling occasion the emergences initially seem to suggest a gradient
of increasing abundance from West to East. However, if parasitism
by Choetospila elegans is taken into account (and assuming that Choetospila
was mainly attacking Sitophilus) this is transformed into an East to
West gradient, consistent with that found in the other trials. It
may be noted that sampling for the preliminary and short-term trials
was also carried out in the wet season, at a late stage in the
succession.
Sitotroga cerealella was only present in appreciable numbers
at the time of the third sample and even then was not sufficiently
abundant to reveal any clear dispersion pattern. It may be noted
however, that this species here penetrated throughout the crib and,
if anything, appeared more abundant within than on the surface (as
compared with its poor penetration noted in the studies quoted in
Section 4.1).
The remainder of the significant pest species (for which the
data are given in Figure 4.4a) - f)) show differences in detail
but overall tend to show a preference for" surface (or corner), rather
than interior, positions. This also seems to be true for the commonest
anthocorid predators (Lyctocoris cochici in samples 2 and 3, shown in
Figure 4.4h), and Scolopoides divareti in sample 4) which are pre-
sumably attacking the free-living larvae of some of these secondary'
pest species. The preference for more * exposed 1 (i.e. surface)
positions is reflected in the results of the analysis of variance for
Cathartus quadricollis and Carpophilus dimidiatus given in Table 4.3;
although the 'exposure' effect is not always statistically
significant the treatment means for the surface positions are in all
cases higher.
Superimposed on this preference for surface positions several of
the species show gradients in abundance from East to West. Carpophilus
fumatus and, to a lesser extent, C. dimidiatus, Cathartus quadricollis
and Monanus concinnulus tend to be more abundant on the East side in
the earlier samples. In the later samples (3 and 4), however, the
FIGURE 4.4
Distribution patterns of secondary pest species and natural
enemies at different stages of the storage season (Long-Term
Distribution Trial).
a) Carpophilus dimidiatus (Col., Nitidulidae)
b) C. fumatus
c) Cathartus quadricollis (Col., Silvanidae)
d) Monanus concinnulus ( " )
e) Gnatocerus maxillosus (Col., Tenebrionidae)
f) Cryptolestes Spp. (Col., Cucuj idae)
g) Choetospila elegans. - (Hym., Pteromalidae)
h) Lyctocoris cochici (Het., Anthocoridae)
Data are numbers of adult insects/500g shelled grain @ =7% m.c.
(arithmetically corrected from sampled values).
Samples were taken after 2 weeks (1), 6 weeks (2), 3 months (3)
and 7 months (4) in store.
.90.
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FIG.4.4 o) Gnatocorus-maxtII o <3 us f) Cryptolastas spp.
9 3 .
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FIG.4.4 g) Chootosplla ologans h) Lyotooorts cochloi
.94.
TABLE 4.3 Results of Factorial Analysis of Variance for dispersion
of a) Carpophilus dimidiatus and b) Cathartus quadricollis
in long-term distribution trial.
Figures given are probabilities for the null hypothesis with main
effect means in brackets.
Design is as given in Figure 4.2a); complete Anovar tables are
given in Appendix VI.
a) Carpophilus dimidiatus
Effect
'Exposure' (Outside/Inside)
East/Mid/West
Interaction
Sample 1 P (x)
0.58 (11/10)
0.07
Sample 2 P <x)
0.70 (36/33)
0.06
Sample 3 P (x)
0.07 (33/24)
0.24 (11.5/11.5/7) (46/28/30) (22/32/31)
0.96 0.95 0.54
Sample 4 P (x)
<0.01 (173/98)
0.25
(165/129/114)
0.25
b) Cathartus quadricollis
Effect Sample 1
'Exposure' (Outside/Inside)
East/Mid/West .
0.16 (23/16)
0.08 (28/14/18)
Sample 2
<0.01 (87/38)
<0.01 (101/26/60)
Sample 3
0.62 (67/60)
0.Q8 (69/39/82)
Sample 4
0.13 (76/46)
0.03 (56/29/98)
Interaction 0.29 0.06 <0.01 0.60
.95.
TABLE 4.4 Results of factorial analysis of variance for dispersion
of insects in Sample 4 (Long-term distribution trial)
(Design is 3 x Factorial as in Figure 4.2b, but note that
for the adult counts, in the absence of replication,
treatment mean squares are tested against the three way
interaction MS.).
Figures are probabilities for the null hypothesis with main
effects means in brackets
'Effects' ADULTS EMERGENCES
Cryptolestes Gnatocerus Cryptolestes Choetospila pusillus maxillosus pusillus elegans
Level (A) (Upper/ Lower
0.01-0.025
(69/103)
>0.5 0.34
(8/9)
0.66
(14/14)
East/Mid/ West (B) 0.01-0.025
(71/71/117) 0.05
(139/110/237) <0.01 (5/8/12)
<0.001 (6/16/21)
Exposure (C) >0.5 0.5 0.12 (11/8/9/6)
0.50 (14/15/12/15)
Interaction A and B >0.5 0.4 0.61 0.59
A and C >0.5 >0.5 0.25 0.82
B and C 0.01-0.025 >0.5 0.67 0.34
A, B and C - - 0.73 0.82
.96.
west side is preferred both by Cathartus and by all three species
that build up at this stage (Gnatocerus maxillosus, Palorus subdepressus
and Cryptolestes pusillus), though not by Carpophilus dimidiatus.
This East to West gradient is also indicated by the emergences of
Cryptolestes pusillus, the only secondary pest species whose immatures
were retrieved in sufficient numbers for their distribution to be
assessed. The results of the analyses of variance for these last
species in sample 4 are given in Table 4.4.
It would be dangerous to draw general conclusions from the
results of a limited trial, based on only four instances from a single
crib. However, as far as they go, the results show a degree of
internal consistency and generally accord with what is known of the
behaviour of individual species from laboratory studies (as will be
discussed later in this Chapter). On a practical level, the rather
uniform distribution patterns shown by Sitophilus over most of the
storage season suggest that the build-up of that species should be
satisfactorily represented in more limited single-point samples (as
used in the successional studies). The tendency of the secondary
pest species to aggregate near the surfaces of the crib means, however,
that fluctuations in the populations of these species may not be
fully reflected in samples taken only from the centre of the crib.
4.5 Short-Term Changes in Insect Distribution
The possibility of daily changes in insect distribution or abundance
was investigated by intensive sampling over a short period of time.
Samples (4 cobs from each of 32 points) were collected on three
occasions from each of three cribs. The cribs were sampled in rotation over
a period of approximately five weeks; this spread in sampling time
represented a compromise between the desire to complete sampling quickly
97.
a) Latin Square Rnovar
CRIBS
I Samp11ng
occasion
1
2
3
p • m .
a. m «
m i d .
II
mid.
p* m,
a. m<
III
a . m «
mf d .
p • m<
b ) Factorial Rnovar
^» ^ ^ Faotors
R time of day (3)
B posItlons-East/West (4!
C posl tlons—"^vertical
Blocks - <3)
sampling occasions
S 'X ».
FIGURE 4.5
Experimental design and analysis for the Short-Term Distribution
Trial:
a) allocation of sampling times and sampling occasions in the 3
replicate cribs. Analysis of variance on this design was carried
out on crib totals (ie. all sampling positions combines) for adults
of major species.
b) analysis for effect of time of day and position within crib on
insect numbers.
under uniform environmental conditions and the need to allow the
insects to return to their undisturbed pattern of distribution between
samples. Maize harvested in January was used for this trial and
sampling carried out in June and July when the full complex of second-
ary pests had developed. This time of year was chosen as offering an
extended period of fairly consistent weather conditions.
On each occasion one crib was sampled in the morning (07.00-08.00 hrs)
one at mid-day (12.00-13.OOhrs) and one in late afternoon (16.00-17.00)
(though not on the same day); cribs were allocated according to a latin
square design (Figure 4.5a). Crib totals were analysed (on this design)
to test for differences between times of day, _crib$. and. sampling
occasions. Totals were then broken down factorially (Figure 4.5b))
to test for any consistent position .effect (i.e. distribution pattern)
or interaction between time of day and positions (i.e. indicating insect
movement within the crib). It should be noted that in this analysis
sampling occasions have been used as major blocks but that inter-crib
variation cannot also be separated. Inherent in this design there is
the risk that a sampling occasion and crib interaction could mask or
enhance a 'treatment' effect (i.e. of position or time of day). For
practical reasons emergences could only be assessed for three of the
nine samples and only two of the three cribs are represented; an analysis
of these data for 'position' effects was carried out but the results
cannot be regarded as properly representative:
The data from all samples are included in Appendix II. The
distribution patterns of the more important species are shown schematically
in Figures 4.6 (adults) and 4.8 (emergences).
The analysis of crib totals indicated significant differences
only between totals for different sampling occasions (Table 4.5). , In
.99.
the case of Sitophilus zeamais and Carpophilus dimidiatus there was
a progressive decline in numbers in all cribs over the three sampling
occasions while for Cryptolestes spp. there was a considerable increase.
For the two former species it is not clear whether the decline represents Jk
a natural feature of the ecological succession (as is presumably the
case for the increase in Cryptolestes spp.) or, as seems more likely,
the decline is due to depletion and disturbance by the sampling itself.
No differences are indicated either between cribs or for different
times of day. However, it should be noted that this analysis is rather
insensitive.
In the factorial analysis of variance, by contrast, nine species
out of the thirteen tested show a significant effect of time of day
on insect abundance (assessed at 1% level; see Table 4.6); it should
be noted, however, that Sitophilus zeamais, the most abundant species,
was among those unaffected (Table 4.6a). Such differences could rep-
resent real increases in the numbers of insects in the cribs at par-
ticular times of day but,, on balance, it seems more likely that they
reflect changes in the efficiency of the sampling technique under
different environmental conditions. It seems unlikely that most of
the species are sufficiently active actually to leave and return to the
crib in appreciable numbers, though it should be noted that three of
the species in question (the pteromalids Choetospila elegans and
Cerocephala dinoderi and the anthocorid Cardiastethus pygmaeus, do
fly strongly and are often observed in considerable numbers outside the
crib. The treatment means (Table 4.6b)) show that for eight of the
nine species which exhibit a 'time of day' effect the highest insect
numbers were recorded at mid-day or in late afternoon when temperatures
are highest and relative humidities lowest. It seems plausible that
at these times many of the insects will avoid exposure to the unfavourable
100.
TABLE 4.5 Analysis of crib totals from short-term distribution trial:
Probability levels from Latin square anovar (Figure 4.5(a)).
Species
Source of
Variation Sitophilus Sitotroga Carpoph. Cryptotestes Guat. Choe. zeamais cerealella dimid. spp. max. elegans
Sampling 0.05-0.1 >0.1 0.05-0.1 0.05-0.1 >0.1 >0.1 occasion
Cribs >0.1 >0.1 >0.1 >0.1 >0.1 >0.1
Time -of day >0.1. >0vl - - - - >0.1 >0.1 >0.1. ->0.1- ,.
.101.
Figure 4.6 (in part)
a) Sltophllus zeamals (max. density - 808 insaota • 500g)
b) Sitotroga osrealella (max. density ~ 58 Inseots / S88g)
FIGURE 4.6 Distribution of adult inseote within orlbe (Short Tarsi
Distribution Trial). Mean numbers of Inseots per 588g of
shelled grain • 17X ot.o. In samples from 32 points In
eaoh or lb (3 replloatee) at different times of day. The
extent of shading within eaoh square Indloatee ths den-
sity of Inseots in the samples from that position as a
fraotlon of the maximum density for that epeolee (given
In Individual oaptlone*.
102.
Figure 4.6 (oontfnued)
o> Ctrpophtlue dlmfdlatus ( m«x . density - 198 fnseote • 388g)
d) Grtttooerue omxtlToeue (ntx. denetty - 188 fneeote / 388g)
e> Ptlorue eubdepreeeue (max. denetty - 289 tneeote / 588g>
«.m ®i d - d» y P • Rl*
103.
Figure 4.6 (continued)
•f) Crypto Testes spp- (max. density - 108 Insects ' 580g)
» t d - d» y p • Bl«
Figure 4.6 (oontlnued)
104.
h> Lyotooorle ooohlot (max. density " 10 tneeote / S00g>
O Choetoeplle elegene (nmx. denelty " 30 Ineeote / S00g>
J) CerooepheTe dlnoderl (mtx. denelty - IS Ineeote / 500g)
eld-dey p.m.
105.
Figure 4.7
600 Vertical
posItlons
700
Vt 600 3
X a 5 0 0 o +>
cn 400
o c 300
200 -
d 8 cd 1 cd 2
be 7
ab 3 ab 5 ab 4
6
2 3
East-West position
FIGURE 4.7 Distribution of Sltophllus (Short Term Distribution Trial)i
interaction plot shouting the effect of East-West position
(factor B) and svert1oa1' position (faotor C) on adult abun-
danoe (see fig. 4.5). Data are mean numbers of Inseots per
500g of shelled grain, based on nine samples from eaoh
position. Separations are from a Newman-Keuls test (5% level).
TABLE 4.6 Dispersion of adult insects within cribs: effect of time of day and sampling position on insect abundance (Short-term distribution trial) a) Probability levels from 3-factor analysis of variance as specified in Figure 4.5ti
Source of Sitophilus .Sitotroga Carpoph. Gnato. Palor. Crypto. Typhaea variation zeamais cerealella dimid. max. subdep. spp. sterc. (No. of levels)
A. Time of day (3) 1 0.68 <0.001 0.39 0.01 <0.001 0.001 0.90 B. East-West position (4) <0.001 <0.001 <0.001 <0.001 <0.001 0.05 0.002 C.f Vertical'position (8) <0.001 <0.001 •<0.601 <0.001 <0.001 <0.001 <0.001 AB 0.96 0.29 0.56 • 0.74 0.83 0.97 0.19 AC 0.64 0.05 0.98 0.89 0.01 0.89 0.54 BC <0.001 0.06 0.17 0.07 0.77 0.03 0.12 ABC 0.99 0.67 0.96 0.75 0.95 0.95 0.19
Litargus Choeto. Cero. Cardiast. Lycto. Scolopoides grain grain bait. eleg. dinod. sp. cGchici divareti m. c. temp.
A 0.07 <0.001 <0.001 0.01 0.001 0.001 <0.001 <0.001 B <0.001 <0.001 <0.001 <0.001 0.66 0.03 0.03 0.002 C <0.001 <0.001 0.06 <0.001 <0.001 <0.001 <0.001 <0.001 AB 0.79 0.86 0.81 0.03 0.87 0.99 0.47 <0.001 AC 0.46 0.86 0.25 0.67 0.81 0.77 0.48 0.001 BC 0.20 0.92 0.67 <0.001 0.99 0.98 0.45 0.99 ABC 0.58 0.81 0.72 0.49 0.99 0.99 1.00 0.94
TABLE 4.6 b) Treatment means from factorial analysis of variance. Separation of means on the basis of Newman-Ketrls test (5% level)
1 Treatment1 Sitoph. Sitotr. Carpo. Gnato. Palqr. Crypto. Typhaea Species zeamais cereal. dimid. max. subdep. spp. sterc.
Time of a.m. 11 a. 41 b 84 a 31 a day mid. 19 b 31 a 86 a 30 a
p .m. 12 a' 38 ab 107 b 41 b
East-West E 308 a 10 a 54 a 30 a 77 a 37 a 2.6 b position M.E. 380 b 13 a 49 a 34 a 87 ab 28 a 0.7 a
M.W. 482 c 13 a 53 a 37 a 103 be 34 a 1.1 a W 538 d 20 b 76 b 47 b 105 c 37 a 1.2 a
Vertical 1 470 cd 25 c 45 ab 25 a 21 a 34 a 0.3 a position 2 433 be 21 c 59 be 60 c 68 b 55 b 2.3 a
3 395 ab 15 b 54 be 45 b 76 b 33 a 1.6 a 4 327 a 15 b 29 a 30 ab 70 b 26 a 0.3 a 5 387 ab 11 ab 42 ab 32 ab 75 b 29 a 0.3 a 6 334 a 11 ab 66 c 36 ab 120 c 52 b 1.6 a 7 544 e 7 ab 102 d " 40 ab 1-64 d 24 a 4.0 b 8 527 de 8 a 67 c 28 a 148 d 21 a 1.0 a
Litargus Choeto. Cero. Cardia.. Lyctocor Scolo. grain. grain bait. eleg. dinod. sp. cochici divar. m.c. temp.
Time 3 a 13 a 5 a 5 a 4 a 4 a 17.1 a 24.0 a 4 a 12 a 6 a 5 a 6 b 4 a 17.6 b ' 26.0 b 5 a 16 b 8 b . 7 b 4 a 7 b 16.9 a 27.7 c
E-W 6 b 11 a 4 a. 4 a 4 a 7 a 17.3 ab 26.1 b 2 a 12 a 5 a 3 a 4 a 4 a 17.0 a 26.0 ab 2 a 16 b 8 b 3 a 5 a 4 a 17.1 ab 25.9 a 7 b 16 b 7 b 13 b 5 a 6 a 17.4 b 25.8 a
Vert. 4 ab 10 a 5 a 10 b 2 a 4 ab 17.5 be 25.6 a 9 c 11 a 6 a 13 c 4 ab 10 c 17.7 c 25.8 ab 6 b 13 a 6 a 8 b 4 ab 5 ab 17.7 b 25.8 ab 2 a 13 a 5 a 3 d 3 a 3 a 16.8 a 25.9 be 1 a 13 a 5 a 3 a 3 ab 2 a 16.8 a 26.1 be 4 ab 13 a 7 a 4 a 4 ab 8 be 17.0 ab 26.2 c 4 ab 18 b 7 a . 3 a 8 c 7 abc 17.5 be 25.8 ab 3 ab 18 b 7 a 2 a 6 b 2 a 16.5 a 26.2 c
108.
TABLE 4.7 Summary of adult insect distribution pattern (short-term distribution trial). Symbols indicate whether insect numbers increase (+) or decrease (-) within the crib in the direction specified. A blank indicates no discemable trend in that direction; symbols in parentheses indicate that trend is unclear or that analysis of variance indicates no significant differences.
*Note that Sitophilus was more abundant at top and bottom and less abundant at the intermediate levels.
Species CO CO CO CO P <D cO CO CO CO P CO CO CO CO •u nH i—i 4-1 CO a)
p ca I-I i-t P P P CO co CO CO •H CO a> •H T-* OOt-i •H 4-1 J-i co <u <u a) CO p (X & -H 4-1 M •H •H •H co o a) & CO <U o CO J-l 1-1 • «0 M P 4-1 CO CO ex M CO O •H 0 4-1 •H U eu •H o r-t p a. o a. a) o 60 cO o e a> a) cO O a a. O pu cO cd o o i-H u a) 4-1 ex CO CJ U a) 4-1 cO O TJ •H . O •H 0 M O a o a) cu •H 4-1 •i-l o
ft CO CO 4-1 <D 00 o o T3 P. 4-1 X r—1 CO 4-> co •P n u 0 ca X rH rO ft a a) 4-> .—i o a) M c M ca O 0 O > •i-t <u •H <u ca •H C co to p u >» v •H cO rC i—i <u »H CO S 0 CJ •H CO N CO o u 13 o 6 p* CO o H CO . l-J rO V a) CP h4 0 CO X)
Direction or part of crib
East to West
North to South
top to bottom
interior to surfaces
+ ( - ) + + +
+ ( + )
V
Figure 4.8 (In part)
109.
a)Si tophi 1 u s N z e a m a i s ' <fl c o m p l e t e l y s h a d e d s q u a r e c o r r e s p o n d s to 29 i n s e c t s p e r 1 0 0 g )
< 4 3 2 ) < 4 2 3 ) < 4 3 3 )
b ) S i t o t r o g a c e r e a l e l l a <fl c o m p l e t e l y s h a d e d s q u a r e c o r r e s p o n d s to 10 i n s e c t s per 1 0 0 g )
FIGURE 4.8 Distribution of emergenoee (Short Term Distribution Trial).
Numbsre of ineeote emerging In one week from eamplee of 108g
of shelled grain. Extent of shading within eaoh square showe
number of Ineeots in aaoh sample as a fraction of ths maxi-
mum number recorded for that epeotee (ae Indicated In Indi-
vidual oaptlone). Samples were oolleoted from orlb 2 on the
third sampling oooaston (423) and from or1b 3 on the ssoond
(432) and third (433) samp1ing oootstone.
110.
Ffguro 4.8 (oontfnued)
c > C r y p t o l e s t e s s p p . <fl c o m p l e t e l y s h a d e d s q u a r e c o r r e s p o n d s to 20 i n s e c t s per 1 0 0 g )
<432) < 423 > <433)
d ^ G n a t o c e r u s m a x i l l o s u s <fl c o m p l e t e l y s h a d e d s q u a r e c o r r e s p o n d s to 20 i n s e c t s per 100g)
<432) <423) <433)
• ^ P a l o r u s s u b d e p r e s s u s <fi c o m p l e t e l y s h a d e d s q u a r e c o r r e s p o n d s to 20 insects per 1 0 0 g )
y S i s s s
T/ s y -
s a s> ^
^ — ^
x s. y y ^ y y y j y
v ^ s y ~7
<432) I Z <423)
^ V 1
^ J? S s
y <433)
111.
Ftgur* 4,8 (oontinuod)
f ) C h o e t o s p i l a e l e g a n s <fl c o m p l e t e l y s h a d e d s q u a r e c o r r e s p o n d s to 15 i n s e c t s per 1 0 0 g )
<432) <423) < 4 3 3 )
g ) C e r o c e p h a l a d i n o d e r i <fl c o m p l e t e l y s h a d e d s q u a r e c o r r e s p o n d s to 19 i n s e c t s per 1 0 0 g )
<432) <423) < 4 3 3 )
112. TABLE 4 . 8
Distribution pattern of emergences (short-term distribution trial) based on 2 factor analysis of variance.
a) Probability levels, b) Treatment means with separations based on Newman-Keuls test (except column marked * where separations are based on Duncan's New multiple range test) c) Summary of trends : "+ " indicates increase, "-" decrease, in directions specied.
Treatments Sitoph. Sitotr Crypt. Gnato. Palorus Choeto. Ceroceph. species zeamais cer. spp. max. subdep. elegans dinod.
a) East-West <0.001 0.18 0.82 0.15 0.59 0.03 0.05 position
'Vertical* 0.01 0.65 <0.001 0.11 <0.001 0.01 0.13 position
interaction 0.64 0.95 0.77 0.44 0.98 0.57 0.09
b) East-West 10.5 b 1.8 4.4 5.8 2.8 4.7 a 2.3 position 8.3 ab 2.0 5.0 6.8 3.0 4.6 a 2.4
7.7 a 2.8 4.7 8.7 3.3 5.5 ab 2.8 12.8 c 2.8 5.2 7.4 3.8 6.7 b 3.6
'Vertical* 9.2 a 2.6 7.2 be 5.3 0.9 a 3.8 a* 2.1 position 9.0 a 2.2 6.7 be 7.7 '2.4 ab 4.8 ab 2.1
8.7 a 2.1 5.3 abc 5.8 2.8 ab .4.8 ab 2.8 7.7 a 3.3 2.8 a 5.9 1.3 ab 4.1 ab 2.3 9.3 a 2.5 4.4abc 10.5 3.6 b 4.6 ab 2.8
10.8 ab 2.2 6.8 be 7.6 3.9 b 6.4 bed 3.7 13.3 b 2.7 3.3 ab 6.7 6.9 b 7.3 d 2.7 10.5 ab 1.6 2.1 a 8.2 3.8 c 7.2 cd 3.8
c) East to + (+) (+) (+) + + West
North to South
top to bottom + - + + (+)
Interior to + + surfaces -
113.
conditions by spending more time within damaged grains, or in the
interstices between grains, in which situation they will be more
readily collected in samples.
Only three species (Sitotroga cerealella, Palorus subdepressus
and Cardiastethus sp.) show changes in distribution with time of day
(as indicated by the Time and Positions interactions, AB or AC, in
Table 4.6a)). The interaction means, however, reveal that these do
not represent insect movement from one part of the cribs to another.
Rather, the time of day effects are only experienced, or are more
strongly in evidence, in some sections of the crib; thus, for instance, «
Card iaS te thuS" shows" a noticeable increase rn numbers~(in the afternoon) -
only for samples for the west of the crib. . It should be noted that these
effects too might be sampling artefacts.
Turning to spatial distribution, it is evident from the results
of the analysis of variance (Table 4.6) that, at the time of this trial
at least, all species showed consistent, non-uniform patterns of dis-
tribution within the cribs: both factors expressing position effects
(B and C) in the analysis show significant differences for almost all
species (0.1% or 1% level). It must immediately be pointed out, however,
that the distributions cannot be described in terms of simple gradients
along the two factors. Factor C in particular represents both vertical
positions within the crib and, because the sampling tunnels are stepped
alternately (see Figure 4.5b)), different degrees of exposure to the
surface; any North-South trends cannot be properly tested, due to the
design of the experiment. Both main effects and interactions have,
therefore, to be taken into account to build-up a meaningful description
of the distribution pattern; the trends in distribution have been
summarised, as far as possible, in Table 4.7 but it must be recognised
that this presentation involves a degree of simplification and (debatable)
114. interpretation.
The analysis for Sitophilus zeamais shows highly significant
differences (p < 0.1%) for both factors B and C and their interaction.
There is a consistent increase in abundance from East to West, though
the effect is stronger at the top and bottom of the crib than in the
centre (Figures 4.6a) and 4.7). Emergences too show higher levels on
the West than the East, but in this case the interior positions are
lower, rather than intermediate (Figure 4.8a) and Table 4.8). The
adults tend to be more abundant at the top and bottom of the crib than
in the centre (Figure 4.6a) and separation of means Table 4.6b)).
Sitotroga cerealella was only present in small numbers at the
time of this trial and virtually disappeared during the course of it.
This species, like Sitophilus, showed an increasing East to West gradient
(both adults, Figure 4.6b) and Table 4.6b), and emergences* Figure 4.8b
and Table 4.8); however the largest numbers in this case were found at
the top of the cribs.
Of the four common secondary pest species, the two Tenebrionidae,
Gnatocerus maxillosus (Figure 4.6d,) and Palorus subdepressus (Figure
4.6e)), showed increasing gradients in abundance from East to West,
though more erratic than in the case of Sitophilus. Carpophilus dimidiatus
(Figure 4.6c)) was also more abundant in samples from the West than
from the East but less abundant in interior samples. Cryptolestes spp.,
mainly C. pusillus, (Figure 4.6f) and, to a greater extent, the two
commonest Mycetophagidae, Typhaea stercorea and Litargus balteatus,
were markedly concentrated on the surfaces of the cribs, though patchily
distributed. Both Palorus and Carpophilus were most abundant in samples
from the bottom layer of the crib.
Both of the common parasitoids, Choetospila elegans and Cerocephala
115.
dinoderi (Pfceromalidae), showed consistently increasing gradients
from East to West and from top to bottom for both adults (Figures
4.6 h) and i)) and emergences (Figures 4.8 j) and g)). Their distribution
corresponded approximately with that of Sitophilus which is regarded as the
main host of the former species; the biology of Cerocephala does not
seem to be properly known. The two most abundant Anthocoridae, Lyctocoris
cochici(Figure 4.6g)) and Scolopoides divareti, occurred rather erratically
in the three cribs. They were most abundant in surface samples and
their distributions followed most closely those of Carpophilus dimidiatus
and Palorus subdepressus; the larvae of both of these species were
subsequently shown in the laboratory to be suitable prey species for
the Anthocorids.
4.6 Distribution of Losses within the Cribs.
The methods used to assess weight loss have-been described briefly
in Section 3.5 and are considered more fully in Appendix V. " For the
distribution trials marked and weighed cobs were included at loading
both in the sections to be used for insect sampling and in the undis-
turbed parts of the crib. In the long-term trial those in the sampled
sections were weighed (whole) on each of the four sampling occasions
to provide an estimate of the progress of damage during the storage
season. At the end of the trial all the loss assessment cobs (i.e. from
both sampled and unsampled sections) were shelled and the weight loss
assessed on the basis of the remaining clean, sieved grain. In the
. short-term trial the loss assessment cobs were not exposed during insect
sampling and only a single estimate of loss was made at the end of the
trial when all cobs were shelled and weighed. For the long term trial
losses were assessed on the basis of four replicate cobs at each of
the forty-eight 'positions' in the crib and for the short-term trial
on three cobs at each of the 64 positions (in each of three cribs).
The crude weight losses observed have to be corrected for diff-
erences in moisture content (both with time and position in the crib)
so that they can be presented on a comparable dry weight basis. This
presents considerable problems, as already mentioned in Section 3.5.
The grain and core moisture contents had to be estimated at the beg-
inning of the trial, and at intermediate sample points in the long-
term trial, from cobs other than those actually used for loss assess-
ment. Even at the end of the trial, when determinations could be made
on subsamples from the loss assessment cobs themselves, the number of
cobs involved meant that only a proportion could be tested individually.
As a result there is an uncertainty in the moisture content correction
for each cob. To simplify calculations, no attempt has been made to
allow for this source of variation in the presentation or analysis of
results. While this should not bias the overall results, the existance
of appreciable inter-cob variation in moisture content" does mean that
the estimates of loss for individual cobs are subject to errors of
several percentage points.
The estimated weight losses (dry weight basis) recorded in the
long-term trial are presented schematically in Figure 4.9; no consistent
weight loss was detectable by the time of the first sample (after two
weeks in store), so these data have not been included. At the second
sample, after six weeks1 storage, damage levels were still low; there
is no clear pattern of distribution, although there is evidence of higher
damage in some corner and surface positions. In the third and fourth
samples, after three and seven months in store, respectively, there
is clearer evidence of more serious damage to grain on the surfaces
of the crib; however the small number of samples from the interior of
the crib make it difficult to assess this critically. In the fourth
sample there is evidence of vertical stratification with more damage to
F i g u r e 4.10
Sample 2 Sample 3
>5 . ^ 8 y
{ / i s . f i ? u
Sample 4
FIGURE 4.9 Progreee of weight loee In different parte of a elngle orlb (Long Term Dletrlbutlon Trial)
Height loes of oobe (dry wstght baete) after elx weeke, three months and ssvsn months
In store. Figures are meana of eetlmatee for four oobe In eaoh poeltlon and are oorreoted
for moleture content ohangee with time in etore and poeltlon within the orlb.
F i g u r e 4 . 1 0
. 6 / 3 6 . 5 / 4 4 T 0 X 4 2 . 5 / j ? .3/36.4/59. 2/^2»9
CRIB 1 CRIB 2 CRIB 3
FIGURE 4.10 Distribution of weight loss within orlbs. (Short Term Distribution Trial).
Final weight toes (dry weight basis) - figures are means of estimates
from three oobs In eaoh position.
119.
TABLE 4.9
a)
Source (Name) df Sums of squares Mean square F Ratio F-Prob
Total 575 21478.26 37.35 Blocks 2 2371.65 1185.83 A Top/Btm 3 2183.75 727.92 24.962 <.001 B South/Nth 3 497.09 165.70 5.682 .001 C West/East 3 1251.87 417.29 14.310 <.001 AB 9 339.52 37.72 1.294 .25 AC 9 1271.88 141.32 4.846 <.001 BC 9 548.90 60.99 2.092 .03 ABC 27 671.36 24.87 .853 .68 Block Error 126 3674.14 29.16 Sampling Error 384 8668.10 22.57
All F tests were formed-with the Block Error
b) North 34 .4b East 32.4a Top 36.0c 32 .4a 31.9a 32.6b • 32 .3a 32.9a 30.7a
South 33 .9b West 35.7b Bottom 33.7b
TABLE 4.9
Summary results of analysis of variance: Effect of position within
crib on grain weight loss.
a) Summary anova table with probabilities for null hypothesis
'no differences between means'.
b) Treatment means and separations on Newman-Keuls test at 5% level.
Data were analysed as a 4x4x4 factorial design for the 64 sampling
positions within the crib with three determinations (cobs) at each
point (minor replications) and three cribs (blocks).
120.
cobs in the top and bottom sections of the crib than to those in the
middle two. In the top half of the crib at the time of the third
sample, and more erratically at all levels at the fourth, samples from
the West face of the crib tend to be more damaged than those from the
East. There is no evidence of reduced damage in the parts of the crib
from which insect samples had been collected (indicated by arrows in
Figure 4.9).
The data from the short-term trial are presented in Figure 4.10.
None of the loss assessment cobs in this trial were disturbed in the
insect sampling program and so all figures were included in a simple
factorial analysis of variance foTL.ppsitjton.,effeotS., jthe factors1
being the three primary axes of the crib. The.results of the analysis
are summarised in Table 4.9 with the main effect treatment means and
separations from Newman-Keuls tests. The analysis, and implied null
hypotheses, need to be interpreted with caution, bearing in mind the
variation inherent in the moisture content corrections used.
As in the long-term trial, the overall picture is of higher levels
of damage on the surfaces of the crib, especially in the corners, and
a tendency towards more severe damage on the West face. There is some
vertical stratification, with the top layer being most damaged and the
third layer least. It should be noted, however, that the trends are
not uniform and that inter-cob variation is appreciable (approximately
+ 10% within positions).
The contribution of various sources of loss (i.e. insects, moulds
rodents, etc.) were not investigated quantitatively. There was visible
evidence of higher levels of mould activity on the surfaces of the cribs
in the form of much more extensive discolouration of grains (as noted
in Section 2.4); there can be little doubt that the more severe losses
121.
in the surface samples are attributable, at least in part, to this
source. Higher levels of Sitophilus and several secondary pest species
were, however, also noted in these positions and the increased numbers
of insects will also have contributed, both directly and in interaction
with the fungi, to the loss of grain. Rodents were present, and caused
severe losses, in some of the experimental cribs but their damage was readily
identifiable and was not a contributory factor in this trial. It may
be noted that the estimates of loss provided by these figures, although
high and representing in practice total loss of the grain, are well
within the range recorded in various surveys of on-farm cereal storage
in Africa (Hall, 1970).
f 4.7 Species Interactions and Habitat Selection
The data presented in the preceding sections have indicated that
insect populations are not uniformly distributed within the crib and
that the distribution patterns of individual species can be described,
at least approximately, in terms of consistent 'preferences1 for par-
ticular parts of the crib. This information is of itself important in
its implications for sampling, whether this is directed towards assess-
ment of losses or of insect infestation: surface samples cannot, in
general, be expected to provide a good indication of the state of the
grain in the entire crib. The more interesting question remains,
however, of whether these observed distribution patterns can be inter-
preted in terms of the behavioural responses of the insects to envir-
onmental factors or to one another.
Multivariate techniques have been used by Sinha and co-workers
(1969, 1977 and 1980) to investigate the factors influencing the dev-
elopment and distribution of infestation of bulk grain by insects, moulds
and mites. In the current study quantitative data were not collected
on a number of potentially crucial environmental and biological factors,
122.
particularly relating to the type, of grain damage and to the development
and distribution of fungal activity and mites. Given the limitations of
the environmental data gathered and the manifest complexity of the
infesting populations, it would be inappropriate here to attempt a
similarly sophisticated interpretation of the maize crib ecosystem.
Rather, attention may be drawn to a number of points of interest in-
dicated by simple correlation and the kind of influences on insect
distribution discussed in general terms. Data from the short-term
distribution trial are summarised in the form of correlation matrices,
comparing the abundance of the thirteen commonest insect species and
levels of-grain moisture and total weight loss, in Table 4.10 and a
schematic outline of the observed-environmental-gradients in Figure •• <
4.11.
In order to simplify consideration of the correlation matrices
(Table 4.10), all values of the correlation coefficient which do not
differ significantly from zero at the 5% level according to a bivariate
significance test (i.e. suggesting that the variables are uncorrelated)
have been diagonally 'hatched*; in addition, some figures that are
significant (at the 5% level) have been similarly 'eliminated1 where
comparison with the corresponding values for the other two cribs indicate
that correlation is not consistent. Attention may then be focussed
on those variables showing consistent correlations, either negative
(vertical hatching) or positive (left unhatched). The matrix has also
been divided on the basis of the known biology of the insect species.
Sitophilus and Sitotroga forming the first group (A - 'primary pests'),
both have larvae that develop within an individual cereal grain and
feed entirely on it. The second group (B - 'secondary pests') all
have free-living larvae but are much more diverse in their feeding
habits: some feed mainly on the grain but others are primarily detritus-
FIGURE 4.11
Summary of trends in environmental factors
(Short-Term Distribution Trial)
(Means based on data from 3 cribs)
a) Weight loss of grain (dry weight basis)
(based on estimates for 3 cobs from 64 sampling points
in each of 3 cribs - data in fig .4.10)
b) Grain moisture content (fresh weight basis)
. (based on determinations made at end of trial; one de-termination, based on pooled grain from 3 cobs, from
- e a c h of 64 sampling ponts in each of 3 cribs)
c) Grain temperatures at different times of day
(figures are means of 3 readings, taken on different ' days, at each point)
(For details of methods see text and earlier full data).
123.
*) weight loss • X
3 6 . 0 3 4 . 4 3 4 . 4 3 8 . 1
3 2 . 3 3 3 . 3 3 3 . 4 3 2 . 7
3 2 . 5 3 0 . 9 3 1 . 5 3 2 . 9
3 4 . 9 3 0 . 6 3 0 . 3 3 3 . 9
H 36.0
3 2 . 6
3 0 . 7
3 3 . 7
top
bottom
N
b) motsturs content / X
1 7 . 7 1 7 . 7 1 7 . 4 1 7 . 9
1 7 . 9 1 7 . 2 1 6 . 9 1 7 . 2
1 8 . 0 1 7 . 1 1 6 . 7 1 6 . 8
1 7 . 9 1 6 . 9 1 6 . 7 1 6 . 9
H 18.1
t7.2
17.1
1 7 . 0
top
bottom
S -v N
F i g u r e 4 . 1 1
TABLE 4 . 1 0
Correlation matrices showing associations between insect
species and with two environmental parameters,
(data from Short-Term Distribution Trial)
Each correlation coefficient is based on 96 samples - 32 sampling
positions each sampled three times. Data for the three cribs have
been analysed separately.
Correlation coefficients not significantly different from zero
(at 5% level) have been diagonally 'hatched' and those showing a significant
negative correlation vertically 'hatched' (see text).
Species in parentheses (T. stercorea and L.cocbici in CRIB III)
were present only in very small numbers.
Species have been classified as:
A - primary pest species
- B - secondary pest species. _
C - parasitoids (Hymenoptera, Pteromalidae)
D - predatory bugs (Heteroptera, Anthocoridae).
TABLE 4.10
CRIB I
CO CO 0) CO P 4-1 CO 3 CU r-l CO p r-l b0 •H <u • H O rC i—i CU CO ja U Pa O O p p. 4-> 0 4-1 0 U o O P, P- 4-1 O 4-> • 4-> U £ cfl rH • H •H nJ C Ctf tn C/3 O U O
Sitophilus A Sitotroga Carpophilus B Cryptolestes Gnatocerus Palorus Typhaea Litargus Choetospila C Cerocephala
0.22
W / / / / W .
.32
.39 .68
.41 y//M. 1 .21
1
Cardiastiethus D Lyctocoris Scolopoides mTC^ Wt. loss
TABLE 4 . 1 0 (Cont inued)
(0
n M to M M
Sito
phil
us
Sito
trog
a
Carp
ophi
lus
Cryp
tole
stes
Gnat
ocer
us
Palo
rus
Typhaea
Lita
rgus
Choe
tosp
ila
Cero
ceph
ala
Card
iast
ethu
j
Lyct
ocor
is
Scol
opoi
des
u X Da
mage
Sitophilus 1 .24 Sitotroga A 1
. 6 7 ^ ^ .67 .32 .65 .47 .46 .23 % ^ .33 .28
.35
.34 Carpophilus B 1 .27 .59 .69 .59 .30 Cryptolestes 1 . 2 9 % ^ .20 .49 Gnatocerus 1 .38 .25 .42 Palorus 1 . W ' / Z / M Typhaea 1 .39 Litargus 1
.74 .49
.56 .41
.69 .56
.24 .54 .40
. 3 7 % ^ / .54
.50 .34 .33 JPBII .73 .26 W / & .60 .42
.!*'///&, .34
.45
.36
.35 V M ' .31
.30
.55 y / / M
.35 V/to
.71
.58 Choetospila C 1 .56 Cerocephala 1
//Mr, .^V/tt// ////& .29 y/py,
.27 v m
.43
Cardiastethus D 1 '//J8&?//JW* Lyctocoris 1 .33 Scolopoides 1
.39 .38 .37 .26
M.C. 1 .44 Wt. loss
TABLE 4 . 1 0 (Cont inued)
CRIB III
CO CO <U co p 4-1 p CtJ rH CO 1—1 to •H d) 'H O & i-l & M P< O P- 4-1 O 4-1 0 o P- P. 4-1 4-1 M ^ •H •H cd CJ o
Sitophilus A Sitotroga _ Carpophilus B Cryptolestes Gnatocerus Palorus (Typhaea) Litargus
24 1
w m i
Choetospila C Cerocephala Cardiastethus D (Lyctocoris) Scolopoides M.C. Wt. loss
or fungus-feeders or may be partially predatory. The third and fourth
groups comprise, respectively parasitic Hymenoptera and predatory
Heteroptera.
Considering first the mainly phytophagous species (groups A and B),
it is noticeable that the dominant primary pest, Sitophilus, which one
may assume to be the main agent of insect damage, is consistently pos-
itively correlated with the two most abundant secondary pests, Palorus
and Carpophilus. Sitotroga, on the other hand, although very slightly-
correlated with Sitophilus (0.01<p<0.05), is m general not correlated
with the secondary pests and shows negative correlations with Palorus
(strong) and Carpophilus (weaker). Both primary pests show a weak
correlation with grain moisture content in two of the cribs and a
similarly weak correlation with damage in all three.
—It-may-be recalled that-both^Sitophilus and Sitotroga were more
abundant towards the west of the crib and that Sitotroga, which was
present only in small numbers, was concentrated strongly towards the
top. As may be seen from Figure 4.11, the West and top of the crib
tended to be moister, more damaged and, except in the late afternoon,
cooler than the remainder (all of which may be associated with the
observation that on eight of the nine sampling occasions the prevailing
wind was from the West). It seems likely that the two species are
favoured by, and are responding to, the same environmental conditions
but that Sitotroga is being largely excluded by competition. Ayertey
(1979, 1980) has shown that, at insect densities comparable to those
found here, Sitophilus causes severe mortality to Sitotroga larvae
developing within the grains and that this effect is more acute in
already-damaged grain (Ayertey, 1979 and de Lima, 1978). The eggs
(and newly-hatched larvae) of Sitotroga are also exposed to predation
by insects and mites. These factors may be related to the low or
1 2 8 .
negative correlation coefficients that Sitotroga shows with several
secondary pest species and to the observation that, although Sitotroga
adult abundance in general increased from East to West, the rate of
recruitment was rather low in North-West and South-West corner positions
where grain damage and insect pest populations were highest.
Turning to the secondary pest species there is a 'gradient' of
associations: Palorus, at one extreme, is most closely correlated with
Carpophilus, less so with Typhaea and Gnatocerus and not at all with
Cryptolestes and Litargus; Carpophilus is correlated with Palorus and
Typhaea and less with Gnatocerus and Litargus; Typhaea is most strongly
strongly correlated with Carpophilus and Litargus and less with Gnatocerus,
while only Gnatocerus and Litargus are correlated with Cryptolestes.
The same associations can be seen in terms of the spatial distribution
of the species, summarised in Table 4.7. All the secondary pests .
except Palorus show a tendency to aggregate near the surfaces of the
crib; Palorus, Gnatocerus and Carpophilus show gradients of increasing
abundance from East to West while Palorus and Carpophilus are both
abundant towards the bottom of the crib. If these observations are
compared with the environmental data it may tentatively be suggested that
Palorus is able to exploit the driest parts of the crib, Carpophilus
is quite versatile infesting both dry and moister parts but preferring
the latter, while the remaining species are largely confined to the
moister surface conditions; Cryptolestes and Litargus are most strongly
limited in this way, Litargus being moreover associated particularly
with the most heavily damaged, mouldy areas.
The two Preromalid parasitoids, Choetospila and Cerocephala,
are correlated with one another, with Sitophilus, Palorus and, to a
lesser extent, Carpophilus. This is consistent with the possibility
that Choetospila is aggregating in areas where Sitophilus, its main
host, is most abundant. (It may be noted that Choetospila was preseit
at much lower densities than its host and that the rate of parasitisi
was not high enough to depress the host population). Dinoderus was
originally described as the host of Cerocephala, hence C. dinoderi,
(Gahan, 1925) but Dinoderus was only occasionally recorded from the
study cribs and then not in sufficient numbers to have supported the
parasitoid population. Attempts to rear Cerocephala in the laboratoy
on a variety of hosts proved unsuccessful.
Among the Anthocorid predators, Scolopoides was uncorrelated
with the primary pest species but correlated with all the secondary
pests except Palorus; Lyctocoris was correlated with Sitophilus, Paltry s,
Carpophilus and Typhaea while Cardiastethus was correlated with Sitotrg^r»ga
and Litargus and more weakly with Sitophilus, Gnatocerus and Cryptolts * es.
Cardiastethus was confined almost entirely to the top half of the crib
especially the surfaces, Lyctocoris occurred throughout the crib but
nymphs were found mainly near the bottom, while Scolopo'ides occurred
and reproduced in surface samples at all levels. The distribution of
these predators, in the crib then, provides strong evidence of some i
degree of ecological separation between them even though, in the labcr
atory, both Scolopoides and Lyctocoris were able to reproduce success-
fully on cultures of all the secondary pests which were offered to ttenmz*
(Carpophilus, Palorus, Gnatocerus and Cryptolestes). Cardiastethus
was not reared in the laboratory despite several attempts with a varie~e=^;y
of hosts.
In conclusion it should be pointed out that the foregoing c o m m e n s s
imply an overly simplistic interpretation of insect distribution in
relation to environmental conditions. Three additional and related
considerations must be borne in mind. Firstly, conditions throughout
the crib were, at the time of the study, quite favourable to insect
L30.
development and most of the phytophagous species were maintained
quite easily in single species culture on similar grain and under
comparable physical conditions in a nearby open-air laboratory. Second-
ly, insect infestation was very heavy with total pest densities often
exceeding one insect per grain of maize, implying intense competition
both between and within species. Under these conditions (i.e. equable
environment but high insect numbers) any full description of insect
distribution should be formulated in terms of competitive exclusion
under subtly differing conditions rather than simple preferences for
a particular habitat. Thirdly, although the environmental conditions
have been described in terms of large-scale gradients, on the scale
of individual cobs the habitat was very heterogeneous: moi-sture and
mould development in particular were" 'patchy* and the overall 'gradients'
often reflected the changing prevalence of patches of more or less
heavily damaged grain. The processes that determine insect distribution,
such as microhabitat selection, competition for food, shelter and ovi-
position sites, and the search for hosts or prey, were thus occurring
on a structurally complex, heterogeneous substrate. The data discussed
in this chapter were collected under only one set of environmental
conditions and it would clearly be dangerous to try to generalise the
conclusions too widely. The complexity of the situation revealed
should, however, at least serve as a warning against an unduly simplistic
approach to the analysis of stored grain ecosystems.
13L .
CHAPTER 5
THE INITIATION OF INFESTATION
5.1 Introduction
The previous chapter was concerned mainly with the spatial
structure of and relationships within the insect community of the
maize crib at a particular time. The next two chapters will consider
in more detail the changes that occur in this community with the
passage of time. This chapter will describe the initial infestation
of the grain maturing in the field, the transfer of grain to and
colonisation of the crib, and the influence of the preharvest in-
festation and harvesting practice on the first stages of development
of the storage insect community. The next chapter will then follow
the successional changes in the pest complex that occur over the
storage period.
Traditional small farm stores are often situated near to or on
the fields where the maize is grown. Where the grain is stored for
long periods, for subsistence use, storage pests may readily move
directly from infested stores to the new crop growing in the field.
Infested stores have been recognised as the main source of infestation
both in the U.S.A. (Blickenstaff, 1960) and Africa (Giles and Ashman,
1971) although other crops and natural reservoirs may be important
sources for some species and in some situations (Schulten, 1976; Linsley,
1944). Many storage insects fly readily and Sitophilus zeamais, which
seems to have been the species most extensively studied in this respect*
can cover distances of at least 400 - 800 metres (Giles, 1969; Chesnut,
1972) . Sitotroga cerealella, too, can actively infest maize before
harvest and this species was found to be more common in the field than
Sitophilus spp. in several localities in Kenya (de Lima, 1978) and
132.
Malawi (Schulten, 1972, quoted by Schulten, 1976; Dobie, 1974b).
Field infestation by storage pests follows and is probably
aided by, prior damage to the silks, sheaths and grain caused by
'earworms' - i.e. Lepidoptera larvae (Floyd, et al 1958; Comes, 1964; Starks
et al. 1966). Secondary storage pests (Coleoptera may also be
common on maize in the field : Schulten (1976; also quoting Giles
and Leon, 1974) suggests that field infestation by these species
may be low in the dry tropics but higher in more humid areas. Cornes
(1964), working at Ilora, S.W. Nigeria, (a location also used in this
study) recorded a succession in the pre-harvest infestation : the
silks were damaged by (unspecified) Diptera and Heliothis armigera
(Noctuidae); damage "to the ears, especially the tips, by Mussidia sp
(Phycitidae) and Argyroploce (=Cryptophlebla) leucotreta (Olethreutidae)
followed and finally the damaged cob apices and sheaths were invaded
by Sitophilus zeamais, Cathartus quadricollis, Carpophilus spp. and
other storage pests.
Particular attention has been given to the role of the husks
in protecting the cob in the field from infestation by storage pests.
Schulten (1976), reviewing the storage of maize,cobs, noted the impor-
tance of inter-varietal differences in extension of the husk over the
tip of the cob (Eden, 1952a), the tightness of the husk (Freeman, 1955)
and the number of sheaths forming the husk (Eden, 1952b). Damage to
the sheaths by birds, rodents and Lepidoptera larvae may allow storage
insects to enter otherwise well-protected cobs (Floyd et al. 1958; Freeman
1955; Starks.et al. 1966) and resistance to sheath-damage by Lepidoptera
may itself be related to varietal characteristics (Starks and McMillian,
1967). In Africa too, poor husk cover has been associated with in-
creased flight activity (Ajibola-Taylor, 1971) and infestation (Giles
and Ashman, 1971) by storage insects in the field.
133.
Storage of maize 1 in the husk', which is the traditional practice
in many areas, may provide protection against infestation for at least
the first part of the storage season, so long, as husk-cover is good
as in most traditional varieties (Thorshaug, 1975; de Lima, 1978).
The retention of the husks may reduce the effectiveness of admixed
insecticides (F.A.O. 1980). However, Golob (1981) has shown that
insecticides applied at appropriate levels to maize cobs in the husk
can achieve satisfactory insect control while leaving very low re-
sidues on the grain itself. As noted in Chapter 2, cobs to be stored
in the husk may have to be allowed to dry to a lower moisture content
before harvest, with the possibility of more severe field damage,
(Thorshaug, 1975; F.A.O., 1980) and their'irate of drying in store
may be slightly slower (Salmond, 1957; de Lima, 1978).
It is difficult to obtain a good idea of the seriousness of-
pre-harvest infestation both because infestation and damage are recorded
in a variety of different ways and because the various farming practices
in different areas mean that maize is not always harvested at a com-
parable stage of maturity. Estimates of grain damage at harvest vary
from 0.2% in local Malawi varieties (Reader, 1971) to over 10% in some
localities in Southern Nigeria (Adesuyi and Adeyemi, 1970). Surveys
in Southern Nigeria (Cornes and Riley, 1962; Comes, 1963 and 1964;
Patel and Adesuyi, 1975) and in Kenya (de Lima, 1978) indicate that
a wide range of values may be expected even under superficially similar
conditions: De Lima (1978) found less than 5% in most localities but
a few were as high as 10% and above.
Little attention seems to have been given to investigation of
the relationship between preharvest infestation and subsequent pest
problems in store. Giles and Ashman (1971) found that a higher level
of damage at harvest led to a more rapid increase in damage in store
while Pointel (1969) observed a correlation between Sitophilus zeamais
infestation after harvest with Lepidoptera damage in the field. An
understanding of the relative importance of sources of infestation is
clearly of crucial importance to the development of an appropriate
insect control strategy.
Grain placed in a crib after harvest will typically carry a t
degree of 'hidden infestation* (i.e. immatures of primary pests dev-
eloping within the grain) and a number of adults from the field pop-
ulation (although a proportion will have left the grain during harvest
and subsequent handling). To these insects will then be added any
that were infesting the fabric of the store, or residues from the
previous year's crop, and those that actively move to the store, either
immediately after harvest or during the storage period; these latter
may include insects from the field populations, from alternative agri-
cultural or natural habitats and from already-infested stores. Im-
portance has variously been placed on disinfestation of the fabric
of the empty store (with the help of exposure to sunshine, smoking,
or chemical insecticides), reduction or elimination of the pest population
carried into store with the grain (by 'sunning' or fumigation prior to
loading) and protection of the stored grain by admixture of persistent
insecticides, synthetic or natural, at the time of loading or repeated
applications of less persistant insecticides throughout the storage season
(methods reviewed by: Pingale, 1963 and 1964; Hall, 1970; Hindmarsh,
Tyler and Webley, 1978; F.A.O., 1980). The effectiveness of such
measures, and so the stress to be placed on them in a pest control
strategy, depends critically on the sources of infestation and the timing
of any insect movement to the store.
A comprehensive investigation of the relationship between pre-
and post-harvest infestation was beyond the scope of the present study.
135.
Limited observations were, however, made on the pre-harvest infestation
of the maize to be used for storage trials: these were to investigate
the species involved, their distribution and abundance, and their role
in promoting damage. In addition, two experiments were undertaken to
consider particular aspects of the relationship between field and
storage infestation: one, the 'Harvesting-Practice Trial1, was con-
cerned with the effects on the insect populations of, firstly, the timing
of the harvest and, secondly, the removal or retention of the husks;
the other, the 'Pre-Harvest Damage Trial* considered the persistent
effects in storage of the damage caused in the field by insects and
fungi. Some information on the source of colonisation of stores was
also provided by the inclusion in the- long'-term 'Succession Studies' of
cribs loaded with maize fumigated after harvest to destroy the field
infestation; these data will be considered in more detail in Chapter 6.
5.2 Pre-Harvest Infestation
Three surveys of field infestation were carried out: the first
at the time of the wet season harvest of 1978 and the second and third
prior to the dry season harvests of 1978/79 and 1979/80. Field samples
were collected from the maize subsequently used for the long-term
Distribution and Succession Studies, the Pre-Harvest Damage Trial and
the Harvesting- Practice Trial, respectively. The maize for the first
and third surveys was grown using mechanised zero-tillage methods (I.I.T.A.,
1973) while that used for the second survey was grown conventionally
(i.e. sown on tilled ground and hand-weeded). The maize varieties
used were white 'composite' varieties, TZPB and TZB.
In the wet season survey two fields of maize were sampled at the
beginning of August during the week before harvest when the grain (in
both cases) had a mean moisture content of approximately 30%. For the
136.
second survey (in the 1978/79 dry season) the maize from a single
field was sampled twice: the first sample was taken at the end of
November, three weeks before harvest, at a grain moisture content of
57 _+ 8%, and the second the day before harvest, in late December,
at a moisture content of 30 '+_ 6%. The intention in the third survey
was to compare the effects on infestation of harvesting at physiological
maturity of the grain with those of allowing the maize to dry some-
what in the field before harvest in the traditional way. One field
sample therefore accompanied the first harvest at 33 _+ 5% moisture
content and the other the second harvest at 18 +_ 3% moisture- content.
The methods used in this last trial will be discussed- further in Section
5.3.
The procedure for selection of samples was slightly different in
the three surveys, depending on the size and shape of the fields studied,
but in all cases the intention was to obtain a 'stratified' random
sample from the whole field. Transects were initially set, at regular
intervals, perpendicular to the rows of maize as sown. The transects
were then divided into subunits, each of a specified number of rows
(namely the total number of rows, divided by the number of samples
required); walking along the transect, one could then select a sample
(one cob) from each subunit, according to pre-selected random numbers,
simply by counting the number of rows crossed. In the first survey
25 cobs were picked along each of two transects in two fields, in
the second survey ten cobs were picked along each of four transects
on the two sampling occasions and in the third survey ten cobs were
picked along each of six transects at the two harvests.
In all trials cobs were snapped off and sealed immediately into
individual plastic bags. In the laboratory the cobs were dehusked,
137.
all insects collected and a subsample of grain taken for moisture content
determination. No attempt was made to distinguish between insects
associated with different parts of the ear - i.e. silks, husk, grain
or core - but where damage or infestation appeared limited to a particular
part this was noted. As the cobs were dehusked a note was also made as
to whether the sheaths were 'open' at the apex, properly 'closed1 over
the tip of the cob, or, if closed, whether the husks had been 'holed'
by insects. After collection of the insects an approximate count was
made of the number of grains on each cob damaged by insects and moulds;
only damage to fully-formed grains was scored and where moulding appeared
to have followed insect damage to a grain that damaged was attributed
to insects. All samples from field trials had to be handled rapidly to
avoid the excessive build-up of condensation and the perforation of
sample bags by Lepidoptera larvae.
Outline results indicating the degree of field infestation and
damage are given in Table 5.1 and the abundance of the various insect
species is summarised in Table 5.2. In comparing the results of the
three surveys it may be noted that the wet season samples, the 'at harvest'
sample of the 1978/79 dry season survey and the 'first harvest'sample
of the 1979/80 survey were all taken at a comparable stage of the crop
phenology - i.e. at grain maturity.
The general picture provided by these data is of considerable
infestation : only a single uninfested cob was collected in each of
the first two surveys (i.e. two from a total of 180 cobs). However, in
many cases the 'infestation' was of no economic significance, consisting
of only small numbers of Coleoptera, often associated with the husk
or silks rather than with the grain. At the time of harvest 52% of
the cobs in the wet season samples and 68% in the first dry season survey
showed less than ten grains damaged by insects (7% and 22% of cobs,
t
TABLE 5.1 Summary results of field samples: infestation by major pest groups, grainidamage and husk, cover.
Mean grain Survey m.c./%
Cobs collected
Percentage of a)Lep. & Col.
cobs infested by: Lep. only Col. only Uninf.
Mean no. grains/cob damaged by:,
a) insects b) fungi
husk cover: % cobs
intact holed open
WET SEASON (1978)
Field A. 29 50 56 16 22 2 16 48
Field B. 30 50 58 26 14 0 36 44
DRY SEASON 1. (1978/79)
1. 3wks.before harvest.
57 40 67.5 20 10 0 2 8 57.5 30 12.5
2. at harvest 30 40 67.5 0 30 2.5 9 15 27.5 40 22.5
DRY SEASON 2. (1979/80)
1. First har-vest 33 60 48.3 8.3 35 8.3 N/D N/D 71.7 20 8.3
2. Second harvest 18 60 38.3 3.3 • . 5 3- 3 5 N/D N/D 45 35 20
u> 00
139.
TABLE 5.2 Percentage of cobs infested by each species (or group). Figures in parentheses are mean numbers of insects/cob for the most abundant species.
SPECIES WET SEASON 1 DRY SEASON 1. DRY SEASON 2. LEPIDOPTERA: (Field A) 3 wks. pre - harvest first second
harvest harvest harvest
Eldana saccharina } 70(2) 28 18 5 2 Mussidia sp. J 3 - i 28 15 Cryptophlebia leuc 1 75 (1. 2) 33 • 20 10 Pyroderces sp. 1 12 5 45 8 22 indet. 12 — 13 5
COLEOPTERA
Sitophilus zeamais 28 - 45 (3. 5) 7 32 Carpophilus spp. 44 38 (1. 5) 35 (2. 3) 35 (1.2) 45 (2 .0) Brachypeplus spp. 8 8 30 28 13 ' Cathartus quad. 14 ., 55 (2.1). 78 (7. 5) 42 (1.0) 38 (3 .6) Mycetaea hirta 18 35 60 (2. 0 36 (2.4) 67 (1 .9) Litargus'varius' — 5 8 5 -
Gnatocerus max. — — 25 - -
Staphylinidae 16 23 23 18 10 other Coleoptera 6 - 38 12 10
Coleoptera larvae
Nitidulidae 42 13 48 (2. 3) 20 (1.5) 17 . others N/D 10 75 (2. 7) 17 42 (1 • 2)
HYMENOPTERA «
Parasitoids N/D - 8 2 2
Formicidae N/D - 23 10 10
DERMAPTERA 24 63 (1 3) 45 23 17 .
(add. and ny.) HETEROPTERA 2 5 10 2 3 (add. and ny.)
DIPTERA N/D 20 N/D 10 -
(larvae & pupae)
BLATTIDAE N/D - 3 5 2 Other insects N/D - - 2 2
Spiders N/D 5 3
140.
respectively, showed 110 visible insect damage to the grains). Using
an approximate conversion, based on the mean number of grains per cob,
the proportion of grains damaged by insects at the time of the wet
season harvest may be estimated at c.3 and 7% (for the two fields
sampled) and for the dry season harvest at c.9%; the corresponding
figures for mould damage were 8-9% for the wet season harvest and c.3%-
for the dry.
It has already been noted (§2.2) that the distributions of insects
in field samples closely followed the negative binomial distribution
with low values of k (< 1.5 for most species) and this pattern was
reflected in the figures for insect damage. Mould damage was similarly,
or more strongly, 'clumped1: at the wet season harvest, for instance,
10% of cobs had been totally infested with mould (i.e. all grains visibly
infected) while 20% showed no visible mould damage at all. The re-
lationship between fungal development and insect infestation is complex
and appears to depend both on the species involved and the timing of
infection.
Infection by Diplodia macrospora was usually primary (i.e. attacking
cobs not previously damaged by insects or other agents) and, once
established, tended to destroy the entire cob. This species accounted
for all the cobs showing 'total infection* mentioned above. Cobs
attacked by Diplodia usually had intact husks, the sheaths covering the
tip of the cob and adhering strongly to the grain due to the vigourous
growth of hyphae between. Such cobs were rarely infested with insects,
with the exception of small numbers of Mycetaea hirta (Endomycidae), a
species believed to feed mainly on fungi. Another primary pathogenic
fungus, Ustilago maydis, was also recorded but only occasionally. Ustilago
tended to cause severe deformation of the cob and husk and the resulting
'wet' rot was particularly attractive to Nitidulid beetles (Carpophilus spp;
141.
Brachypeplus spp. and Urophorus humeralis).
More limited damage was caused by Fusarium moniliforme (= Gibberella
fujikuori) (probably in association with other species). Infection in
this case appeared to be associated almost always with prior infestation
by insects, especially Lepidoptera larvae. Fungal development was
usually confined to the immediate vicinity of insect-damaged grains
and often affected only the tip of the cob. More severe damage by
these fungi occurred when the sheaths were open or damaged at the apex.
The larvae of Carpophilus spp. and Cathartus quadricollis seemed to be
mainly feeding in the grains already damaged by moulds although Schulten
(1976) states that Cathartus can act as a primary pest (i.e. attacking
undamaged grain) at moisture contents of 30% or above.
Husk cover in the maize varieties used for these studies was
near the average for 'improved' high-yielding varieties-(Olusanya, pers.
comm.) with the sheaths completely closed over the tip of the cob in
most plants. The results indicate that the proportion of cobs with
intact husks declined markedly during field drying, with comparable
increases in both husks perforated by insects and those that opened at
the tip as they dried. It may be noted that in both dry season surveys
the rate of infestation by Lepidoptera declined markedly between the
two samples, reflecting emergence of the single generation of these
species that is completed on the developing cobs. It is the late instar
Lepidoptera larvae, especially Eldana saccharina and Mussidia nigrivenella, i that are mainly responsible for damage to the sheaths. It may be noted
that both insect infestation and the proportion of cobs with 'open'
husks were higher in the first dry season survey than in the second. Poor
husk cover in maize is often associated with adverse conditions during
growth (Quin, pers. comm.) and it may be relevant that the former crop
142.
suffered considerable drought stress. Damage to husks by birds and
rodents was not significant in the fields surveyed, although large
flocks of weaver birds (Ploceidae) were at times observed feeding on
maize fields on the research station.
The Lepidoptera infesting maize cobs in the field are diverse in
biology and in the damage they cause. Eldana saccharina is economically
important mainly as a stem borer, invading the crop at or after tass-
elling and continuing to develop in the maize plants until they are
completely dry (Kaufmann, pers. comm.). The severe lodging that may be
caused by infestation by this species is an important factor to be con-
sidered in assessing the optimum time of harvest. Eldana larvae may
invade the cobs at any stage, entry being via the tip or directly through o
the sheaths, rather than via the stem (a point also noted by Cornes, 1964).
Damage to the grain can be extensive: a single larva may move and feed
superficially down one or two rows of grains..,along the length of a cob,
causing little direct damage but exposing all the grains so affected
to fungal attack and infestation by secondary pests. Mussidia nigrivenella
appears to attack only the cobs (i.e. it is not a stem borer) but shows
a similar pattern of damage. The late instar larvae often tunnel through
the bases of a row of grains, finally pupating 'in situ', and similarly
promoting the destruction of a large number of grains. Several larvae
of the same size are typically found together. The dominance of these
two species (present in approximately equal numbers) at the time of
the wet season survey was responsible for the higher number of insect
damaged grains recorded then.
The other two common species of Lepidoptera, Cryptophlebia
leucotreta and Pyroderces sp. (probably gossypiella), were almost in-
variably found near the apices of cobs. The former was often found
feeding on the silks or on immature grains, while the latter appears to
143.
complete development within a single grain (and, for this reason, was
probably under-recorded); the damage caused by both species is, acc-
ordingly, much more limited. Damage to the cob apex and sheaths by
Cryptophlebia appeared to be associated with some mould development but
this was usually limited to the core. Cryptophlebia feeds on a variety
of other hosts, especially fruits, and only attacks the maize at a
high moisture content before maturity. Pyroderces infests later in
the succession and many adults emerged in the crib after harvest, though
they did not reproduce there. Pyroderces sp. has been found in large
numbers on maize at harvest in Zaria, under much drier climatic con-
ditions (Ayertey, pers. comm.,), and two species have been collected
from maize in Cameronn (Bradley, pers. comm). The dearth ,of records
of this species from other localities may be due to its being mistaken
for Sitotroga cerealella, which is similar in size and form. Other
Lepidoptera recorded in small numbers included Helio.this armigera, Sesamia
calamistis and Busseola fusca, the first feeding usually on the silks
and the latter two on grains or cores.
The dominant Coleoptera in field samples were, as indicated in
Table 5.2, Cathartus quadricollis, various Carpophilus species and
Mycetaea hirta. Among the Carpophilus spp., C. fumatus was by far the
commonest, followed by C. dimidiatus, with smaller numbers of C. zeaphilus,
C. freemani,C. binotatus, C. hemipterus and C. obsoletus; the incidence
of the Carpophilus species was erratic and highly clumped with the less
abundant species sometimes occurring in large numbers on a single cob.
There was no evidence of primary damage by any of these species but they
undoubtedly contributed to losses by completing the destruction of
grains previously only slightly damaged.
Infestation by Sitophilus zeamais in the field was not severe,
despite the proximity of infested stores to the sampled fields. It has
144.
been shown that Sitophilus zeamais can oviposit successfully on maize
with a moisture content as high as 60% (Giles and Ashman, 1971), but
in this study no infestation was observed at this level. The data
confirmed the reported preference of Sitophilus for cobs with open or
damaged sheaths : cobs in these categories comprised 83% of those
infested by Sitophilus as compared with 63% in the sample as a whole,
in the first dry season survey and 79% as compared with 55%, in the
second. A slight preference for drier cobs was also indicated in
the second survey. Sitotroga cerealella was not recorded in any of
the field surveys.
The data from the second dry season.survey were analysed to
investigate the possibility of uneven distribution of insects in the
field. The field surveyed was rectangular, approximately 1.5 hectares
in area, with a- larger area of maize fields to the North, a small patch
of forest and a residential area to the East and plots of rice and
carsava to the South and West. The nearest storage cribs (which con-
tained infested maize) were 200 - 300m away to the South-East. Single
factor analysis of variance was carried out on counts for all the common
insect species, transformed ^og^o (X + 1), using first the transects
and then the position along transects as 'treatments'.
Only Mussidia nigrivenella at the time of the first harvest, and
Sitophilus zeamais at the second showed any indication of differences
in abundance between transects (p = 5% and 10%, respectively) or position
along transects (p = 3% and 6%). Both species showed a progressive
decline in abundance from North to South ('positions along transects')
and from East to West ('transects'); proximity to the main area of maize
fields and, for Sitophilus at least, to the infested cribs might have
been the important factors in the two directions. No 'field edge effects'
145.
as noted by Blickenstaff (1960) and Giles and Ashman (1971) were
discernable but this may be due to the rather small size of the
field surveyed.
5.3 The Effects of Harvesting Practice on Infestation
Current recommendations for the storage of maize 'on the cob'
(F.A.O., 1980) differ most conspicuously from traditional practice
in that the maize is harvested earlier (i.e. at physiological maturity),
the husks are removed and the grain is treated with insecticide. The
high moisture content of the grain at harvest means that only 'well-
ventilated' cribs may be used for storage. The aim of the 'Harvesting
Practice Trial' was to assess the effects of the first two factors
(i.e. early harvest and removal of the husk) on the initial infestation
in store. Assessment was based on counts of adult insects and emergences,
grain moisture content and weight loss. The trial was not intended to
show which practice was preferable in economic terms and no attempt
was made to estimate the total crop losses involved.
The maize from a single large plot was harvested in two parts,
the first at physiological maturity (assessed visually on the basis of
'black layer' formation) and the remainder three weeks later. On each
occasion half the maize was 'husked' (i.e. the husks removed) and
half 'snapped' (i.e. the husk retained). Cobs were then sorted to
remove those unsuitable for storage (see below) and stored in comp-
artments in three cribs. Samples were collected from the field prior
to each harvest, from the sorted piles waiting to be loaded into the
crib and, at the time of the second harvest, from the maize stored in
the crib since the first harvest. Maize from all treatments (i.e.
early - & late-harvested, with and without husks) was then sampled after
an additional one month and two months in store. Table 5.3 summarises
146.
TABLE 5.3 Summary of Sampling regime for Harvesting Practice Trial.
FIRST HARVEST
Field sample 23/11/79
Harvest .26/11/79
Sorting and Loading 27/11/79
6 x 10 = 60 cobs adult counts (individual cobs) moisture content( " " ) emergences
(6 x 500g samples).
all cobs sorted into damage categories and scored.
12 x 5 .' snapped* 12 x 5 'husked':
= 120 cobs
adult counts (sample totals) loss assessment
(24 x 1000 grains)
SECOND HARVEST
Field sample 17/12/79
Harvest 19/12/79
Storage sample
19/12/79
Sorting and Loading 20/12/79
6 x 10 = 60 cobs adult counts (individual cobs) moisture content( " " ) emergences
(6 x 500g samples)
6 x 20 'husked' = 240 cobs
(from early-harvested material in crib) 6 x 20 'snapped' adult counts (12 sample totals)
emergences (12 x 500g shelled grain)
moisture content (12 x lOg subsample)
loss assessment (12 x 1000 grains «
all cobs sorted and scored. 6 x 20 ' snapped' 6 x 20 /husked'
= 240 cobs
adult counts (12 sample totals) emergences (12 x 500g shelled
grain) moisture content (12 x lOg
subsample) loss assessment (12 x 1000 grair
FIRST STORAGE SAMPLE 20/1/80
Storage sample 6 x 20 'early snapped' 6 x 20 'early husked' 6 x 20 'late snapped'
6 x 20 'late husked'
= 480 cobs
adult counts (24 sample total emergences (24 x 500g grain) moisture content (24 sub-
samples) loss assessment (24 x 1000
grains)
SECOND STORAGE SAMPLE 18/2/80
Storage sample as for first storage sample.
147.
FIGURE 5.1 Summary of sampling program from cribs (Harvesting-Practice Trial)
Design:
Lh Eh Ls Es
Lh Eh Ls Es
Ls Lh Es Eh
Ls Lh Es Eh
First harvest:
h h - s s
h h s s
s h s h
s. h s h •
Compartments allocated at random to the four treatments in pairs (as shown), so that each treatment is represented in the top^and bottom of three cribs, (only one crib shown). Anovar:
Blocks (3) = cribs Treatments = 1) Position in crib (upper
or lower half). 2) Harvesting time (Early or
late). 3) Husks (presence or absence'
E/L = Early/Late h/s = husked/snapped
All compartments loaded (as shown): Es and Ls with 'snapped* cobs Eh and Lh with 'husked* cobs
Second harvest: samples
Storage samples samples
Lh1 E h l Ls^ Es^
Lh2 Eh 2 Ls^ ES 2
Ls2 Lh2 Es, i Eh 2
Ls.. L hl Es j Eh
Compartments originally allocated to 'Ls' and 'Lh' are emptied; Subsamples (20 cobs/section) are collected randomly from this material as it is unloaded, for comparison with 'pre-loading' samples. Ls and Lh compartments are loaded with newly harvested 'snapped' and 'husked' cobs, respectively.
First Storage Samples (subscript 1)
Second Storage Samples (subscript 2)
On the first sampling occasion sections in the top and bottom layers are emptied; on the second the sections in the middle layers are emptied. In both cases samples of 20 cobs/section are collected at random from the material as it is unloaded.
the size, timing and purpose of all the samples collected and figure
5.1 the sampling programme from the cribs.
Maize cobs were harvested directly into hessian sacks which
were carried by the pickers and removed from the field as soon as full,
the intention being to minimise the movement of disturbed insects to
the remainder of the crop which was to be harvested later. Harvesters were
spread out across the field, about 10 rows apart, and picked two rows
at the first harvest and the balance at the second. Some maize for
the storage trial was collected from each harvester so that all parts
of the field were represented. . Alternate harvesters collected 'snapped'
and 'husked' cobs: the former group simply broke off the ears whole,
while the second group opened each ear on the plant and twisted off the
cob, leaving the husks attached to the haulms. A time and motion study
carried out on this harvest showed that 'snapped' maize could be
harvested three times as fast as 'husked' maize, and that 'husking* on the
plant, as here, was still significantly faster than the more traditional
method of collecting * snapped' cobs and husking them later (Buchele,
pers. comm.). Cobs were left in piles near to the cribs overnight before
sorting, sampling and loading.
The selection of cobs for loading was intended to simulate the
normal practice of local farmers. Cobs that were severely damaged by
moulds or insects, or any that were seriously malformed (due to poor
pollination or physiological stress), were discarded. In addition,
'snapped' cobs with loose or open sheaths were also excluded. Although
the selection (carried out by the author) was basically subjective,
standardisation was improved by comparison with a set of 'acceptable'
and 'unacceptable' (for storage) cobs used at both harvests. The numbers
of cobs attributed to the various categories are given in Table 5.4.
149.
TABLE 5.4 Selection of damaged and sound cobs for storage -
Harvesting Practice Trial a) First Harvest b) Second Harvest
Accepted for storage: (i) stored (ii) excess (
o
Rejected as unsuitable: i) open sheaths (ii) mould (iii)mal-r insect damaged damaged formed
a) SNAPPED No.
%
1080 235
84%
129 35 41
10% 3% 3%
HUSKED No.
%
1140 869
86%
96 155 68
4% 7% 3%
b) SNAPPED No.
%
536 136
60%
237 51 174
21% 4% 15%
HUSKED No.
%
576 294
57%
241 207 • 222
16% 13% 14%
a*
150.
Although the overall intensity of selection was approximately
the same for 'husked' and 'snapped' cobs, the composition of the 'rejects',
and so, by implication, of those stored, was not identical.' This
occurred because, in selecting snapped cobs, any with open or loose
husks were rejected, irrespective of whether or not they showed visible
insect damage. The larger number of cobs excluded for this reason was,
however, balanced by the inclusion of more Diplodia infected cobs: these
usually had closed sheaths and so snapped cobs infected with Diplodia
were only noticed in the most severe cases where the sheaths as well
as the grain had become discoloured.
Cobs for the field samples were collected individually, as des-
cribed in Section 5.2. For the pre-loading samples and those from the
cribs, all cobs for a particular sample (10 or 20) were collected
together.in a large plastic bag, husked (where necessary) and shelled
inside the same bags. All adult insects were then sieved off and
subsamples drawn from the pooled, mixed grain for estimation of emergences,
moisture content and loss assessment.
The cobs were stored in the sectioned cribs previously used for
the distribution studies (Figure 4.1). Each section was carefully
packed with stable 'stacks' of cobs, those in successive layers being
laid perpendicular to one another, to form a single well-ventilated bulk.
Each section could be unloaded separately, one cob at a time, and a sample
of 20 cobs randomly selected. Two vertically-adjacent compartments
were allocated randomly to each of the four treatments in the top and
bottom halves of each of three cribs (see Figure 5.1). Insects could
move readily from one 'treatment' to another through the wire partitions.
Loss assessment was carried out on the basis of successive estimates
of the mean dry weight of a thousand grains. A subsample of approximately
151.
500g was drawn from the mixed shelled grain from each replicate sample.
This was heated, first at 60°C then at 90°C, for several hours to re-
move excess moisture. A further subsample of 1000 grains was then
counted out, weighed, and the moisture content determined to provide
an estimate of the dry weight.
Simple analysis of variance was used to compare the infestation
and moisture data from field samples with those from the piles of cobs
prior to loading and, at the time of the second harvest, with those
from the cribs. Figures for the individual cobs collected in the field
samples were pooled so that the data compared were based on the same
number of cobs in each case. The selection procedure was, however,
different in the three situations (i.e. 'field1, 'pre-loading' and 'in
stdire') and so the comparison connot be regarded as entirely satisfactory.
The two complete storage samples were analysed factorially (Figure 5.1)
to separate the effects of time of harvest (early or late), removal or
retention of the husks and position in the crib (i.e. upper or lower
half). Samples were taken from different 'layers' of the crib on the
two sampling occasions (see Figure 5.1) and so the data obtained should
not be compared across sampling occasions. Counts of adult insects and
emergences for most species showed 'over-disperison' and so these data
were transformed (Log^) before analysis.
Data comparing the numbers of insects in field samples, prior
to harvest, with those from sorted cobs awaiting loading, 24 hours
after harvest, are given in Table 5.5. There is no evidence that
harvesting and selection have had any effect on the numbers of
Sitophilus zeamais or Mussidia nigrivenella, but secondary pest pop-
ulations have been affected :; Mycetaea hirta has been virtually eliminated
from the post-harvest samples and Carpophilus spp. markedly reduced.
152.
TABLE 5.5 Effects of harvesting on adult insect populations: comparison of adult insect numbers before harvest (field samples) and 24 hours after harvest (snapped and husked). Probability levels from single factor analysis of variance.
a) First Harvest b) Second Harvest
a) Mean number of insects/10 cobs: Species field samples snapped husked prob.(no diff.)
Sitophilus 1.2 2.2 0.3 zeamais (1.6) (2.6) (0.5)
Cathartus 9.5 4.0 9.8 0.21 quadricoll. . (3.8) (4.6) (8.7)
Carpophilus 12.0 2.0 6.5 0.001 spp. (4.1) (2.5) (3.8)
Mycetaea 22.0 0.3 1.0 <0.001 hirta (9.8) (0.5) (1.5) .
Mussidia 6.2 8.5 3.2 0.12 nigriven. (6.1) (3.0) (2,2)
b) Mean number of insects/20 cobs:
field (3) snapped (6) husked (6)
Sitophilus 13.7 12.2 9.5 zeamais (14.4) (14.2) (5.0)
Cathartus 72.2 58 73 quadj-icollis (47.2) (42) (35)
Carpophilus 39.7 5.2 7.7 spp. (5.1) (4.2) (5.9)
Mycetaea 37.3 0.2 0 hirta (14.0)
153.
TABLE 5.6 Effects of Early harvesting on insect populations. Comparison of mean moisture contents (a),emergent insects (b),and adult insects (c),from early-harvested maize (after 3 weeks in store) and late-harvested maize (24 hours after harvest and before loading); data from pre-harvest (field) samples are included in comparison of emergences. Probability levels are from a single-factor analysis of variance. Probabilities in parentheses indicate data showing non-homogeneous variances (Bartlett's Test); Figures in parentheses below means are atandard deviation.
(a) Moisture content (%)
Treatments Early harvested (from crib)
Late harvested (freshly harvested)
husked snapped husked snapped probabilities. 14.7
(0.3) 16.7
(0.5) 17.2
(0.9) 17.1
(0.4) <0.001
(b) Emergences (no. insects emerging from 500g/4 weeks).
Species early husked early snapped late husked late snapped field P
Sitophilus 129 b 84 ab 91 ab 32 a 53 a o.oc zeamais (37) (42) (39) (36> (48)
Cathartus 10 21 18 14 11 0.04 quadricoll (4) (8) (8) (4) (7)
(c) Adult counts (no. insects/kg @ 15% m.c.). Treatments Species Early harvested
(from crib) husked snapped
Late harvested (freshly harvested) husked snapped probabilities
Sitophilus 63.2 65.0 3.8 5.1 (<0.001) zeamais (21.4) (26.1) (2.1) (6.0)
Carpophilus 5.5 3.0 2.2 3.1 0.21 spp. (3.3) (3.0) (1.7) (2.4)
Cathartus 59.7 92.6 • 29.2 24.0 <0.001 quadricollis (31.7) (31.4) (14.0) . (18.3)
Palorus 0.6 7.7 0.1 0.1 (<0.001) subdepressus (0.7) (3.2) (0.2) (0.2)
Gnatocerus 0.6 1.3 0.0 0.0 maxillosus (0.5) (1.0) Mussidia 1.0 1.0 1.1 0.7 0.91 nigrivenella (1.0) (0.9) (1,1) (0.8)
Pyroderces 0.4 1.0 0.9 1.1 (0.15) gossypiella (0.2) (0.8) (0.3) (0.8).
Zeteticontus 1.7 0.3 0.3 0.1 (0.07) la evigatus (2.0) (0.5) (0.5) (0.3)
It may be rioted that for both Cathartus quadricollis and Carpophilus spp.
there is evidence of lower numbers, post-harvest, in snapped than in
husked cobs, even though insects in the latter have suffered less *
direct disturbance. It seems likely that the majority of these more
active insects in fact left the cobs during harvest and that the figures
reflect more rapid recolonisation of the exposed, husked cobs.
The effects of early harvesting are indicated in the data collected
at the time of the second harvest, presented in Table 5.6. The figures
for grain moisture content (Table 5.6a) show that cobs stores in their
husks (i.e. 'snapped') have dried at the same speed as those left in
the field, but that cobs dehusked before storage have dried significantly
faster.
Insects emerged from the grain subsamples (Table 5.6b)) were
collected four weeks after sampling and so mainly reflect oviposition
at a time when the early harvested material was in the crib and the
late-harvested still in the field. Differences are not clear-cut but
there is some evidence that on the early-harvested husked maize re-
production of Sitophilus has been the most successful and that of
Cathartus the least.
The figures for adult insect numbers (Table 5.6c)) indicate
higher levels of Sitophilus, Cathartus, Palorus subdepressus and
Gnatocerus maxlllosus on the early-harvested maize in the crib. It
should be noted that the higher infestation on the maize in the cribs
cannot, after so short a time, be due to higher recruitment but must
be due to preferential colonisation. It is possible that some insects
moved directly from the newly-harvested material to that in the cribs
between the time of harvest and sampling. However, this cannot have
been the case for the two Tenebrionidae, which were rare in the field,
and seems unlikely to be responsible for the great difference in Sitophilus
155.
TABLE 5.7 Effect of time of harvest and retention of husks on grain moisture content. Figures are means of single determinations from six samples (B each being the pooled grain from 20 cobs) with probability levels from a three-factor anovar (as indicated in Figure 5.1); after one month in store ('First Storage Sample1) and two months in store ('Second Storage Sample') - early-harvested material has had an additional three weeks in store.
FIRST STORAGE SAMPLE
Early Late X Husked
. 14.9 | 15.0 15.0
Snapped 15.3 | 15.0 15.1
x 15.1 | 15.0
SECOND'STORAGE SAMPLE
Early Late X Husked
13.5 | 13.2 13.4
Snapped 13.5 | 13.2 13.3
X 13.5 | 13.2
A time of harvest B husked/snapped C position in crib AB AC BC ABC
A time of harvest B husked/snapped C position in crib AB AC' BC ABC
0.12 0.08 0.005 0.02 0.68 0.31 0.31
0.03 0.90 0.01
0.90 0.21 0.37 0.37
156.
numbers. Of the maize stored in the cribs, the snapped cobs were
much preferred by Palorus and Gnatocerus and possibly slightly pre-
ferred by Cathartus quadricollis.
The first complete storage sample was taken four weeks after
the second harvest (i.e. when the early-harvested maize had been in
store for seven weeks and the late-harvested for four) and the second
storage sample another four weeks later. By the time of the former sample
all treatments had dried to effectively the same grain moisture content
(and continued to dry uniformly thereafter) although consistent, very
small differences were detectable on both occasions (Table 5.7).
The probability levels from a factorial analysis of variance
of the insect counts (transformed, L o g ^ ) are given in Table 5.8; where
significant differences between treatment means are indicated the
~ actual-means are given in Tables 5.9 (adults) and 5.10 (emergences).
Emergences from the second sample correspond approximately to oviposition
at the time of the first sample while those from the first sample re-
present oviposition over a more extended period between the second harvest
and the first storage sample. The 'breeding-out* period for the first
sampling occasion was long enough to allow some additional parasitism of
Sitophilus in the laboratory. In order to provide a better approx-
imation to the original pest distribution the figures for Anisopterom alus
calandrae and Chcetospila elegans emergences have been added to those
of Sitophilus.
The data indicate that the retention of the husks has provided
no protection against insect infestation. On the contrary, Sitophilus
appears to have reproduced initially more successfully on the snapped
maize (Table 5.10a), possibly due to its slightly.higher moisture content,
and both adult counts and emergences indicate that the snapped maize
was preferred by most of the secondary pest species.
1 5 7 .
TABLE 5 . 8
a) , ADULT COUNTS (FIRST STORAGE SAMPLE)
Sitophilus Cath. Oryzae. Carpoph. Gnato. Pal. zeamais quad. mere. spp. max. subdep.
A Early/Late 0.18 0.13 0.32 0.02 0.16 <0.01 B husked/snapped 0.27 <0.01 <0.01 0.92 <0.01 <0.01 C position in crib 0.37 0.20 0.05 <0.01 0.52 0.06 A and B 0.34 0.23 0.27 0.35 0.03 <0.01 A and C 0.57 0.95 0.91 0.87 0.05 0.09 B and C 0.01 0.02 0.65 0.11 0.57 0.95 A, B and C 0.85 0.03 0.70 0.73 0.61 0.67
b) EMERGENCES (FIRST STORAGE SAMPLE) Sitophilus Cath. Gnato. Anisopt. Choeto zeamais quad. max. cal. eleg.
A Early/Late 0.07 0.57 0.05 0.25 B husked/snapped 0.58 <0.01 0.32 0.43 C position in crib 0.24 0.44 0.09 0.39 A and B <0.01 0.92 0.37 0.02 A and C 0.51 0.58 0.46 0.70 B and C " 0.02- 0 .33 - — <0.01 0.14 A, B and C ' 0.74 0.02 0.39 0.79
c) EMERGENCES (SECOND STORAGE SAMPLE)
Sitophilus Cath. Gnato. Anisopt. Choeto. zeamais quad. • max. cal. eleg.
A Early/Late 0.79 0.02 <0.01 0.82 0.61 B husked/snapped 0.82 <0.01 <0.01 0.32 0.96 C position in crib 0.73 0.38 . 0.56 0.60 0.84 A and B 0.30 0.09 0.87 0.58 0.84 A and C 0.95 0.32 0.73 0.78 0.53 B and C 0.40 0.66 0.84 0.33 0.29 A, B and C 0.63 0.29 0.49 0.70 0.45
TABLE 5.8 Effects of Time of harvest on insect infestation 1
1. Probability levels from analysis of variance - effects of time of harvest, removal of husks and position in crib on insect numbers.
a) adult insects - first storage sample b) emergences - first storage sample c) emergences - second storage sample
Data used were: a) numbers of insects/kg shelled grain @ 15% m.c. b) insects emerging from 500g grain fresh weight in 23 days c) insects emerging from 500g grain fresh weight in 8 days
Data were transformed Log- n before analysis.
158.
TABLE 5.9 Effects of time of harvest and removal or retention of husks on adult insect infestation (first storage sample).
Figures are mean numbers of insects per kg of shelled grain at 15% m.c. (each figure based on six samples of 20 cobs).
a) Cathartus quadricollis
Husked Snapped Mean Early 155 426 290 Late. 126 259 192
Mean 140 1 342 b) Oryzaephilus mercator
Early 1 5 3 , Late 1 1 4 2
Mean 1 1
c) Carpophilus spp.
Early 21 1 1 9 20 Late 12 1 10 11
Mean 16 1 14
d) Gnatocerus maxillosus
Early 0 1 6 3 Late 1 1 3 2
Mean 1 1 4
e) Palorus subdepressus
Early 1 | 23 12 Late 1 3 2
Mean 1 13
159.
TABLE 5.10 Effects on emergences of time of harvest and removal/
retention of husks at the time of a) the first storage
sample and b) the second storage sample.
Figures are mean no. of insects emerging from 500g (fresh wt.)
of grain in a) 23 days and b) 8 days; figures are based
on six replicates of each 'treatment*.
a) FIRST STORAGE SAMPLE
Sitophilus zeamais husked J snapped mean
Early 174 | 309 241 Late "212 1 154 183 Mean 193 \ 231
Cathartus quadricollis
Early i
3 1 11 7 Late 4 1 9 6 Mean 3 | 10
Anisopteromalus calandrae
Early 1.2 I 1 1.2 1.2
Late 0.9 0.4 0.6 Mean 1.0 | 0.8
Choetospila elegans i
Early Late
0.2 0.3
0.6 0.1
0.4 0.2
Mean 0.2 0.3
b) SECOND STORAGE SAMPLE
Cathartus quadricollis htisked 1 snapped mean
Early 7 I 19 13 Late 5 1 8 6 Mean 6 i 13
Gnatocerus maxillosus
Early i
1 2 1 24 18 Late 6 1 12 9 Mean 9 1 18
160.
TABLE 5.11 Effect of time of harvest on losses. Figures are mean
dry weights of 1000 grains (standard deviation in parentheses)
with probability levels from 3 factor anovar; after a) one
month in store; b) two months in store. (Grain weight in grammes)
FIRST STORAGE SAMPLE
Early Late X Husked
234.3 (6.23)
248.4 (15.5)
241.3
Snapped 217.9 (17.3)
240.7 (9.0)
229.3
X 226.1 244.5
SECOND STORAGE SAMPLE
Early Late Husked
218.5 (7.8)
249.3 (4.8)
233.9
Snapped 219.4 (13.3)
240.0 1 (8.0)
229.7
218.9 | 244.6
A Time of harvest 0.005 B Husk cover 0.045 C (position in crib) 0.697 ABTime and Husk 0.438 AC 0.19 BC 0.99 ABC 0.78
A Time of harvest <0.001 B Husk cover 0.29 C(position in crib) 0.37 ABTime and Husk 0.20 AC 0.10 BC 0.98 ABC 0.74
161.
The time of harvest seems to have had little persistent effect
on Sitophilus infestation : despite the heavy initial colonisation
of the early-harvested material, infestation was effectively uniform
by the time of the first full storage sample. At the first storage sample
Carpophilus spp. adults were more abundant on both types of early-
harvested material while Palorus subdepressus was concentrated strongly
on the early-harvested, snapped cobs. The data on emergences suggest
that reproduction of both Cathartus quadricollis and Gnatocerus maxillosus
was most successful on this material; it should be noted however that
recruitment of these secondary pest species will not be accurately
estimated by emergences from samples of shelled grain.
The overall losses suffered in the various treatments could not
be assessed because the baseline samples proved unsatisfactory, probably
due to some loss of dry weight involved in the heat-drying of the early,
high-moisture content samples. The data for the two storage samples,
provide a comparative indication of performance, but there is a poss-
ibility that the differences indicated are the result of differential
selection at loading, rather than more severe losses in storage. Mean
grain weight was lower in the early-harvested maize and for the cobs
stored in their husks (Table 5.11a)). The inclusion of more Diplodia -
damaged cobs in the selection of snapped cobs almost certainly con-
tributed to their lower mean grain weight but there is no evidence for
the spread of Diplodia infection in store (Table 5.11b)). . The im-
portance of insect infestation cannot be assessed on the basis of this
data.
On the evidence of this trial and of the data quoted in Chapter 3,
the storage of maize in the husk does not appear to reduce the level of
storage pest infestation although the latter figures showed that serious
162.
infestation is confined to a smaller number of cobs. The increase of
primary pest species does appear to be delayed by leaving the maize
in the field; however, this may not provide a net advantage if losses
from other sources, such as lodging or bird and rodent damage, are
severe in the field. Such factors may be expected to vary considerably
from one locality to another. The dynamics of pest population increase
in store will be discussed further in the next chapter but it seems that
under the conditions of the study site, maximum equilibrium levels were
reached so rapidly that little benefit could be expected over the period
of the storage season from minor changes in initial conditions.
5^4 Persistent Effects of Pre-Harvest Damage ., . , .
The experiment described in this section, the Pre-Harvest Damage
Trial, was set up to investigate whether field infestation might be
affecting storage pest populations indirectly by providing a more
favourable substrate for their development at the beginning of the
storage period. It was noted in Section 5.2 that reproduction of
Silvanidae and Nitidulidae in the field appeared to be confined to grains
already damaged by Lepidoptera and/or associated moulds; it seemed
possible that this damage might continue to support secondary pest
species in store until damage by primary storage pests provided an
alternative substrate. It has also been shown that some grain damage
may reduce mortality of first instar Sitotroga cerealella larvae by
aiding their penetration into the grain (Ayertey, 1979). Sitophilus
zeamais may oviposit preferentially on damaged grain.
The grain for the Pre-Harvest Damage Trial was harvested in late
December and all cobs were dehusked. Cobs were then selected which
showed ho visible insect damage and an equal number which had suffered
limited damage by Lepidoptera" and fungi. Heavily damaged cobs were not in-
cluded, those selected in the 'damaged' group having no more than 15%
163.
damaged grains. The cobs selected were not intended to be in any
sense representative of the total population : the intention was to
compare the effect of the presence of small foci of damaged grain on
the infestation of otherwise sound cobs. The progress of infestation
was assessed on the basis of samples taken before storage and after
approximately one, two, three and four months in store. On each
sampling occasion adult insect populations, recruitment, grain moisture
content and weight loss were estimated.
The cobs selected were divided into five groups of eighteen cobs
of each type (i.e. damaged and undamaged). Two cobs were taken from
each group to provide an estimate of the initial 'latent1 infestation.
All adult insects were removed from these cobs and a file of grain
removed along the length of each as a subsample for moisture content
determination. On the 'damaged' cobs Lepidoptera attack and mould
infection were mainly confined to the apices. To investigate whether .
Coleoptera infestation was similarly limited the cobs were cut in half
and the apical and basal portions caged separately. Emergences were
scored after 10, 20 and 30 days. Of the remaining (16) cobs in each
group, sections were cut from the bases of four and the core and grain
moisture content determined separately from each. All cobs were then
numbered individually with indellible ink and weighed. Each group of 16 cobs
was packed tightly into a coarse netting bag (commercial onion bags)
and the bags stacked in a suitably modified crib, in the arrangement
shown in Figure 5.2, to form a single bulk.
On each sampling occasion three cobs were removed from each bag.
Two were sealed in a plastic bag and set aside whilst the third was
weighed, a section cut off for moisture content determination, and the
remainder replaced in the crib. From the former two cobs all adult
a) vertical section b) plan views
wire partitions
unsamplsd cobs (hatohed)
loose
bagged
upper layer
sampled oobs (plain)
1 - 5 ^positions'
C / D field damage
C " clean • h
D m damaged
FIGURE 5.2 Rrrangement of material for Pre-Harvest Damage Trial
a) vertical ssotlon through whole or1b
b) plan view of sampled layers
middle layer
lower layer
165.
insects were collected for later identification and scoring; the
cobs were then shelled individually, the grain and core weighed, and
subsamples taken from each for moisture content determination and
estimation of emergences (from lOOg over 8 days). The difference
between the weight of each cob when removed and its weight initially,
when suitably corrected for moisture content changes, provided an
estimate of the progress of weight loss.
At the fourth (i.e. final) sampling occasion the four cobs from
which sections had initially been cut for moisture content determination
were also weighed, shelled, and their moisture content again determined.
The overall dry weight loss could more accurately be estimated from
the weight change of these cobs because their initial and final moisture
contents were individually known.
Collated data on adult insect numbers, recruitment, weight
loss and moisture content are presented in Tables 5.12 - 5.15. The
composition of the insect population was different on each of the four
sampling occasions and so the data for each have been analysed separately.
Differences between mean numbers of adult insects on damaged and
undamaged cobs were tested using t-tests, but undue reliance should not
be put on the results: the samples were collected systematically rather
than randomly and the markedly uneven distribution of insects in the
crib may have affected the sample variance. Recruitment samples
were replicated and so these could be analysed factorially to separate
the effects of grain damage and position within the crib. Recruitment
data were transformed (Log^^(X + 1 ) ) before analysis. The data for
progressive weight loss and moisture content changes have been analysed
over all four samples, using sampling occasions, position in the crib
and grain damage as factors in the analysis of variance.
Emergences from the samples collected at the beginning of the
166.
trial showed that initial infestation by both Sitophilus zeamais and
Cathartus quadricollis was much heavier on the damaged cobs (Table 5.12).
The numbers of Sitophilus emergences from basal and epical halves of
the cobs were approximately equal but Cathartus emergences were higher from
the apices. Emergences of Zeteticontus laevigatus, a parasitoid of
Cathartus, closely followed the distribution of its host.
Adult counts for all the common pest species indicate that the
damaged cobs were more heavily infested throughout the four months of
storage (Table 5.13). Mean numbers of Sitophilus zeamais, Carpophilus
spp. and Cathartus quadricollis were higher on the field-damaged cobs
on all four sampling occasions (although, in most cases not significantly
so, on the basis of the individual t-tests). During the fourth month
in store, at the onset of the wet season, several secondary pest species
(Cryptolestes pusillus, Typhaea stercorea, Palorus subdepressus and
Gnatocerus maxillosus) appeared in appreciable numbers; these species
too, together with their Authocorid predators, preferentially infested
the field-damaged cobs.
Emergences of the primary pest species and their parasitoids were,
however, not significantly different in samples•from damaged and undamaged
grain (Table 5.14). It is possible that differences were simply not
detected, due to insufficient sample size, but it is also possible that
less infested cobs (i.e. those that had not been damaged initially) were
preferred for oviposition or that mortality of immatures was lower on
these cobs. Recruitment of the secondary pest species again could not
be assessed from these samples because of the loss of immatures during
shelling and sieving.
Summary results of the analysis of moisture content and weight
167.
TABLE 5.12 Effects of field damage by Lepidoptera on initial
infestation of maize by storage pests (Pre-Harvest Damage Trial)
a) Probability levels from 3 factor analysis of variance:
(Factor A : successive periods over which emergences were scored.
Factor B : presence or absence of grain damage.
Factor C : portion of cob (i.e. apical or basal half). )
Data were numbers of insects emerging over successive 10 day periods.
corrected for sample size and transformed L o g ^ .
Source of Species : Sitophilus Cathartus Zeteticoutus Variation df zeamais quadricollis laevigatus
A Time 2 <0.001 0.14 <0.001
B Grain damage 1 <0.001 <0.001 <0.001
C Tips/bases 1 0.27 <0.001 <0.001
AB 2 t r~>
0.40 0.51 0.42
AC 2 .0.95 0.27 <0.001
BC 1 0.90 <0.001 <0.001
ABC 2 0.51 0.48 0.33
Sampling error 108
b) Mean number of insects emerging/50g shelled grain over 10 days.
Undamaged damaged
tips bases tips bases
Sitophilus 0.7 0.4 5.9 4.2 zeamais
Cathartus 0.5 0.4 6.9 1.5 quadricollis
Zeteticoutus 1.0 0.3 6.4 0.6 laevigatus
...
TABLE 5.13 Effects of field damage on subsequent infestation in store. Mean number of adult insects per 250g of shelled grain @ 15% after one, two, three and four months in store. Figures in parentheses are standard deviations, figures below each line are estimated values of the t statistic.
Sampling Occasion
Species:
Sampling 1
G nd rt 0) 0) t>0 nH eg CJ p
ca Q
Sample 2
a is rt a) <u t>o rH CO U |
Q
Sa;nple
§ <u i-t CJ
OJ
Damaged
Sample 4
§
0) rH o
Damaged
Sitophilus 66(31) 115(58) 88(29) 105(36) 116(48) 127(37) 176(42) 191(38)
zeamais 1.7 0.9 0.4 0.6
Sitotroga 3.8(3.6) 2.2(1.9)
cerealella 0.9
Cathartus 12.8(11.4) 106(60.6) 8.4(6.4) 30.6(17 .2) 5.2(1.9) 22.4(18.2) 3.8(3.9) 11.2(3.2)
quadricollis 3.4 2.8 2.1 3.2
Carpophilus 7.2(4.3) 17.6(10.9) 27.4(11.4) 62.0(43 .3) 18.6(8.8) 21.2(6.1) 174.4(32.3) 210.6(85.3) spp. 2.0 1.7 0.5 0.9
Cryptolestes 14.8(9.1) 20.0(7.6) pusillus 1.0
TENEBRIONIDAE 7.0(8.3) 1.2
14.2(10.3)
Typhaea 5.8(4.7) 9.0(3.5) stercorea 1.2
ANTHOCORIDAE 0.6(0.5) 5.8(3.6) 3.2
Critical values of the t distribution (8 degrees of freedom) 5% : 2.3 . 1% : 3.4.
i
ON 00
TABLE 5.14 Effects of field damage on subsequent infestation in Store. Mean numbers of primary pests and their parasitoids emerging from lOOg of shelled grain in eight days after one, two, three and four months in store. Probability levels are for 2 factor analysis of. variance of the data transformed L o g ^ C X + 1) .
(For the Anovar the numbers of parasitoids emerging have been added to the numbers of the appropriate host to provide a better estimate of the underlying pest distribution)
Sampling Occasion: Sound or damaged: Clean
1 damaged Clean
2 damaged'
3 Clean damaged
4 Clean • damaged
Species:
Sitophilus 23.2 17.9 9.9 11.5 3.4 3.4 29.6 31.3
Sitotroga 1.6 1.8 3.0 3.4 9.4 7.4 22.0 17.4
Cathartus 0.3 0.2 0.1 0.2 0 0 3.7 2.7
An. cal. 0.6 0.1 0.1 0.1 0.2 0.1 1.7 1.0
Hab, cer. 0.3 0.1 1.5 2.6 , 2.4 2.6 0.5 0.4
Anovar. df Sito. zeam.
Sitot. cer.
Sito. zeam.
Sitot. cer.
' Sito. - zeam.
Sitot. cer.
Sito. zeam.
Sitot. cer.
grain damage 1 0.05 0.28 0.39 0.93 0.46 0.90 0.23
Position in crib 4 0.04 0.34 • 0.65 0.67 0.70 0.33 0.81
Interaction 4 0.32 0.70 0.63 0.52 0.20 0.16 0.37
Sampling error 10
i
ON VO
1 7 0 .
TABLE 5.15 Progressive changes in mean moisture content and mean dry weight loss over the storage period with probability levels from a three factor Anovar.
Sampling occasions : (months in store) a) 1 2 3 4
Moisture undamaged 11.9 11.5 13.5 14.9 content
% damaged 11.9 11.4 13.5 14.8
11.9 11.4 13.5 14.9
Weight undamaged - 9.7 14.7 22.1 loss
% damaged - 10.7 14.9 26.2
- 10.2 14.8 24.1
b) Source bf variation
moisture df
content P
weight df
loss p
A sampling,occasion 3 <0. 001 - 2 <0. 001
B grain damage 1 0. 11 1 0. 05
C position in crib 4 0. 13 . 4 0. 41
AB 3 0. 94 2 0. 17
AC 12 0 .01 8 0 01
BC 4 0. .90 4 0 .10
ABC 12 0 .55 8 0 .12
Sampling error 80 30
TABLE 5.16 Final weight loss (four months in store) for maize damaged in the field by Lepidoptera (Pre-Harvest Damage Trial).
Overall weight loss (final) %.
a) Position in crib 1 2 3 4 5 X
undamaged 18.5 21.9 18.4 20.4 18.2 19.5
damaged 20.0 17.5 27.6 23.2 16.7 21.0
X 19.2 19.7 23.0 21.8 17.5 I
b) Source of variation
df P
0.13 0.01 0.002
A grain damage 1 B position in crib 4 AB 4
Sampling error 30
171.
loss data are given in Table 5.15 and 5.16. The moisture content of
the grain increased markedly over the third and fourth months of
storage, approaching the tfet season. Although the moisture^content
changes were different for the different parts of the crib (p = 0.01 for
the 'sampling occasion X position' interaction) there was no difference
between 'damaged' and 'undamaged'. The increase in weight loss over
the season was also different for the various positions in the crib.
There was some indication of higher weight loss in initially-damaged
cobs in some parts of the crib but the overall difference in weight
loss would not have been economically significant.
The results of this experiment confirm the impression that the
initial field damage to cobs is important for a considerable period
in maintaining high populations of secondary pest species but has little
effect on the primary pests (and so, apparently, on weight loss). Initial
recruitment of pests was higher from field-damaged cobs. However, insects
from this source may, as previously noted, contribute relatively little
to the population colonising the crib.
5.5 Sources of Storage Infestation. i
The studies described here confirmed in general terms the
observations of workers in other countries regarding the importance
of insect infestation in the field, particularly by Lepidoptera, in
causing direct damage to the grain, promoting mould damage and .allowing
the early establishment of storage insects. Under the conditions of
the study the transfer of the field infestation with the harvested, grain
appeared to be less important than active migration of insects in
colonising the newly-stored maize. This was also confirmed by the
succession studies, to be described in the next chapter, in which grain
fumigated before harvest to eliminate all field infestation became,
172.
within a few weeks, as heavily infested as material in nearby cribs
that had not been treated. The separation of damaged cobs at loading
and the storage of cobs in their husks had little effect on the damage
ultimately suffered by the maize in store. In considering the im-
plications of these results for improved storage strategies it is
important to recognise the extent to which the experimental conditions
differed from those found on local farms and the practical constraints imp-
osed by particular farming systems.
The incidence of various stem-boring Lepidoptera was found to be
variable from one field to another on the study -site, was generally
higher on the field station than on farmers' fields-.-outside> ^nd changed n ,
markedly through the year (Kaufmann pers. comm). Moreover, the storage
of large quantities of maize on the field station over several seasons, •
in connection with insecticide trials, may have produced locally high
levels of storage pests. Against this it should be noted that levels
of field and storage infestation recorded in this study were comparable
to those found on local farms in some localities in Kenya (Giles
and Ashman, 1971; de Lima, 1978) and Nigeria (Cornes and Riley, 1962;
C o m e s , 1963, 1964). Poor store sanitation and similarly high levels
of insect infestation were also noted in the current study on commercial
farm settlements in the Ibadan area.
The initial infestation of stored maize could be reduced by
measures that directly reduce the field infestation, that lower the
numbers of insects transferred with the harvest from field to store or
that break the cycle of infestation from already infested stores to
the new crop.
Trials of insecticides to control Lepidoptera infesting cobs in
the field have generally proved this method to be too expensive or
173.
ineffective (Conies, Donnelly and Adeyemi, 1966). There is clearly
some scope for resistance breeding to increase the inherent resistance
of the grain and to improve the protection provided by the husk (Kirk
and Manwiller, 1964; Starks and McMillian, 1967). The breeding of
varieties with two smaller, better-covered ears has been proposed by
Giles and Ashman (1971) as a means of maintaining overall yield. The
general problem remains, however, that selection for longer husks will
tend also to select for preferential development of other vegetative
parts of the plant at the expense of the reproductive, and thus to reduce
'plant efficiency 1 and yield potential (Quin, pers. comm.).
The benefit from reduction of the initial infestation by fumigation
or relatively short-lived insecticides (Rawnsley, 1968) or by the sep-
aration and differential treatment of damaged cobs (de Lima, 1978) will
be lost if active reinfestation of stores is significant. Clearly,
the storage pests have a considerable capacity for movement from
sources of infestation. It has been proposed that cross-infestation
may be reduced by removing stores at least 800m from the nearest source
of infection (Giles, 1969), but the handling of the crop involved would
make this impractical for many local farmers. Measures directed at this
point in the cycle of infestation can only be successful if combined
with greatly improved sanitation in both field and store to reduce
sources of reinfestation.
Control measures directed against insects in infested stores
would have to be taken long before harvest of the new crop if cross-
infestation is to be prevented. Giles and Ashman (1971) have drawn
attention to this problem, noting that Sitophilus zeamais is able to
survive for long periods on newly pollinated cobs (until they are dry enough
to allow successful oviposition), on crop residues buried in the field
174.
and on dumps of maize cores left after shelling (Mossop, 1940). There
are particular problems in preventing cross-infestation from old
stocks of maize to new where the grain provides the main staple food.
These possibilities and their associated problems will be con-
sidered again later in the context of a wider discussion of control
strategies.
175.
CHAPTER 6
THE INSECT COMMUNITY : COMPOSITION AND SUCCESSIONAL CHANGES
6.1 Introduction
Published records from a number of African countries (Cornes,
1965, 1967, 1968; Giles, 1965; Forsyth, 1966; Walker and Boxall, 1974;
Haines, 1974; Walker 1979) indicate that a considerable variety of
insects may at times be found in rural maize stores. There is, however,
little quantitative data on the incidence or pest status of individual
species in such stores.
De Lima (1978), in an extensive study in Kenya, has considered
the dynamics of Sitophilus zeamais and Sitotroga-cerealella under a
variety of environmental conditions and insecticide treatments. He
- also considered the succession of species-that occurred on small
quantities of grain in experimental cribs and in the laboratory over a
three and a half year period. In the latter studies Sitotroga and
Sitophilus both increased rapidly during the first months of storage.
Sitotroga then declined steeply to extinction while Sitophilus remained
the dominant pest for more than two years. Secondary pest species,
Tribolinum castaneum, Cryptolestes ferrugineus and Oryzaephilus sur-
inamensis, Carpophilus dimidiatus, Rhizopertha dominica and Gnatocerus
cornutus, appeared and became abundant in successive seasons as the
grain became increasingly damaged.
Corne s and Riley (1961), studying maize cribs in Southern Nigeria
which had been initially treated with Malathion, observed a marked
succession of species over a much shorter storage period. Sitophilus
'oryzae' (=S. zeamais)^ and Cathartus quadricollis were brought in to
1. Sitophilus oryzae and S. zeamais were for a long period considered to be 'strains' of a single species. Richards (1944) showed that the two strains were physiologically distinct and that progeny of crosses between them were sterile. The nomenclature of the two species in. the literature remained confused until settled by Kuschel (1961).
176.
the cribs with the maize from the field; Cillaeus sp., Carpophilus
dimidiatus and Khyzopertha dominiea appeared during the first month
in store, Gnatocerus maxillosus and Tribolium castaneum during the
second, third and fourth months, and Cryptolestes spp. and Araecerus
fasciculatus during the fourth and fifth. The pattern of population
changes was slightly different at the three localities studied, although
in all cases the dominant pest, Sitophilus, increased to a peak after
three to four months in store, declining slightly thereafter. At
Ilaro the secondary pest species increased steadily over the storage
period. However, at Ilaro Cathartus and Carpophilus reached a maximum
in the second month of storage and then declined, while Gnatocerus
reached a peak in the fifth, and last month in store.' -
Data from large-scale shelled maize stores in Nyasaland (Salmond,
1957) showed some changes in"pest incidence over the storage season
as did samples of shelled maize from local markets in Nigeria (Caswell,
unpublished data). Successional changes in insect populations have
been observed in bulk stores of other commodities both in temperate .
conditions (wheat, studied by Coombs and Woodroffe, 1963, 1968, 1973;
Sinha, 1974) and in the tropics (groundnuts, considered by Smith, 1963;
Prevett, 1964).
Laboratory studies on the environmental and nutritional re-
quirements of storage insects and their reproductive potential can
provide strong evidence of the likely ecology and pest status of
particular species in real stores (Howe, 1963). It is difficult in
such studies, however, to take account of the cyclically changing con-
ditions (Howe, 1956a) interactions with other members of a potentially
large pest complex, and dispersal behaviour to and from alternative
environments which may be important in particular situations. Field
workers, involved for instance in insecticide testing, have perhaps
been too ready to accept 'conventional wisdom' and have felt it
unnecessary to collect quantitative data on the incidence of particular
species. In the absence of a clear understanding of the 'natural*
factors limiting pest population growth the results of insect control
studies may easily be misinterpreted.
In the present study an effort was accordingly made to consider
as many members of the pest complex as possible, including those which
do not initially seem to be economically significant. To simplify
discussion, only the incidence of the more abundant species will be
described in this chapter. However, a complete list of the species
identified, with notes on the taxonomy and previous records of species
of particular interest, is given in Appendix I and collated data on
the incidence of all species in Appendix III.
6.2 Treatments and Sampling Techniques
Cribs for the Succession Studies were set up as indicated in
Table 6.1. Maize of the white dent variety TZPB, grown at the IITA
study site, was used for all cribs and that for the Wet Season trials
came from a single field. The cobs were harvested at approximately
30% moisture content and all were dehusked. Fumigation (where in-
dicated) was carried out in hermetically sealed drums using phosphine
("Phostoxin" tablets) for four and a half days. During fumigation
the drums were kept in the shade to avoid excessive heating but slight
fermentation nevertheless occurred.
The cribs of fumigated maize at Ibadan were set about 100 metres
away from the untreated ones, to reduce direct cross-infestation, and
178.
the cribs for the dry-season harvest were subsequently sited c.15 metres
from the untreated TWet Season* ones. The storage site was isolated
from other grain stores but near to a store of yams and surrounded by
fields of maize, cassava and cowpea. The cribs at Ilora Farm Settle-
ment, near Oyo, were sited near to farmers' cribs that had been used
for maize storage in previous seasons and which still contained some
infested residues. There were maize fields on one side of the Ilora
study site and a residential area, including maize stores, nearby.
The cribs used were the standard half-tonne units used in pre-
viously-described trials and were modified as shown in Figure 6.1a .
The cobs to be used for insect sampling were packed inside a central
tunnel of wire netting supported on wooden 'laths'. The sample cobs
could be removed through a wire 'door' iii one of the vertical faces,
- without disturbing the cobs in the remainder of the crib.
Separate sets of cobs were designated for sampling of adult insects,
of emergences and moisture content, and for loss-assessment. Those for
adult insect sampling were placed in a flexible nylon netting 'trap bag'
(2.5cm mesh), approximately in the centre of the crib, and the other
cobs packed around them as shown in Figure 6.1b). The same cobs were «
used throughout the trial for the adult counts, although the quantity
of cobs in the sample in the wet season cribs had to be reduced from
c3kg initially to c.lkg by the middle of the storage season (in order
to keep the handling time for samples within reasonable bounds as the
insect numbers increased). For the dry season cribs the sample was
divided between three trap bags (per crib) which were collected and
scored separately in an attempt to obtain an estimation of the population
variances. In practice the samples proved too small, given the con-
siderable inter-cob variation in infestation, to provide a useful
FIGURE 6.1
Modifications to cribs used for the Succession Studies:
a) general view showing structural modifications - 'cut-away*
section shows construction of sampling tunnel;
b) arrangement of cobs for sampling (note that cobs within
the sampling tunnel were removed at frequent intervals
but that loss assessment cobs in the 'unsampled* sections
were only collected at the end of the experiment).
179.
FIGURE 6.1
a)
uf re meah
(South f a c e )
sampling tunnel
wooden 1aths
wi re mesh
b )
E aooees to
samples
unsampled
oobs for damage assessment
m m cobs for adult insect s amp lee
oobs for recruitment & m . o . samplee
estimate and so the data were pooled.
Three cobs were collected from each cob on each sampling occasion
to provide individual subsamples for estimation of grain and core
moisture content and recruitment. Moisture contents were determined
using the routine oven method (Appendix IV) . Recruitment was assessed
on the basis of the number of insects emerging from lOOg of shelled
grain over one week under ambient conditions. Samples were kept
during this week in gauze-topped containers in the shade of an open-
walled shelter on the study site. Cobs removed from the cribs for these
samples were replaced with cobs from below the top surface of the un-
sampled remainder of the same crib. Introduced cobs were marked
with indellible ink and were not subsequently used as samples them-
selves until they had been in the centre of the crib for at least
•three months.
Weight loss (on a dry weight basis) was estimated, as in the Long-
Term Distribution Studies, by repeated weighing of individually identified
cobs (Appendix V), with an appropriate arithmetic correction for the
changed moisture content on each occasion. Ten such cobs were included
in the sampling tunnel of each crib. In all trials loss-assessment
cobs, weighed only at the beginning and end of the experiment, were
also included at loading above and below the sampling tunnel, 15 cobs
in each position. These cobs, as described in Section 3.5, were in-
tended to show, by comparison with those in the sampling tunnel, whether
the repeated sampling had reduced insect populations as evidence by
the weight loss. For the dry season trial the initial moisture content
of each loss-assessment cob was estimated from a section cut from the
base of the cob and in all trials the final core and grain moisture
content were determined individually. Moisture content corrections
for the intermediate sampling occasions and for the 'baseline' of the
TABLE 6 . 1
Cribs
A & B
C & D
E & F
' treatment'
untreated
fumigated initially
fumigated initially
locality Time of harvest/loading
IITA, Ibadan August 1978
IITA, Ibadan August 1978
Ilora Farm Settlement
August 1978
L & M untreated IITA, Ibadan January 1979
TABLE 6.1 Cribs on which Succession Studies were based. Letters
designate individual cribs and are used to identify
data in Appendix III. Cribs A - F are indicated as
'Wet Season' in the following figures, cribs L & M as
'Dry Season'.
182.
wet season trials depended on the routine determinations made from
the 'recruitment sample 1 cobs.
6.3 The Abundance of Major Insect Groups and Changes in the Physical
Environment
The data presented in Figure 6.2 show the extent to which Coleoptera
were numerically dominant in-the insect community in the study cribs.
Lepidoptera (almost entirely Sitotroga cerealella) only became well
established in the untreated cribs and, even there, were only abundant
.for part of the storage season. Adult parasitoids were observed in
the cribs from the beginning of the storage season and increased in
abundance fairly steadily throughout. Heteroptera, including both- :M_ ~
predatory Anthocoridae and 'phytophagous 1 Lygaeidae, became abundant
in the early part of the storage season, declined sharply and then
increased again in the final samples.
When the Coleoptera population is broken down by families
(Figure 6.3) it is apparent that, although Curculionidae (i.e. Sitophilus
zeamais) formed the greater part of the insect population over most
of the storage season, other families became abundant at times, their
incidence following markedly different patterns. Nitidulidae and
Silvanidae were present in large numbers in the early samples, declined
to a different extent in the various cribs, and then increased again
at the end of the trials (except at Ilora where the observations were
terminated earlier). Tenebrionidae and Cucujidae showed a contrasting
pattern, being rare ot absent at the time of harvest but increasing
steadily over the storage season.
Although there are marked differences in the ecological require-
ments of some of the component species (as discussed later) the per-
formance of the Coleoptera families may be broadly related to changes
183
10000 „
<9 1000
\ 9 C. 0 1 100 c +» o o tt c
UNTRERTED CRIBS - Ibadan TOTRL INSECTS
CoTeoptora
Paras tto Ids
Heteroptera
10 I
10000 _
w 1000 JC \
C. O | 100 c •»» o o 0 c
FUMIGRTED CRIBS - Ibadan TOTRL INSECTS
Colooptora
Parasttoids
. i "
10 i
K
i • *
^ Hotoroptera
Lepfdoptera 1 1 i
10000
a* 1000
\ v c ©
I 100
+> o ©
5 10
FUMIGRTED CRIBS - Ilora
1 1
! — i TOTRL INSECTS Coleoptera
t... .f Parasftofds
Hotoroptora J L
Rug Sep Oct Nov Dec Jan Feb Mar Rpr May Jun Jul
184.
10000 UNTRERTED CRIBS - Ibadan
Dry Season Harvested TOTAL INSECTS 1000
Coleoptera
100 Parasitoids • I • I
10 f A
Lepidoptera
I Heteroptera
1 J ^ I Rug Sep Oct Nov Dec Jen Feb Mar Rpr May Jun Jul
FIGURE.6.2 Incidence of major insect groups through the storage season.
Data are mean numbers (for 2 cribs) of insects per kg. of
shelled maize at 13% moisture content (arithmetically
corrected from observed weight and moisture content),
transformed log . e
Bars indicate actual values for two replicate cribs (i.e. range).
Arrow on x -axis indicates time of loading. (Note:
insect populations start from zero in fumigated cribs;
initial populations were not critically determined for
untreated cribs).
185.
Rug Sep Oct Nov Dec Jan Feb Mar Rpr May Jun Jul
186.
FIGURE 6.3 Seasonal incidence of major Coleoptera families
Presentation of data as for Figure 6.2.
FIGURE 6.4 Changes in grain moisture content through the
storage season.
Data are means of six determinations (3 from each crib)
with standard errors.
25
20
15
10
25
20
15
10
25
20
15
10
25
20
15
10
187.
UNTREATED CRIBS - Ibadan Net Season
^ ^ I--'
UNTRERTED CRIBS - Ibadan Dry Season
FUMIGATED CRIBS - Ilora % \ \ \
V p .
Aug Sep Oot Nov Deo Jan Feb Mar Apr May Jun Jul
188
* \
* \
* s
25
20
15
10
UKTOERTEH CKlflB - Xbsdan
A - r "
.r
0
25
20
15
10
i i i i i i « i i
FUHIGRTED CRIBS - Ib«d««l Hit S m o n
A A I
0
25
20
15
-{
FUMIGflTEB CRIBS - Ilora Hit Siuon
10
5
0 . i - i - r .i"
Bug Sip Oct Nov Deo Jin Fib Mir Rpr May Juri Jut
FIGURE 6.5 Increase in grain damage over the storage period.
Weight loss of cobs (dry weight basis). Data are mean
values for 20 cob& (10 from each replicate crib); bars in-
dicate mean for each crib separately.
in the physical environment. The changes in numbers of Nitidulidae
and Silvanidae followed approximately the fall and rise in grain
moisture content (Figure 6.4), dying out completely under the slightly
drier conditions at Ilora. Cucujidae and Tenebrionidae, in contrast,
increased throughout the dry period and on into the following wet
season. The figures for weight loss (Figure 6.5) describe only one
aspect of the changing condition of the substrate (as discussed in
Section 2.6). However, from the known ecology of the species of
Cucujidae and Tenebrionidae involved, which are characteristically pests
of damaged or milled grain products, it seems likely that their
increase may be related to increasing grain damage.
6.4 Incidence and role of individual insect species.
6.4.1 Primary Pest Species
The data for Sitophilus adults are given in Figure 6.6. and for
emergences in Figure 6.7. Adult populations in all cribs of Wet
season maize increased rapidly for the first three to four months but
much more slowly thereafter. The populations in dry season cribs
reached a maximum within one month of loading and no further increase
occurred until the onset of the new wet season. It is immediately
apparent that initial fumigation failed to suppress adult population
build-up. There is, indeed, some evidence that the initial rate of
increase was faster in the fumigated cribs and that the level of the
first 'plateau' was higher.
No emergences were recorded in the samples from the fumigated
material during the first month in store, indicating that the fumigation
had successfully eliminated all developing immatures. Emergences in
the unfumigated cribs reached a peak after 3 - 4 months, while the
fumigated cribs showed peaks after 2-3 months and 4-5 months in store.
190.
10888 „
« 1888 J£ \
C o JO £ 3 C
188
4» o o e
5 10
1 18888
ts 1888 JC \ • t e I 188
•»» o o • c
UNTREATED CRIBS - Ibadan Both seasons Hot Season harvested
Dry Season harvested
• *
18 L
FUMIGATED CRIBS - Ibadan fc Ilora Net Season . ' Ibadan
A
i Ilora
i / "7T7T Rug Sop Oct Nov Dec Jan Feb Mar Rpr May Jun Jul
FIGURE 6.6 Seasonal incidence of Sitophilus zeamais (adults).
Data presentation as for Figure 6.2.
FIGURE 6.7 Seasonal incidence of Sitophilus zeamais and
Sitotroga cerealella (emergences)
Data are means of emergences during one week from
6 samples of lOOg each (3 from each crib), with
standard errors.
30
20
10
0
30
20
10
0
30
20
10
0
30
20
10
0
191.
UNTREATED
Ha* Saaaon
CRIBS - Ibadan
A •*',4 -fc- + A SttophtTua
/
X X ^ ^ . »* ' * 1 .... I Bttotroga i i
UNTREHTED CRIBS - Ibadan Dry Season
H H/D
1-
1 \ St tophi 1us
Sltotroga
I I I I I I Ii H I j-— i » i
FUMIGRTED CRIBS - Ibadan
/ Y 7
/
I r-fi,
\ i Sitophilus
I Sltotroga • , • • • > • ; ' J » I L
FUMIGRTED CRIBS - Ilora
/ i "f ^ ^ Sitophilue
/ » I I 1 I I » I I I I
Rug Sep Oot Nov Deo Jan # Feb Mar Rpr May Jun Jul
192.
Without more detailed information these patterns cannot be inter-
preted with confidence but it seems likely that the fumigation has
produced semi-synchronous 'generations' while those in the unfum-
igated cribs were more continuously overlapping. If this is the
case, it is unclear whether the increase in the final samples represent
a response to more favourable conditions at the beginning of the wet
season or simply the emergence of a third population peak. It should
be noted that adult populations remained high throughout the storage
period and that egg production in Sitophilus zeamais is spread over .
an extended period, though reaching a maximum during the second or third
week of life (Dobie, 1974). Accordingly the 'peaks' in emergences must
represent variation in oviposition orsurvivalof^ i-Temafeures..^atAei^.ihan
the progeny of discrete generations.
Oviposition by Sitophilus zeamais is known to be favoured by
high grain moisture content and fall off sharply at a relative humidity
between 60 and 70% on wheat, (Howe, 1952b),equivalent to 12-14% moisture
content for maize. Such low moisture contents were reached in January
and February in the study cribs. These adverse conditions cannot,
however, have been responsible for the decline in emergences as this
occurred earlier when conditions for oviposition and development were
within the favourable range. The second 'peak' of emergences occurred
in the fumigated material at a time when emergences from the unfumigated
maize had already begun to decline, even though grain moisture content
in the two treatments were almost identical.
McFarlane (1978) has noted the adverse effects of periods of
high temperature (in excess of 30°C) on the reproductive success of
the closely related S. oryzae in some lowland localities in Kenya. Air
temperatures did exceed 30°C for a short time on most afternoons during
193.
the dry season in Ibadan (see Chapter 2). However, this can also be
rejected as the major cause of reduced recruitment, given that all
cribs at Ibadan were exposed to similar conditions yet showed 'peaks'
at different times.
Parasitoids which are known to attack Sitophilus spp. were
present throughout the storage period (see 6.A.5 below). Life table
analysis would be required to demonstrate conclusively that parasitism
was not playing a significant role in limiting recruitment, but the
very small number of parasitoids emerging in samples, especially
during the early part of the storage season, suggests that they were
not important.
The major factor limiting Sitophilus populations appears to be
its own response to crowding. Markedly reduced oviposition (and/or
reproductive success as assessed by the number of progeny produced
per female) have been noted in both Sitophilus zeamais and S. oryzae
at densities similar to, or below, those observed in the study cribs
at Ibadan (Birch, 1945; Khare and Agrawal, 1963; Dobie, 1974). The
pattern of adult population increase in the cribs,, corresponds quite
closely with that observed by Ayertey (1976) in crowded .laboratory /
cultures of-S. zeamais. Ayertey, in the same study, observed that
populations starting from a lower initial density reached a higher first
'peak' of abundance. There is some evidence, both from the present
work and from a recent study in Malawi (Golob, unpublished data), that
this effect also occurs in the field. In this study the fumigated
cribs (i.e. starting totally uninfested) showed slightly higher adult
insect numbers than the untreated ones after three months in store,
while in Malawi Sitophilus populations in insecticide treated cribs
(nkokwes) overtook those in untreated ones when the insecticide had .
194.
broken down and the insects began to increase with the onset of
the wet season.
Oviposition by S. zeamais on previously uninfested grain has
been shown by Dobie (1974) to be clumped rather than random. Without
further information on the operation of this effect at various densities
its importance cannot be evaluated. However, it could clearly affect
the severity of density dependent mortality due to larval competition.
In conclusion, mechanism appear to exist which could explain the
observed changes in Sitophilus populations in terms of intra-specific
interactions; changes in the environment may modify these effects but are
probably not the primary cause of population fluctuations.
Numbers of adult Sitotroga cerealella in the cribs are indicated
by the curves for TLepidoptera' in Figure 6.2 and data for emergences
are included in Figure 6.7. Sitotroga became abundant only in the
untreated cribs, though appreciable numbers also appeared in the
fumigated cribs at Ibadan at the beginning of the wet season. Sitotroga
was recorded in samples from the cribs at Ilora but never became es-
tablished there.
Other Lepidoptera were recorded from the cribs but never in
large numbers. Mussidia nigrivenella and Pyroderces sp. occurred at
the beginning of the storage season, but did not reproduce successfully
in store. Live larvae of Mussidia were found on maize that had been
initially fumigated, indicating that considerable oviposition had
occurred in the cribs, but most larvae appeared to become desiccated
and died before completing development. Ephestia cautella (adults and
larvae) and, occasionally, Plodia interpunctella (adults only) were
found in the cribs later in the storage season. Both species were
195.
easily maintained in single-species cultures on damaged grain under
ambient conditions; their failure to become well-established in the
cribs is thus probably due to competition or the activity of natural
enemies.
Emergences give a better indication than adult counts of the
status of Sitotroga because the adult flies readily (and so tends to be
under-recorded in cob samples) and because it lives only a few days.
Although less abundant than Sitophilus in the cribs used for the Succession
Studies, Sitotroga is clearly a potentially serious pest under similar
conditions: in two cribs set up for a preliminary study and filled
with traditional yellow maize varieties, Sitotroga became for a time
the most abundant species in recruitment samples with up to 20 adults
emerging per 50g of shelled grain in one week (mean of 10 samples from
one sampling occasion).
The population dynamics of Sitotroga are rather different from
those of Sitophilus. Although the total number of eggs that may be
laid is comparable, those of Sitotroga are laid over a much shorter
period: 50% on the day of emergence and 90% within the first three
days under laboratory conditions (Ayertey, 1976). Moreover, ovi-
position is unaffected by the moth's own population density or that of
Sitophilus.
The factors affecting competition between Sitophilus zeamais and
Sitotroga cerealella have been considered in some detail by Chestnut
and Douglas (1971), Ayertey (1976, 1979, 1980) and de Lima (1978).
Without more detailed information it would be pointless to speculate
on the possible role of competition in limiting Sitotroga populations
under the conditions of this study. It is of interest to note, however,
that the increase in Sitotroga numbers observed in the final samples
from the Ibadan cribs occurred under conditions which, from the studies
196.
quoted above, might be considered very unfavourable to it, with high
populations of Coleoptera and severely damaged grain.
In the case of Sitotroga, the importance of natural enemies
to limiting populations cannot be discounted: rates of parasitism
appear to have been higher than for Sitophilus (see 6.4.7, below)
and a mite, Blattisocius tarsalis, which is known as an efficient predator
on Lepidoptera eggs (Graham, 1970; Haines, unpublished data) was often
found, both in the cribs and phoretic on Sitotroga adults.
6.4.2 Secondary Pest Species ~ Coleoptera
The contrast has already been noted (6.3) between the incidence
of the Nitidulidae and Silvanidae, which were most abundant at the
beginning and end of the storage season, and that of the Tenebrionidae
and Cucujidae, which increased throughout. These general trends
however, conceal considerable differences in the ecology of individual
species.
The Nitidulidae recorded included small numbers of Brachypeplus spp.
(mainly B . pilosellus) and abundant Carpophilus spp. The taxonomy
of Carpophilus spp; associated with stored products has been well
established by Dobson (1954, and subsequent notes), but the detailed
examination required to separate the species of the 'dimidiatus group'
precluded their specific identification in later samples. Although
many .Carpophilus spp. were recorded (see Appendix' I), only two were
present in large numbers: C. fumatus, the commonest species in the field
was initially the more abundant in store but died out after approximately
three months (Figure 6.8), while C. dimidiatus appears better adapted
to storage conditions and was present throughout the seaspn at Ibadan.
C. fumatus reappeared in some cribs (for instance those used in the
- FIGURE 6.8 Seasonal incidence of G3rpophilu^,^fi, (N,itidulidae).
Presentation of data as in Figure 6.2.-
197.
1080
100
10
1 I
UNTREATED CRIBS - Ibadan
Hit Ssuon Total CarpophITus
.1
1000
100
10
C. funatus
• * ' * 1 •
C.dlsldlatus
i i i
UNTREATED CRIBS - Ibadan
Dry Ssason Total Carpophilus
.1 • V « t t
1000 „
1O0
FUMICRTED CRIBS - Ibadan
1000
100
FUMIGRTED CRIBS - Ilora
Tot a1 Carpoph11us
10 Total Carpoph11us
C.dlmldlatus
C.-fumatus
• * • •
Rug Sep Oot Nov Deo Jan Feb Mar Rpr May Jun Jul
FIGURE 6.9 Seasonal incidence of main species of Silvanidae
(Data presentation as in Figure 6.2).
198.
1000
100 L
10 I
1 I
UNTREHTED CRIBS - Ibtdmn
Hat Sauon
— , f ~
K . J -
Cathartua
.1
1000
100
10
K -
quadrtoolIt*
Monanua oonofnnulua
Oryzaspht tua naroator
UNTREATED CRIBS - Ibtdan Dry StMon
Cath.quad.
1 I
.1
1000
100
10
1 I
FUMIGATED CRIBS - Ibadan
' f' " \
• Cath.qutd.
Oryz.msro.
Monan.oon.
\
.1
1009
109
10
\
FUMIGATED CRIBS - Ilor*
A ,
i' i A .1 lL >r
\ \
N 1 • w -'r "-> | Ctth.qutd.
/ \ i Oryz.Mro. \ Honan.oon. * •
Aug Sop Oot Nov Dso Jan Fob Mar Apr May Jun Jul
199.
Distribution Studies) late in the succession. C. pilosellus, although
only occasionally recorded from the Succession Studies cribs, was re-
corded in appreciable numbers on damaged grain in a preliminary trial.
C. fumatus has been recorded from maize cobs in the field in
Nigeria (Cornes, 1964) and Kenya (Aitken, 1975) and from stored maize
in the West Indies (Dobson, 1959) . C . dimidiatus is known as a serious
pest of dried fruits and has been recorded from cocoa, groundnuts,
palm kernels (Dobson, 1954) and various cereals in Africa (Haines,
1974). Neither species is recognised as a significant pest of stored
maize, although the large numbers present in the cribs at Ibadan must
have contributed to the damage.
The two commonest species of Silvanidae, Monanus concinnulus
and Cathartus quadricollis, show patterns of incidence closely analogous
to that of the Carpophilus spp. : "M. concinnulus was common only under
the humid conditions at the beginning and end of the storage season
(and never became established on the dry-season harvested material),
while C. quadricollis, was present in-considerable numbers throughout
(Figure 6.9). The increase in numbers which occurred in the fumigated
cribs at Ibadan during December and January is surprising in a species
normally favoured by more moist conditions but may reflect invasion of
the cribs by insects driven from the fields by the dry season harvest.
Both C . quadricollis and M . concinnulus have a worldwide distribution on
a variety of commodities (Aitken, 1975) and C . quadricollis has been
described as a field pest of maize by Cornes (1964) and Schulten (1976).
Neither is usually considered to be a pest of stored maize but, as with
the case of Carpophilus dimidiatus, Cathartus must have made some con-
tribution to damage here.
A third Silvamid, Oryzaephilus mercator, which is known as a
common pest of oilseeds and their products from West Africa and elsewhere
(Aitken, 1975), increased to some extent in the middle of the storage
period (Figure 6.9) and was found in most cribs in small numbers late
in the succession. Conditions would appear, on the basis of its biqlogy
(Howe, 1956b),to be suitable for the development of 0 . mercator and
its failure to achieve pest status was probably due to competition with
other secondary pests. The same may be true of Ahasverus advena, a .
species often found on damaged grain where it feeds on both moulds
and the grain itself (Woodroffe, 1962; Hill, 1964). Although fre-'
quently recorded, Ahasverus never-became established in the study cribs.
The vast majority of Cucujidae occurring in the cribs were
Cryptolestes spp. (For incidence, see the curve for Cucujidae in
Figure 6.3). Small numbers of Placonotus politissimus were recorded
at the beginning and end of the storage season but did not become
established in the cribs. Cryptolestes species can only reliably be
separated on the basis of internal genitalia (Lefkovitch, 1962) and
so the species were not determined in most samples. The limited number
of determination that were made indicated that the population consisted
almost entirely of C. pusillus with occasional C. ferrugjneus » This
is in accord with the observation of Howe and Lefkovitch (1957) that
C. pusillus is the dominant species worldwide in the humid tropics,
probably due to a faster rate of increase under these conditions. Aitken
(1975) notes that both species can be very serious pests of cereals
and are able to build-up on grain that shows little initial damage.
By the end of the storage period at Ibadan C. pusillus was the second
most abundant species (after Sitophilus) in most cribs and was still
increasing rapidly.
Twenty species of Tenebrionidae were recorded from the cribs,
including five species of Palorus and four of Tribolium. Palorus
FIGURE 6.10 Seasonal incidence of main species of Tenebrionidae
(Data presentation as in Figure 6.2).
201.
o» JC \ 9 C 0 n e 3 C 4> O •
c
1000
100
10 L
i I
.1
UNTRERTED CRIBS - Ibadan
Wet Sauon Gnatooarue max.
Id A-
% . • Palorue aubdap. / N • r
Trlbolfun oaat.
/
/
(9 JC N « c.
4» o
100
10
i L
JC N.
3 C 4» o e » c
.1
1000
100
10
UNTRERTED CRIBS - Ibadan
Dry Saaaon
1. . I Pal .aub. • Gnato.«ax,
• V
1 1
\ 9 tm
o
3 C •»
o o c
.1
1000
100
10
FUMIGATED CRIBS - Ibadan
• v i v . x A \ T y'
. .a Pal .sub
Gnato.nax. A K
- i Trfb.oaat.
! t i j / N / 1
i i -I • iii?
• • r /
.1
FUMIGRTED CRIBS - Ilora
Trlb.oaet.
_ Gnato.max. E _ > Palor.aub.
+ ' / i
• "
t I'
i i i i Rug Sop Oot Nov Doo Jan Fob Mar Rpr May Jun Jul
2 0 2 .
subdepressus and Gnatocerus maxillosus were both abundant, the former jtn-
i
creasing earlier in all the wet season cribs, and both appeared to
reach a 'plateau1 during the dry seasofi (Figure 6.10). Tribolium
castaneum was present in all cribs but did not become as abundant as
P. subdepressus and G. maxillosus, while T . confusum was present in
smaller numbers. Palorus subdepressus is well known as a 'minor pest' •
of cereals and cereal products in humid tropical areas (Halstead, 1967a).
However, the comparative insignificance of the Tribolium spp. is perhaps
surprising : on the basis of laboratory studies of biology it has been
suggested that Tribolium spp. are likely to be more successful than
both Gnatocerus maxillosus (Aitken, 1975) and Palorus subdepressus
(Halstead, 1967a). . ,
Competition between T. castaneum and T. confusum has been ex-
tensively studied in the laboratory (e.g. Birch et al., 1950; Sokoloff
and Lerner, 1967) but competition with other T.enebrionidae does not
seem to have been considered. P. subdepressus is sometimes found under
bark in natural habitats and is probably indigenous to West Africa
(Halstead, 1967 a and b), while both P. subdepressus and G. maxillosus
were found in the fabric of empty maize cribs on the study site. The
comparative success of these species in the cribs may be due to the
existence of significant source populations in the surrounding en-
vironment. Halstead (1967a) notes that P. subdepressus is often found
in association with Sitophilufi and that Sitophilus frass is a favourable
diet for it. The accumulation of frass in the cribs may have been
advantageous to P. subdepressus and may explain the greater abundance
of this species at the bottom of the crib.
Of the remaining Tenebrionidae only Palorus ficicola, Sitophagus
hololeptoides, Palembus ivoirensis and Palembus ocularis occurred in
sufficient numbers to suggest that they had become established in the
2 0 3 .
cribs. The records of Palembus spp. are of some interest in that
Giles and Graham (unpublished) had previously predicted that one of
the species, P . ocularis, could become a pest of maize.
Araecerus fasciculatus (Anthribidae) was recorded from all cribs
but was only common during the first half of the storage season. As
mentioned earlier, Cornes and Riley (1961) recorded this species later
in the succession on cribs treated initially with Malathion. Araecerus
is important in West Africa mainly as a pest of cocoa and coffee but it
was obviously also breeding successfully on maize at Ibadan. Boshoff
(pers. comm.) noted considerable populations of this insect on cribs
at Ibadan that had been treated previously with Pirimiphos-methyl
("Actellic", I.C.I.).
Rhyzopertha dominica (Bostrichidae) is a major pest of cereals
throughout the tropics but only occurred occasionally in.the^study cribs.
It did, however, increase to become one of the more abundant pests
in some cribs on the same site in which grain-drying was promoted by
lighting a slow-burning fire below the platform (a traditional practice
in the area to the South of Ibadan - See Figure 2.1). This is consistent
with the suggestion that Rhyzopertha cannot compete with Sitophilus •
under humid conditions, but replaces it in hotter, drier environments
(Howe, 1958; Aitken, 1975).
Finally, three species of Mycetophagidae were frequently recorded
from the cribs. Litargus 'varius occurred only in the early part of the
storage season while Litargus balteatus and Typhaea stercorea are both
mould feeders that are recorded frequently from produce stored under
poor conditions (Aitken, 1975) and the ecology of L.'varius*is probably
similar.
The taxonomy of this species is uncertain - see Appendix I.
2 0 4 .
6.4.3 Predatory Coleoptera
Several species of Staphylinidae, Carabidae and Histeridae were
found in the cribs. The Staphylinidae occurred mainly at t he beg-
inning of the.storage period, Coenonica sp. being commonest in the field
but rarely recorded from the cribs, while. Coproporus sp. and Oligota
chrysopyga were comparatively common in store. 0 . chrysopyga is known to be
predacious on mites and, from the biology of closely related species,
both Coenonica sp. and Coproporus sp. are also most likely to be pre-
dators on small arthropods (Hammond, pers. comm.). Carabidae were found
sporadically throughout the storage season.- Two species, Coptoderina
laticollis and Catascopus senegalensls were recorded sufficiently fre-
quently to be regardfed ^s -more-Irhan just 'accidental'-.-visi-tora to the -
cribs. Histeridae were found mainly at a late stage in the succession
and the commonest species was Platysoma castanlpes.
None of the species was sufficiently abundant to be of any prac-
tical importance as a predator on the grain pests and no attempt was
made to investigate possible predator-prey relationships.
6.4.4 Heteroptera
Four species of Anthocoridae, three Reduviidae and two Lygaeidae
were frequently recorded from the cribs. Some Anthocoridae and Reduviidae
are well known from stored products as predators on the larvae of
Coleoptera and Lepidoptera and on mites. The potential of one of the
species, Xylocoris flavipes, as a biological control agent has been
widely studied (see, for instance, Jay et al., 1968; Awadallah and
Tawfik, 1972; Le Cato and Davis, 1973; Press et al., 1974, 1975; Le Cato,
1976; Arbogast, 1978). One lygaeid, Aphanus sordidus, is well known as §
damaging pest of groundnuts (Gillier, 1970) and has been recorded from
cocoa, copra and sorghum (Haines, 1974).
Lyctocoris cochici (Anthocoridae) and Mizaldus sp. (Lygaeidae)
205.
built up rapidly in the cribs at the beginning of the storage season
and then disappeared completely (Figure 6.11). From their close temp-
oral association it seemed possible that the former was preying on the-
latter. However, Lyctocoris did occur in some cribs in the absence of
Mizaldus and, in later laboratory observations, Lyctocoris was never
seen to attack live Mizaldus. (adults or nymphs) although it fed
readily on the larvae of several Coleoptera, including Carpophilus
dimidiatus, Palorus subdepressus and Lasioderma serricorne. Lyctocoris
cochiciiand Xylocoris afer (Anthocoridae) both reproduced successfully
on laboratory cultures of Coleoptera under ambient conditions but the
latter species never became abundant in cribs.
Cardiastethus sp. (probably always C. pygmaeus), Anthocoridae,
was recorded in all cribs and became established for short periods in
the fumigated cribs at both localities. Population 'peaks! occurred .
during the first two months in store, during the dry season in the
fumigated cribs and then, with the onset of the new wet season, in the
cribs of dry-season harvested maize (Figure 6.11). This pattern of
incidence does not correspond with that of any of the common Coleoptera
pests and Cardiastethus was not observed to feed on any of the Coleoptera 1
immatures offered to it in the laboratory (though 'probing' of the
substrate was observed). It seems likely that this species is attacking
mites or psocids, whose seasonal incidence was not investigated.
Scolopoides divareti occurred only in very small numbers at Ilora
but in all Ibadan cribs became common during the second half of
the storage season (Figure 6.11). In the laboratory it was observed
to feed on larvae of Palorus subdepressus, Carpophilus dimidiatus and a variety
of other Coleoptera and was easily maintained on cultures of these
species (with repeated additions of the prey species).
FIGURE 6.11 Seasonal incidence of main species of Heteroptera.
(Data presentation as in Figure 6.2).
206.
<9 JC \
c.
i 3 C 4» O
JC \ 9 C.
1 3 C 4* o
\ c.
1 3 C 4> o o • c
a» JC \ e t e 41 e 3 C
4* o o » c
1000
100
10
.1
UNTREATED CRIBS - Ibadan
Wet Season
Mtzaldus sp.
rt
¥ Lyotooorfs
ooohfof
Soolopotdss dlvarstt
100
10
.1
UNTREATED CRIBS - Ibadan
Dry Stason
Lyoto.oo.
•*-7r
r ±
h Card f a.
Sool.dfv.
i t
100
10
.1
1000
108
10
FUMIGATED CRIBS - Ibadan
,/f V
Lyoto.oo.
Mtzald.sp.
i V N s 't I
v * • . I •
.« Sool.dtv,
y Card1astathua ap.
L , ,
.1
FUMIGATED CRIBS - Ilora
1 /
mr Mfzaldus sp.
Lyoto.oo.
1 i i
\ i i
\
Card!aststhus sp,
Soolop.dlvar.
Rug Sop Oct Nov Deo Jan Feb Mar Rpr May Jun Jul
207.
Of the Reduviidae, adults and nymphs of Cethera musiva and
Emesopsis nubila (Emesinae) were found only sporadically but Peregrinator
biannulipes built up considerably, late in the succession in some cribs.
In:the laboratory both Cethera and Peregrinator fed on tenebrionid and
nitidulid larvae and were successfully reared on Tribolium castaneum,
although development was very slow.
Both Lyctocoris cochici and Scolopoides divareti appear to have
a considerable reproductive capacity under crib conditions and are
potentially important control agents of secondary pests although the
rather brief appearance of each species in the succession suggests particular ,
ecological limitations. There -is evidence in the results of this •»
study as to what the limiting factors might be.
The role of the Lygaeidae is not clear. From the biology of
related species they are likely to be phytophagous (sensu lato) rather
than predatory and species of one of the genera (Dieuches) are common
in leaf litter habitats (Deeming, pers. comm.). Neither Dieuches armatipes
nor Mizaldus sp. was successfully maintained on maize (or Coleoptera
cultures on maize) in the laboratory, although the former survived for «
some time and oviposited, especially when provided with a source of
moisture. Mizaldus sp. was observed to feed on dead (damaged) Coleoptera
larvae, but these may simply have been a source of moisture. The eggs
of D. armatipes were extensively parasitised by a scelionid, Telenomus sp.
6.4.5 Hymenoptera
At least 28 species of parasitoids were collected from the maize
cribs, although of these only 13 could be identified to species level.
The incidence of the commonest species is shown in Figures 6.12 and 6.13.
Adults of Anisopteromalus calandrae, Choetospila elegans, Cerocephala
FIGURE 6.12 Seasonal incidence of main species of Preromalidae.
(Data presentation as in Figure 6.2).
208.
\ 9 C. •
i 3 C
4>
o c
CO j£ \ 9 C. • I 3 C 4>
o c
\
c 0 1 3 C
4» O • 9 C
10
I I
UNTRERTED CRIBS - Ibadan
Hat Saaaon
btrAi i ~ - -
I
I -
{ Choatoap 11a aTagana
Carooaphala dtnodart
Habrooytua oaraalal li
Rn f aopt a roaa 1 ua oatandraa
100
10
UNTRERTED CRIBS - Ibadan Dry Saaaon
K h - —
V
— * Choat.alag.
Cero.dfno.
1 Rnfao.oal.
Habro.oar.
Choat.alag,
»
\ 9 (L 0
1 3 C
4* o o 9 C
100 L
10
.1
FUMIGRTED CRIBS - Ilora
|— •— "f" * Choat.aleg.
Rnlao.oal•
Cero.dfno.
Rug Sap Oot Nov Dao Jan Fob Mar Rpr May Jun Jul
FIGURE.6.13 Seasonal incidence of various parasitoids.
(Data presentation as in Figure 6.2).
209 .
\ m c •
1 3 C 4> o
JC
\ m c
4» O
N c
3 C 4» O
109
19
.1
UNTREATED CRIBS - Ibadan
Hat 9 >««o n
Zststloontus lasvlgatus
r
iU
3 BsthylIdas / ' ^ I Eup*1 nus
urozonus
100
10
1 I
.1
UNTREATED CRIBS - Ibadan
Dry Season
1 £
A
Mssopotobus J BethylIdas
Eup.uroz.
109
19
FUMIGATED CRIBS - Ibadan
Zstst.
» JC \ m c « 1 3 C •>
O 0 • c
.1
1009
109 .
10 .
J. y 1
r.lasv. I
Y ' t *
BsthylIdas
s . _
Eup.uroz.
.1
FUMIGATED CRIBS - ITora
* r " "
Bsthyl Idas
Eup.uroz.
i i
* " ' n Zstst.Iasv.
Rug Sep Oot Nov Deo Jan Feb Mar Rpr May Jun Jul
211.
it was thought to attack Nitidulidae (Noyes, identifier's comment).
At Ibadan it was found to be attacking Cathartus quadricollis and, assessed
on the basis of total emergences, achieved 30 - 40% parasitism at the
time of harvest.
Among the Bethylidae, Cephalonomia spp. are known to parasitise
a variety of stored products Coleoptera, but especially Oryzaephilus and
Cryptolestes spp. Their steady increase over the second half of the
storage period is consistent with the latter species being the main
host. Rhabdepyris zeae, which showed a similar progressive increase
in abundance, is well known as a parasite of Tribolium spp. (Haines, 1974),
but its host range does not seem to have been critically investigated
and it may have been attacking one of the commoner Tenebrionidae here.
Both Holepyris hawaiiensis (Bethylidae) and Bracon hebetor (Braconidae)
are common pests of Phycitidae in tropical stores (Haines, 1974) and
Antrocephalus mitys has also been associated with storage Lepidoptera in
Africa and Asia (Boucek, identifier's comment). Ephestia cautella was,
like the parasitoids, present in small numbers in most of the cribs and
may have been the host species; there is, however, no particular evidence
for this.
Scelionidae of the genera Gryon and Telenomus were frequently re-
corded from the cribs and were at times abundant. Scelionids were often
.found in the eggs of Dieuches spp. in insect samples and, judging from
this, the rate of parasitism appeared to be considerable. Telenomus sp.
successfully reproduced on eggs of D. armatipes in the laboratory, but
the parasite species could not be determined.
The investigation of the suitability of parasitoids, especially
Pteromalidae, for use as biological control agents against storage pests
212.
was begun many years ago (see, for example, Cotton, 1923, on Aniso-
pteromalus calandrae, and Noble, 1932, on Habrocytus cerealellae).
Small-scale trials under both laboratory and ambient field conditions
have indicated that, in confined environments,parasitoids can limit
populations of both Sitophilus spp. and Sitotroga (Williams and Floyd, 1971;
De Lima, 1978). However, studies of full-scale stores have in general
shown that, although parasitoids were present and at times abundant, they
failed to maintain the pest populations below the economic injury level
(Kockum, 1953; Salmond, 1957; De Lima, 1.978) .
In the current study no attempt was made to quantify the impact of
parasitism and the recruitment samples were too small to assess properly
the rate of parasitoid emergences. The results from such samples are
difficult to interpret both because one cannot be sure of the species
of the host with which an emerged parasite was associated and because of
the difference in development time between parasite and host (which means
that the parasites emerging in a particular unit of time will not be
those associated with the 'generation' of hosts that emerges during that
period).
Taking the total numbers of hosts and parasitoids emerging and assuming 1
the separation of hosts discussed earlier, the rates of parasitism achieved
here appear to have been of the same order as those observed by De Lima
(1978), namely less than 5% for Sitophilus and usually less than 10% for
Sitotroga. Higher'apparent rates of parasitism were recorded for Sitotroga
in cribs other than those used for the Succession Studies and for Sitophilus
in the final sample from the wet season cribs at Ibadan when total emergences
of Sitophilus (for the four cribs combined) were 149, those of Choetospila
56 and those of Cerocephala 21. Although adult parasitoids were caught
in all cribs from the fourth week of storage onwards appreciable numbers
213.
of emergences were not recorded until after 2| months at Ilora and after
more than three months at Ibadan.
Several species of Formicidae were recorded but only two, Pheidole sp.
and Myrmicaria sp., appeared in the cribs in significant numbers. Colonies
were not formed in the cribs themselves. Foraging ants were observed to
carry away fragments of grain and/or frass, dead insects and, occasionally,
live insect larvae. In the period just after harvest Myrmicaria sp
appeared to collect significant numbers of Mussidia nigrivenella larvae,
but this was not examined critically. Graham (1970) noted the importance
of predation by Pheidole megacephala on larvae of Epnestia cautella
in a warehouse in Kenya, but there seems to be no information on the
—general importance of predation by ants in rural stores.
6.4.6 Diptera
Diptera were frequently collected from the cribs but were never
numerous. The .most consistently recorded was Medetera sp. (Dolichopodidae)
which was often seen in small numbers on the outside of the cribs.
Larvae which probably belonged to this species were found in infested grain ,
samples and may have been predatory on immatures of the grain pests.
The larvae of some Medetera species are known as predators on larvae and pupae
of wood boring Coleoptera (Dyte, pers. comm.). Drosophilidae and
Sciaridae were occasionally found, probably associated with the mouldy
grain, and Phlebotomus sp (Phlebotomidae) were recorded in several samples from
Ilora, where they may have been feeding on the rats which were often
found in the cribs.
6.4.7 Psocoptera
Psocoptera became abundant in the cribs but were difficult to census
(mainly because of the time and care required to extract them from the
grain) and most species could not be identified. Psoquilla marginepunctata
214.
was by far the most abundant species and was sometimes seen to cluster
on,the outside of the cribs in large numbers. Liposcelis sp. (possibly
more than one species) was also common.
Psocids are frequently recorded from stored products but little
seems to be known of their role in storage ecology. Nineteen species
have been found in stored products in Zaire (Badonnel, 1974). In the
ck . laboratory Liposcelis bostryophilus has been shown to feed on the eggs of
A
Plodia) interpunctella (Lovitt and Soderstrom,. 1968) and of beetles (Williams,
1972; Shires, unpublished report) as well as a variety of plant materials.
Williams (1972) reports 5% predation on eggs of an anobiid beetle in
wood under natural conditions. Since Psoquilla marginepunctata has been
recorded quite frequently from tropical stores (Hairies, identifier's
comment), its biology would seem to merit investigation.
6.4.8 Other Insect Groups
Metabelina abdominalis (Dictyoptera, Blatellidae) and Diaperasticus
erythrocephala (Dermaptera, Forficulidae) were found in maize cobs both
in the field and during the early part of the storage season. The former
species reproduced in small numbers in the cribs. Spongovostox gestroi
(Dermaptera, Labiidae) became quite abundant in some cribs (especially
those used for the Short-Term Distribution Studies) on damaged grain late
in the succession. Metabelina is presumably a scavenger but the Dermaptera
may be wholly or partly predatory.
6.5 Other Arthropods
As Haines (1974) has remarked, infestations of mites in tropical
stores are probably more frequent than the few records in the literature
might suggest. In the current study mites were observed on the maize
from field samples and became abundant in store. Small collections of
mites were made at various stages in the study using a paraffin extraction
215.
process (Thind and Griffiths, 1979) but the large numbers in samples
and the difficulty of identifying them meant that this could not be done
frequently or quantitatively. This was an unfortunate omission in that
mites may well have had important effects on the insect pest population
via egg predation or parasitism.
The most abundant species appeared to be Tyrophagus putrescentiae
(Astigmata, Acaridae). This species has a cosmopolitan distribution in
a variety of habitats, including many stored products where it appears
to feed both on carbohydrates and moulds (Hughes, 1961). Other common
mites were prostigmatans of the genera Tarsonemoides, Tydeus and Anystis
and the me sostigmatans Proctolaelaps, Typhlodromus and Amblyseius. It
is difficult to generalise on the ecological roles of mites because
individual species are often very versatile, while closely related species
may have totally different habits. It is likely, however, that the
mesostigmatans at least were predators on -phytophagous (or mycetophagous) mite
Carpophilus spp. caught in the cribs were frequently observed to be
carrying Pseudotarsonemoides sp. (Prostigmata), a genus which has been \
found in association with bark beetles (Lynch, identifier's comment).
On Carpophilus the mites were usually clustered along the soft membranes
between selerites, but it is not clear whether they were ectoparasitic
or phoretic; the mites were also found free on frass in samples. Pyemotes
sp. (Prostigmata) was on one occasion found parasitising Sitotroga in a
laboratory culture recently started from moths caught in the cribs,
while Paracarophenax sp. (Prostigmata), another species that may well
be an insect parasite, was found in at least two samples from the cribs.
The frequent occurrence of Blattisocius tarsalis in association with
Sitotroga and its possible importance has already been noted; this
species was also found free in grain samples.
216.
The effect of the mites in degrading the substrate must have
been trivial in comparison with that of the insect pests. However,
as Haines (1974) points out, in temperate climates mites can be of
economic importance on stored produce and may become so in the tropics
as insect control is improved.
Small numbers of pseudoscorpions were occasionally found in the
cribs. Pseudoscorpions are frequently recorded from stored products
where they prey on mites (Champ, 1966), but in the conditions of this
study they,were not common enough to have had any significant impact.
Spiders were often present in samples from the cribs, but also in small
numbers. Zelotes sp. (Aranea, Gnaphosidae) was frequently recorded but
the majority of other species have not, as yet, been identified.
6.6 Vertebrates
Rodents are one of the most important^ causes of losses in stores,
both because of the considerable quantity of grain consumed' (up to 10%
of body-weight per day) and the much larger quantity that is contaminated
and damaged (Dyks tra, 1973). Rat guards were fitted to the study cribs but
proved ineffective, especially at Ilora. No attempt was made to
quantify losses due to rodents and cobs damaged by rats were not included
in the loss assessment data below (6.10). The species usually noticed in
the cribs was the multimammate rat, Mastomys natalensis.
Birds did not appear to be a serious cause of loss from cribs
although Laughing doves, Streptopelia senegalensis, were frequently
seen feeding from the top surface of the maize.
Agama lizards, Agama agama, and skinks, probably Mabuya sp., were
frequently observed in the cribs. Examination of the faecal pellets of
the former species indicated that its diet in the cribs consisted mainly
217. of ants, Myrmicaria sp., but included a small proportion of Sitophilus.
6.7 Grain Weight Losses
The progress of weight loss for the wet season cribs has already been
indicated (Figure 6.5). The considerable variation in the initial
moisture content of cobs in this trial resulted in estimates of dry
weight loss with large standard errors (not shown). As a result successive
estimates of weight loss cannot be compared critically. The mean values
(Figure 6.5) suggest, however, that-the weight loss over the .first two.
months in store is comparatively small but rises steadily thereafter.
The observation that weight loss continues to increase rapidly over the
second half of the storage period, when reproduction of Sitophilus and >
Sitotroga is comparatively low, provides at least circumstantial evidence
for the importance of the secondary pests in continuing damage.
"Hie final losses for individual cribs are given in Table 6.2.
Differences between the four Ibadan cribs were tested using a single factor
analysis of variance with each crib as a 'treatment 1. There were sig-
nificant differences between replicates (i.e. A vs. B, C vs. D) but the
fumigation appeared to have had no effect. The Ilora cribs had to be tested
separately because they were terminated after a shorter storage period.
'T' tests on these cribs and on the dry season cribs at Ibadan showed no
significant difference (at 5%) between the replicate cribs. The dry
season cribs suffered slightly greater damage than the wet season ones
over a considerably shorter storage period. The faster rate of damage
may be due at least in part to the higher rate of Sitophilus reproduction
over a longer period in the dry season cribs, but the missing data
points (Figure 6.7) make it difficult to assess this difference.
The weight loss in the cobs placed outside the sampling tunnels
could not be assessed for the wet season cribs because a number of cobs
2 1 8 .
TABLE 6 . 2
Crib Treatment Total time in store/ Final wt. loss/ s.d.
weeks %
A ) ) )
Wet season, untreated 38 22.5 a 3.8
B
) ) ) Ibadan 38 26.7 b 2.3
C ) ) )
Wet season, fumigated 38 27.9 b 3.6
D
) ) ) Ibadan 38 19.8 a 4.5
E
F
) ) )
Wet season, fumigated
Ilora
r-. r
CN
CM
14.8 ) 6.7 ) N.S.D.
16.4 ) 7.0
L
M
) ) )
Dry season, untreated
Ibadan
2 6£
2 6£
31.8. ) 4.6 ) N.S.D.
28.4 ) 1.5
TABLE 6.2 Mean loss in dry weight at the end of the storage period.
(calculation assumes that weight loss in the core is negligible). 4
Single factor anovar indicated significant differences between
means of cribs A - D (P<0.001). Separations are from a Newman -
Keuls test with « = 0.05.
Cribs E and F and cribs L and M were not significantly different
on a t test at the 5% level.
2 1 9 .
TABLE 6 . 3
'Cribs' -
a) 'levels' CRIB L CRIB M Mean
above tunnel 33.3 34.6 34.0 (unsampled) + 4.7 + 3.1
inside tunnel 31.8 28.4 30.1 (sampled) + 4.6 + 1.5
below tunnel 25.9 24.0 25.0 (unsampled) + 3.5 + 2.2
b) Source of variation degrees of freedom probability,
Total 59
Cribs 1 0.14
Levels 2 <0.001
Interaction 2 0.10
Error 54
TABLE 6.3 Weight loss in dry season cribs at termination:
Effect of position/sampling.
a) mean weight loss (jf standard deviation) on a dry weight
basis for different positions in two replicate cribs.
b) probability levels from a 2-factor analysis of variance,
were severely damaged by rats and because in others the identification C u«""
numbers were obscured. Improved rat guards and a different marking
system were used in the dry season trial. The results for this trial
are presented in Table 6.3. The mean weight loss for cobs within the
sampling tunnels is intermediate between the values for cobs immediately
above and below (as compared with the results of the preliminary trial
in which the sampled cobs showed a lower value - Section 3.5). This
suggests that although there is a 'position 1 effect, sampling at this
intensity (approximately monthly) has not significantly reduced the
insect population.
The levels of weight loss recorded here represent very severe damage
to the maize, although (as noted in Section 4.6) the data are within
the range recorded in surveys of rural cereal stores (Hall, 1970). Grain
in the condition reached at the end of the study would not normally be
used for human consumption but might be fed to poultry. However, as
Adams and Harman (1977) have pointed out, such figures for loss at the
end of the storage season greatly overestimate the effective losses that
occur in subsistence stores: in normal circumstances consumption of
the grain would reduce the quantity left in the crib so that only a small
proportion of the original total would be subject to the severe damage
encountered over the latter part of the storage season.
6.8 Conclusions
The occurrence of different patterns of incidence of insect species
in the cribs with the passage of time demonstrates the need to consider
both the detailed ecology of individual species and their interactions
with other members of the insect community. The seasonal incidence o f ,
say, Monanus concinnulus or Carpophilus fumatus, might have been broadly
predicted from a knowledge of their requirement for a moist and/or mouldy
221.
substrate, combined with the obvious climatic observation that there
is a marked dry season at Ibadan. However, it is much less obvious,
for instance, what factors might have enabled Cryptolestes pusillus
and Gnatocerus maxillosus, species which are regarded as sensitive to
low humidities, to increase almost throughout the storage period while
Tribolium castaneum and T. confusum did not, despite their greater
drought-tolerance and proven ability to eliminate other species of
secondary storage pests in laboratory competition experiments.
The evidence produced by this study is not sufficiently detailed
to answer such questions with any confidence. It is not known, for
instance, whether the increasing populations of particular secondary
pest species were partly or wholly due to successful reproduction within
the cribs or whether they reflected immigration from other habitats. There is,
however, evidence that different species may be limited by quite different
factors. The observation that Sitophilus reproduction'is apparently
limited mainly by its own adult population density (rather than, say,
unfavourable substrate conditions during the dry season) has particularly
serious implications for control strategies: one may expect any breakdown
of control measures to be followed by very rapid recovery of pest pop- «
ulations as individual surviving insects are able to realise their full
reproductive potential.
Similarly, there is at least circumstantial evidence that a number
of potentially serious pest species are present in the environment but
are not achieving pest status due to competition. If control measures
were to act selectively against existing pest species there is at least
a possibility that they would be replaced by others. This phenomenon is
well known from field crops, such as cotton, that are also subject to attack
by a variety of insects, and there is some evidence that it has occurred
on stored produce in Kenya (Graham, 1970). Interspecific differences in
tolerance to insecticides are already well documented for storage
pests (Champ and Dyte, 1976), but a similar outbreak of 'new* pests
could occur if, for instance, cultural controls were successfully used
to interrupt the cycle of Sitophilus infestation. The implications of
these possibilities will be further considered in the discussion which
follows.
2 2 3 .
CHAPTER 7
DISCUSSION:CHARACTERISTICS OF THE MAIZE CRIB SYSTEM AND
IMPLICATIONS FOR CONTROL STRATEGIES
Stored grain has been described as "a man-made ecosystem with
relatively simple structure and non-regenerating food energy supply
...... Such an ecosystem is unstable and is composed of species with
high growth and reproductive rates and low specialisation" (Sinha, 1973) . i
Entomologists studying grain storage under commercial conditions are
usually concerned with insect pests at low population densities
developing on an abundant food supply. The rate of population growth
may be constrained by harsh environmental conditions, especially by
very low moisture contents and high temperatures, or the associated
physical and physiological problems_of exploiting the food supply (i.e.
dry, undamaged grain). Equilibrium population densities are rarely,
if ever, attained. As Solomon (1953) has pointed out, although studies
of storage pest population dynamics in the laboratory have shown a
variety of crowding effects, the densities at which these effects become
important are rarely achieved under good storage condit ions. They are
encountered in practice only around 'hot spots 1 or on limited grain
residues left in empty stores.
It is interesting to consider the extent to which the maize cribs
studied here differ from such 'conventional 1 stored grain systems. The
features in common include the limited duration of the habitat (i.e. the
length of one storage season) and the non-regenerating food supply.
Although the insect populations reach high densities after a short period
of storage, their development occurs in an environment of seasonally
changing physical conditions and the insects are themselves progressively
2 2 4 .
changing the nature of their substrate: stable interactions between
species and equilibrium population levels cannot, therefore, be
expected. The insect community in cribs is remarkably diverse and,
although some of this diversity is undoubtedly maintained by the inter-
change of insects with other habitats, there do seem to be various
features of the crib system which, from stability arguments, appear to
favour the maintenance of ; diversity.
One such feature is the spatial heterogeneity of the substrate
which is greater than that in bulk shelled grain. The variation in
grain conditions from one part of the crib to another, imposed by the
outside environment, and, at a smaller scale, the heterogeneity provided
by small foci of damaged and mouldy grain have already been described.
These provide a variety of microhabitats favourable to insects with
different ecological requirements. Moreover' the storage of maize on
the cob provides a structural complexity not encountered in bulk grain.
Crombie (1946) showed that the provision of 1refuges 1 (consisting of
sections of fine glass tube) allowed Oryzaephilus surinamensis and
Tribolium confusum to coexist on flour, by providing protection for the
pupae of the former species. It may be imagined that in cobs of maize
a variety of such refuges might exist and provide shelter for vulnerable
stages such as eggs or pupae.
The superficial similarity of many storage pests is perhaps mis-
leading. There is an obvious difference between the requirements of
primary pest species, whose larvae develop most successfully within
intact grains, and those of secondary pests whose larvae feed mainly
from damaged surfaces or on frass. Co-existence of two such contrast-
ing species, Rhyzopertha dominica and Oryzaephilus surinamensis under
laboratory conditions was demonstrated by Crombie (1945). However,
within these broad groups a variety of more subtle differences exist
in feeding habits, moisture content preferences and so on. The
importance of such niche separation in allowing coexistence of com-
peting species has been discussed by May (1975) .
Another important condition for coexistence that is fulfilled,
at least by Sitophilus, is that populations are limited more strongly
by intraspecific effects than by interspecific ones. The strong
inhibitory effect of high Sitophilus densities on its own reproduction
may prevent it from eliminating other less numerous species over the
duration of the storage period.
It is possible to speculate that the activity of predators and
parasitoids is contributing to the maintenance of species diversity,
(see Hassell 1979). While these agents do not appear to be limiting
Sitophilus populations they may be having a significant effect on the
secondary pest species (between which competition is likely to be most
acute). The mite Blattisocius tarsalis is known to be polyphagous
(attacking the eggs of both beetles and moths), the three commonest
Authocoridae were shown in the laboratory to attack a variety of
Coleoptera larvae and the Bethylidae also appear from published records
to show at least limited polyphagy. No information seems to be avail-
able on the 'switching 1 or aggregation behaviour of these species on
mixed populations of hosts; however, given the demonstrated voraciousness
of both the mites (Haines, unpublished data) and of Anthocorids similar
to those found here (e.g. Jay et al. 1968), it seems at least plausible
that the activity of natural enemies is contributing to the maintenance
of diversity.
2 2 6 .
The picture of the storage community which emerges is one of
considerable complexity. There appears to be a good deal of special-
isation among the grain-feeding species while the consistency of the
spatial distribution patterns and of the temporal succession observed
suggests the existence of well-established relationships between species
analogous to those found in co-evolved communities. Although a similar
spectrum of Arthropod groups may occur in temperate stores (see, for
instance, the food webs proposed by Sinha (1973), these systems are
usually dominated by only one or two species. The greater species
diversity in crib stores may be partly explained by the features discussed
above in combination with more equable environmental conditions: in part-
icular, the floury endosperm of the dent maize at a comparatively high
moisture content provides a favourable substrate for a wide range of
species.
It is important to recognise, however, that the crib, unlike a
closed bulk grain store, is an integral part of a farming system and,
indeed, of a farming system that retains great ecological diversity by
the alternation of crops with semi-natural 'bush-fallows'. It is widely
accepted that traditional cereal varieties resistant to storage pests
have evolved by a process analogous to natural selection over an extended
period of time: only grains sufficiently resistant to withstand prolonged
exposure to insect attack over thfe storage period remained viable and
were propagated in the following year's crop. While there is little
direct evidence that insect communities have been similarly selected in
traditional agricultural systems this is at least a possibility. More-
over the crib habitat is similar in several respects to the 'natural'
food stores of other organisms in which some at least of the pest species
probably evolved.
2 2 7 .
One might postulate that the inherent 'irritability 1 of storage
insects (Surtees, 1965), which would lead in small open stores to dis-
persal, and the effects that reduce net fecundity at high densities
(Solomon, 1953) are in fact adaptations which allow the insects to
survive for an extended period on a strictly limited resource. In
some tropical ecosystems plant production may be nearly continuous
through the year: in such an environment a seed feeder, like Sitotroga
cerealella, which is short-lived, quite strongly dispersive and which
maintains a high rate of reproduction (even at high densities) can
survive by moving from one habitat of limited extent or duration to
another, saturating and destroying, each."-However, in.other tropical
environments, especially the savannah regions that include or are •
dominated by grasses (i.e. cereals), the productive phase is limited to
a short pefiod of the year in which rainfall is concentrated. An -insect
which depends on surviving for at least the several months of dry season
in an accumulation of grain must possess mechanisms that will allow it
to maintain its population without destroying the substrate.
The strong intra-specific limitation of reproduction shown by
Sitophilus may be an example of such an adaptation. Self regulating
mechanisms may be found in other species but it is also possible that
many storage species, especially those described as 'secondary* pests,
depend for their survival under natural conditions on the slow rate at
which they are able to attack dry, undamaged grain. Although they
appear in the laboratory to have very high inherent rates of reproduction
these would not normally be achieved on whole grain in the field:
reproduction only becomes sufficiently rapid for these species to reach
high populations when they are provided with an unusually favourable
substrate (such as moist, dent maize).
2 2 8 .
Although these considerations may seem to be of largely academic
ititerest they are in practice crucial to an understanding of the
development of storage pest problems with changes in the agricultural
system. Traditionally, only inherently resistant cereal varieties
have been stored and natural protective structures such as pods (grain
legumes) glumes (rice) and husks (maize) have been retained. Many
cereal-dependent agricultural systems developed in areas where a dry
period (or, at higher altitudes, a dry and cold period) of sufficient
severity to slow pest attack followed after harvest. Additional
protection may have been afforded to small quantities of grain by
'smoking* or by the addition of ash, sand or tbxic plant products.
Under these conditions losses due to insects, may have been appreciable
but they were-sustainable. In humid areas,, where losses to cereals
would have-been too severe,- staple foods were provided mainly by root
and tree crops.
In some areas these conditions still prevail, but in others changing
economic and social circumstances have resulted in considerable changes
in agricultural practices. Increases in human populations and in the
degree of urbanisation have resulted in an urgent need to increase agric-
ultural production and in particular to change from subsistence farming
to systems that produce a marketable surplus. This has been expressed
in various ways but most importantly in the selection of new crop varieties
in the extension of cereal production into more humid areas and in
'intensification' (in the sense that more land has been brought into
production with a concomitant reduction of fallows).
New varieties of cereals have been selected mainly for good agronomic
characters and maximum yield at harvest. Although techniques for ident-
ifying post-harvest resistance have been established and a considerable
2 2 9 .
body of information accumulated on existing varieties (Dobie, 1974,
1976) these have not widely been exploited in breeding-programmes.
Plant breeders have apparently believed the extra investment of time
and effort required for post-harvest screening of separate progeny
to be unwarranted.
Historically, agricultural communities were to some extent excluded
from very humid areas by factors such as human disease or excessive pest
and disease damage to field crops. It has only become feasible com-
paratively recently to grow significant quantities of cereals and grain
legumes in high rainfall areas. While devices such as the storage of
partially fermented grain in water of the use of 'smoking cribs 1 have
enabled the preservation of small quantities of grain, these methods
cannot" cope with significantly higher production, hence the attempt
to introduce highly-ventilated cribs for natural-drying. -The disastrous
levels of storage damage described from the experimental cribs at Ibadan
indicate the problems that may be expected if additional pest control
measures are not taken in these circumstances.
The effect of intensification on storage pest problems does not
seem to have been widely discussed. The biological system described
here depends for its long-term stability on high mortality among the
insects that disperse from stores. With intensification there is an
•increasing chance that dispersing insects will find a new crop or store
on which they can reproduce successfully, leading to much increased
endemic pest populations.
The question then arises as to whether the techniques of crib
storage and the accompanying crop production system can be modified to
cope successfully with these problems or whether a fundamental change
2 3 0 .
in storage practices is required to return losses to an acceptable
level.
In the situation considered in this study the following features
seem particularly important. Firstly, the pest species appear to
have a considerable capacity for dispersal which is apparent at the
time of colonisation of the cribs and may well be important during
the course of the storage season (although this was not demonstrated
directly) Secondly, conditions are adequate for rapid build-up of
the major pest species for most of the storage period and such limit-
ations as there are on population increase appear to be largely due
to interactions within the pest community. There are also other
potential pests present in the environment for which conditions appear
favourable.but which are effectively excluded by competition. Finally,
there~is circumstantial evidence that some~of the species which are
potential storage pests have significant source populations in habitats
not associated with the maize production or storage system.
Insect control strategies could, in principle, be aimed at reducing
the potential rate of pest increase in store, reducing the overall level
of pest populations in the total environment or interrupting the cycle
of infestation from store to field and back. To have any chance of
achieving lasting improvement all three components would have to be
pursued in parallel.
In a crib there can be little or no control over temperature or
humidity and the improvements in drying rates that can be obtained by
modifying the structure are not sufficient in humid areas to affect
significantly insect development. The substrate must therefore be made
less suitable by increasing the inherent resistance of the grain. While
2 3 1 .
varieties as susceptible as that used in this study are widely used
there can be-little chance of reducing endemic pest population levels:
any breakdown of chemical or cultural controls will be followed by
explosive recovery of pest populations. This does not necessarily
imply a return to traditional low-yielding varieties but rather the
selection of the less-susceptible improved cultivars. Resistance
levels in currently available varieties are not sufficient to provide
protection on their own under high pest pressure but would make a
significant contribution to a wider strategy.
There may be some potential for exploiting improved husk cover.
Although that shown by the varieties used in this study was clearly
ineffective in protecting the maize (affecting insect distribution but
not overall population levels), other varieties can.be better. The
penalties*of lower maximum yield, slower^ drying, the need to delay
harvest and the greater total bulk to be stored are all factors that
argue against the development of better-covered varieties for storage
'in the h u s k 1 . It remains possible, however, that the reduction of
losses in store might outweigh these disadvantages. Possibly good husk
cover is more important before harvest as a means of delaying the
establishment of storage pests. While under current conditions this
early infestation seems unimportant compared with direct cross-infestation
from infested stores, it might become significant if other sources of
pests can in the future be controlled. Certainly it seems important to
exclude, those varieties which show high susceptibility to Lepidoptera in
the field and those with a large proportion of open tips because of the
severe direct damage they suffer in humid conditions.
Biological control seems to have little potential in the immediate
future. Natural enemies cannot be expected to maintain pest populations
2 3 2 .
below injurious levels under current conditions of massive pest in-
vasion and rapid reproduction. If these adverse factors can be
significantly reduced the prospects would be improved, but the
evidence of these studies suggest that the hymenopteran parasitoids
at least are slow in moving to cribs at the beginning of the storage
season. Reports of successful biological control of storage pests
(e.g. Le Pelley & Kockum, 1954; Le Cato et al. 1977) appear to have
come only from 'closed* environments (i.e. warehouses) where there is
little insect movement to and from outside habitats.
The conventional chemical control methods involving the admixture
of more or less persistent insecticidal d u s t s l o a d i n g Q.f the crib
appears to be still effective in some dry areas. In humid conditions,
however, the rapid breakdown of the insecticide renders this method
virtually ineffective. Moreover, insect resistance to the two estab-
lished pesticides for this application, Lindane and Malathion, appears
to be widespread (Champ & Dyte, 1976). Some success has been claimed
for a system involving the repeated spraying of the outside of narrow
cribs with pirimiphos-methyl or synthetic pyrethroids (F.A.O., 1980);
using the former chemical, the economics are favourable and the method
appears robust enough for use at the small-farm level.
In principle, however, it seems ill advised to advocate the use of
considerable quantities of insecticide in a situation where they can be
at best only moderately effective. A total 'kill' is not obtained and
there appears to be little residual action. The current study has
indicated the speed with which populations of major pests may recover
from low levels and the potential for recolonisation from outside sources
under current conditions is considetable. While the use of insecticides
on cribs in humid areas may be necessary in the short or medium term it
must be combined with measures to reduce the general pressure of infest-
ation. More information is also needed .on.the precise biological ef.fects...
of pesticide applications on the insect populations.
Various authors have proposed that the cycle of infestation from
infested stores to the growing crop might be interrupted by ensuring
that there is a distinct interval prior to harvest when no grain is held
in store and when the fabric of storage structures can be properly dis-
infested. Even if this were socially feasible, it seems unlikely to be
effective, given the existence of source populations' of storage pests
(and their considerable longevity) in Alternative* habitats. Proper
crop and store sanitation would undoubtedly improve the situation and
would be a crucial component of any control programme, but a real solution
to the problem in terms of an—integrated pest management strategy cannot
be expected until—a greater part of-the total environment can be controlled.
Storage losses may be reduced by removing the grain to a more easily
controlled environment such as a silo or warehouse. While small-scale
silos have been successfully introduced in some developing countries, in
others they have not been adopted because of high capital cost or the
non-availability of materials, or, in humid areas, because of the recurrent
cost of artificially drying the grain. In such areas there is also a
considerable danger that subsequent moisture migration in the silo can
lead to moulding and, potentially, total loss of the grain.
Warehouse storage offers a more robust system, in which the drying
requirement is less critical and in which pesticides are likely to be
reasonably effective. Good pest control has been achieved, for instance,
by application of pirimiphos-methyl to successive layers of bags stacked
in warehouses (F.A.O. 1980). Cribs could play an important role in such
a system by providing a cheap method of natural drying of grain prior to
2 3 4 .
warehouse storage. For the short period involved it might be pos-
sible to avoid the "use of insecticides in the cribs altogether. JThe
small size of individual farms in many developing countries means,
however, that the introduction of more sophisticated storage tech-
niques may require a change in social organisation, such as storage
by 'collectives 1 of farmers or by larger-scale marketing boards. In
practice such systems have often proved difficult to introduce or
operate.
In conclusion, the immediate prospects for the use of cribs in
humid areas do not appear favourable. Environmental conditions are
ideal for storage pest development and changes in agricultural practices
have resulted in excessive endemic pest population levels. In the face
of this pressure of infestation, measures such as the use of insecticides
on cribs are only a palliative and breakdown of the protection prbvided
by them must be anticipated. Research is needed to investigage the
ecology of rural storage systems in different environments and to identify
points of weakness before further serious pest problems develop. Applied
workers seeking to improve storage techniques must consider crib stores
as an integral part of the particular farming system and must give more
attention to the biological characteristics of the insect community which
they are trying to control. Improved crop sanitation and genetically
resistant varieties should be introduced to contain the immediate pest
problem. However, significant reduction of storage losses cannot be
expected without further major changes in agricultural systems and these
changes may, for economic and social reasons, be very difficult to achieve.
235.
APPENDIX I : An annotated list of species of insects and mites recorded
from maize cribs at Ibadan and Ilora.
Insects are recorded by Orders, according to systematic
convention,but within Orders lower taxa are set out
alphabetically.
Field Store
DICTYOPTERA (det. J.A. Marshall - C.I.E.)
Blattellidae
Metabel'ina abdominalis (Shelford)
DERMAPTERA (det. A . Brindle - C.I.E.)
Forficulidae
Diaperasticus erythrooephala (Olivier)
Labiidae
Spongovostox gestroi (Burr)
PSOCOPTERA (det. C.P. Haines - T.S.P.C.)
Liposcelidae
Liposcelis sp.
Psoquillidae
Psoqwilla marginepunctata Hagen
HEMIPTERA (det. M.S.K. Ghauri - C.I.E.) (HETEROPTERA)
Anthocoridae
Cardiastethus pygmaeus Poppius 1 Lyotocoris cookici Delamar-Deponttevimme & Paulian 1 Scolopo-ides divaret'i Carayon . 1 Xytocoris (Proxylocorzs) afer (Reuter) 2
Lygaeidae
Dieuches avmatipes (Walker) • 2
236.
APPENDIX I : Continued.
Field Store
Dieuches sp. 3 Mizaldus sp. 1 Pachybrachvus sp. 3
Reduviidae
Cethera mus-Cva (Germar) . 2 Emesopsis nub-ila Uhler 2 Pevegrinatcp biannuli,pes Montrouzier 1/2
Tingidae
Arushia sp. 3 gen. & sp. indet. 3
COLEOPTERA
Anobiidae (det. C.P. Haines)
Lasioderma serrioorne (F.) 2
Anthicidae (det. R.B. Madge - C.I.E.)
Anbhious bottegoi Pic 2
Anthribidae . (det. C.P. Haines)
Arae oerus fascLaulatus .Degeer 1/2
. Bostrichidae (det. C.P. Haines)
Bo s tryc hop 1% tes comu tus (01 iv i er) Di-noderus minutus (F.) Heterobostryohus brunneus (Murray) 'Rhyzopertha dominica (F.) Xyloperthella erznitarsis (Imhoff)
Bruchidae (det. C.P. Haines)
Callosobruchus maculatus (F.) 3
Carabidae (det. R.B. Madge)
Catasoopus senegalensi-s Dejean Coptoderina latircollzs (Lafevre) Metallica aeneipennis (Dejean) gen. &. sp. indet.
2
• 2 2
2/3 3
2 2 3 2
237.
APPENDIX I : Continued.
Field Store
Cerylonidae (det. R.B. Madge)
Elytrotetrantus sp. 2
Chrysomelidae (det. M.L. Cox - C.I.E.)
Melixanthus sp. 3
Ciidae (det. R.B. Madge)
gen. & sp. indet. 3
Cleridae (det. R.B. Madge)
Korynetes analis (Klug) " 3 •Neorobia rufipes (Degeer) 2
Colydiidae (det. D.G.H. Halstead - M.A.F.F.)
Microprius oonfusus Grouvelle 2 Pseudobothrideres oonradsi Pope 3
Corylophidae (det. R.B. Madge)
Alloparmulus sp. 2 Arthrolips sp. ' ^ 2
Cucujidae (det. D.G.H. Halstead)
Cryptolestes ferrugineus (Stephens) • 2 Cryptolestes pusillus (Schonherr) • 1 Cucujirius sp. 3 Mario.laemus sp. 3 Plaoonotus majus Lefkovitch 2 Plaoonotus politissimus (Wollaston) 2 Planolestes oorrtutus (Grouvelle) • 2
Curculionidae (det. D.G.H. Halstead)
Cylas puncticoll'is (Boheman) 3 Pseudostenotrupis marshalli- Zimmerman 3 Sitophilus zeamais Motschulsky 1
Elateridae (det. C.M.F. von Hayek - C.I.E.)
Aeoloides sp. 3 Cardiophorus sp. 3
Endomychidae
Mycetaea hirta (Marsham) 1 2
APPENDIX I : Continued.
Histeridae (det. S. Mazur - Inst. Ochrony Lasu Drewna, Warsaw)
Diplostix mayeti (Marseu-1) Platysoma castanipes Marseul Teretrius pulex Fairmair Teretrius oylindricus Wollaston
Hydrophilidae (det. E.A.J. Duffy - C.I.E.)
Enochrus sp.
Lyctidae (det. D.G.H. Halstead)
Lyotus nr. africanus Lesne Minthea rugicollis Walker
Mycetophagidae (det. D.G.H. Halstead)
L-itargus batteatus Leconte L-itargus'var-ius' Typhaea steroovea (L.)
Nitidulidae (R.M. Dobson - Glasgow University; C.P. Haines; R.B. Madge)
Brachypeplus Igabonensia (Grouv.) Braohypeplus' pilosellus (Murray) Carpophilus binotatus Murtay Carpophilus dinridiatus (F.) Carpophilus freemani •Dobson Carpophilus fumatus Boheman Carpophilus hemipterus (L.) Carpophilus maculatus Murray Carpophilus marginellus Motschulsky Carpophilus obsoletus Erichson Carpophilus pilosellus Motschulsky Carpophilus zeaphilus Dobson Carpophilus sp. nov. Haptonchus minutus (Reitter) Lasiodaotylus sp. Urophorus humeralis (F.)
Phalacridae (det. R.B. Madge)
Litotarsus sp.
Platypodidae (det. M.L. Cox)
Platypus hintzi Schaufuss
239.
APPENDIX I : Continued.
Field Store
Scolytidae (det. M.L. Cox)
Hypothenemus obscurus (F.) 2 Xyleborus ferrugineus (F.) 3
Silvanidae (det. D.G.H. Halstead)
Ahasverus advena (Waltl ) 3 Cathartus quadricollis (Guerin) 1 1 Monanus concinnulus (Walker) 1 1 Oryzaephilus mercator (Fauvel) 1/2 Parasilvanus faipairei (Grouvelle) 3 Silvanoprus frater (Grouvelle) 3 Silvanoprus linsidiosus Grouvelle 3 Silvanoprus longicollis (Reitter) 3 Silvanus .inarmatus Wo11aston 2
Stephylinidae (det. R.B. Madge)
Atheta dilutipennis (Motschulsky) 3 Coenonica sp. 1 2 Coproporus sp. 3 2 Gabronthus Ibadalus Tottenham 3 Oligota chrysopyga Kraatz 2 Philontfrus peregrinus (Fauvel) 2
Tenebridnidae (det. D.G.H. Halstead)
Alphitobius diaperinus (Panzer) 3 Alphitobius laewigatus (F.) 3 AZphitobius viator Mulsant & Godart . 2 Gnatocerus maxillosus (F.) 2 1 . Gonocephalum simplex (F.) 3 Latheticus oryzae Waterhouse 3 Palembus ivoirensis (Ardoin) - 2 Palembu$ ocularis Casey '2 Palorus bobiriensis Halstead 2 Palorus carinicollis (Gebien) 2 Palorus cerylonoides (Pascoe) • 2 Palorus crampeli Pic 2 Palorus ficicola (Wollaston) 2 Palorus subdepressus (Wollaston) 1 Platydema sp. 2 Sitophagus hololeptoides (Castelnau) 2 Stomylus sp. 3 Tribolium anaphe Hinton 2 Tribolium castaneum Herbst 3 1 Tribolium confusum Jacquelin du Val 1/2 Tribolium semicostata. (Gebien) -3
240.
APPENDIX I : Continued.
Field
Trogositidae (= Ostomatidae) (det. C.P. Haines)
Tenebroides mauritanicus (L.)
Store
3 (Ilora only)
LEPIDOPTERA (det. C.P. Haines; J.N. Ayertey - I.A.R.; M.J. Cornes - N.S.P.R.I.)
Cosmopterygidae
Pyroderces sp.
Gelechiidae
Sitotroga cerealella (Olivier)
Pyralidae
Eldana sacckarina Walker Ephestia cautella (Walker) Mussidia Inigrivenella Ragonot Plodia inter puna tetla (Hubner)
Tortricidae (Olethreutidae)
CryptopKlebia leucotreta (Meyrick)
DIPTERA (det. J.C. Deeming
M.A.F.F.)
Cecidomycidae
gen. & sp. indet.
Dolichopodidae
Medetera sp.
Drosophilidae
gen. & sp. indet.
Phlebotomidae
Phlebotomus sp.
Sciaridae
Bradysia sp.
- I.A.R.; C.E. Dyte -
.2
241.
APPENDIX I : Continued.
Field Store
HYMENOPTERA
Braconidae (G.E.J. Nixon & I.D. Gauld - C.I.E.)
Apanteles sp. Braoon hebetor Say Chelonus sp. Phanerotoma sp.
Bethylidae (Z. Boucek)
Cephalononria formiciform-is Westwood Cephalonomia sp. Holepyris hawaiiensis (Ashmead) Plastanoxus westwoodi (Kieffer) Rhabdepyr-is zeae Turner & Waters ton
Ceraphronidae (N. Fergusson - C.I.E.)
Aphanogmus sp.
Chalcididae (Z. Boucek - C.I.E.)
Antrocephalus crassipes Masi Antrocephalus nritys (Walker) Antrocephalus sp. Euchalci&ia sp. nr. microgastricidia Steffan
Diapriidae (Z. Boucek)
Triohopria Bp.
Encyrtidae (J.S. Noyes - C.I.E.)
Zetet-icontus laevigatus (De Santis)
Eucoilidae (Cynipoidea (J. Quinlan - C.I.E.) Rhoptromeri-s sp. 2
Eupelmidae (Z. Boucek)
Bruohocida vuilleti Crawford 2 Eupelmus urozonus Dalman 1/2 Maoroneura sp. 2
Formicidae (B. Bolton - C.I.E.)
Dorylus (Anomma) sp. .. 3 . Monomorium sp. (pharaonis-group) 2 Myrmicaria sp. ' 1/2
1/2 1/2 2 2 1/2
1/2
242.
APPENDIX I : Continued.
Field Store
Odontomaohus troglodytes (Santschi) 3 Paohyoondyla senaarensis (Mayr) 3 Pheidole sp. 1/2 Tetramoriwn caldar-ium (Roger) 3
Ichneumonidae (I.D. Gauld)
Allophrys sp. 2
Pteromalidae (B.R. .Subba Rao - C.I.E.)
An-isopteromalus oalandrae (Howard) 1/2 ' Cerooephala d-inoderi, Gahan 1/2 Choetdsp'Lla elegans Westwood 1/2 Habroeytus oerealellae Ashmead 1/2 Mesopolobus sp. 1/2
Scelionidae (I.D. Gauld)
Gryon sp. 2 Telenomus sp. 1/2
ACARINA
ASTIGMATA (det. D. Griffiths -M.A.F.F.)
Acaridae
Tyrophagus putresoentiae (Schrank)
PROSTIGMATA (det.' S. Lynch - M.A.F.F.)
• Anystidae
Anyst-is sp.
Cheyletidae
Cheletomorpha lepidopterorum (Shaw)
Pyemotidae
Paracarophenax sp.
Tarsonemidae
Tarsonemcrides sp. Pseudotarsonemozdes sp.
243.
APPENDIX I : Continued.
Tydeidae
ITydeMs sp.
CRYPTOSTIGMATA (det. D . Macfarlane - B.M. (N.H.))
Scheloribatidae
Soheloribates sp.
MESOSTIGMATA (det. S. Lynch & C.E. Bowman - M.A.F.F.)
Ascidae
Blatt-isoovus tarsalis (Berlese) Proctolaelaps sp. Typhlodromus sp.
Phytoseiidae
Ambtyseius sp.
Note I : In cases where identifier's name is followed by "C.I.E.", specimens were identified through the Commonwealth Institute of Entomology identification service, although in some case the identifiers were British Museum (N.H.) staff. Other institutions abbreviated above are as follows- - T.S.P.C. : Tropical Stored Products Centre (0:D.A.), Slough; M.A.F.F. : Slough Laboratory, Min. of Agriculture; N.S.P.R.I. : Nigerian Stored Prodjcts Research Institute; I.A.R. : Institute of Agricultural Research and Training, Samaru, Nigeria; B.M. (N.H.) : British Museum (Natural History).
Note 2 : Classification in right-hand columns:
1 : abundant or common species. 2 : frequently recorded species 3 : occasionally recorded
(for further explanation - see below)
The above list is not complete (a small number of Hymenoptera and
Coleoptera remain unidentified) but includes all species that were freqently
recorded. The numbers in the right-hand columns are intended to provide
an indication of the status of each species in the insect community, both .
244.
APPENDIX I : Continued.
pre-harvest ('field') and in the cribs ('store'). The categories do not
relate to particular levels of abundance or numbers of records but are
based on a subjective assessment of the information collected in all trials
over a two year period.
In category one are all species that were sufficiently abundant to
cause or contribute to significant grain damage; also included are those
species (mainly predators and parasitoids) which, though present in smaller
numbers seem likely to have played a part in the ecology of economically
important species. Category 2 comprises those species which did not achieve
pest status but which were recorded sufficiently regularly to indicate
that they had become established on the grain (or were at least frequent
visitors from nearby habitats). This category includes a number of species
that are apparently well adapted to the stored grain environment and which
might well achieve pest status under slightly different conditions. .
Category 3 includes species recorded on only one or a few occasions. It
should .be noted that several of the species in the first two categories
only occurred in significant numbers during a limited part of the storage
season and were rare or absent at other times.
The notes which follow are intended to be complementary to the inform-
ation given in Chapters 4, 5 and 6 which dealt with the distributipn and
seasonal incidence of most of the major species. Taxa are considered in
the order presented above. Records for many of the insect groups associated
with stored products in West Africa have not been collated and so comments
on possible new records below must be regarded as tentative. Useful lists
or reviews including such records have been published by Forsyth (1966),
245.
APPENDIX I : Continued.
Cornes (1973), Haines (1974) and Aitken (1975).
Heteroptera
The abundance of Lyctoooris cochici and Scolopoides divareti. is of
interest. Lyctoooris campestris and two species of Xyloooris have been
recorded from stored products in Nigeria (Cornes 1973) but these two
species recorded here do not appear to be well known (Ghauri, p.c.) and
would seem to merit further investigation as potential biological control
agents. Sootopoides seemed in laboratory culture to be more tolerant of _ .
low humidity than Lyctoooris.
Coleoptera
Both Las-ioderma serrioorne (Anobiidae) and Araecerus fasciculatus
(Anthribidae) are versatile storage pests known from a variety of commodities
Both bred successfully on the maize variety used (i.e. a white 'dent* with
a fluor^fendosperm) in single-species culture and appear to be potential
pests.
• Bastrichids, with the exception of Rhyzopertha, are usually regarded
as more important for the damage they can do to the structure of the store
than for that done to the stored commodity. In the experimental cribs,
however, Bostryohoplites, Dinoderus and -Heterobostrychus all appeared to be
feeding on the grain. Their large size and mode of feeding (moving along
'files' of grain, taking little material from each) meant that they caused
considerable damage.
Korynetes analis and Necrobia rufipes (Cleridae) are both recorded
from Nigeria and the latter species is also known as a pest of copra and
246.
APPENDIX I : Continued.
animal products stored under poor conditions. In the maize cribs they
were presumably acting as predators (see Aitken, 1975).
Mioroprius oonfusus (Colydiidae) is widespread in Africa. It is
usually found in association with bark- and wood-boring species but has
been recorded from Nigerian groundnuts (Aitken, 1975). In the cribs it
was presumably feeding on moulds. Corylophidae are known from rotting
vegetation in natural habitats and their frequent occurence in the cribs
probably also reflects the poor storage conditions.
Among the Cucujidae, species of Cryptolestes other than C. pusillus
and C. ferrugineus may well have been present but remained undetected due
to the impossibility of examining critically more than a small fraction of
the total collected. The frequent occurrence of Planolestes cornutus is
of interest because previously it has usually been found in association
with legume pods (Lefkovitch, 1962), and was 'indeed found in this habitat
at the study site.
Cylas puncticollis (Curculionidae) presumably strayed into the cribs
from nearby fields of sweet potato while Pseudostenotrupis marshal'li,
although recorded from various Nigerian stored products, is probably
associated with palms- (Aitken, 1975) . Both Sitophilus zeamais and S.
oryzae have been recorded from Nigeria and both can attack maize in single
species or mixed populations. Only S. zeamais was recorded from the
experimental cribs but the dissections necessary to distinguish the
species were only carried out routinely during preliminary investigations:
a small proportion of S. oryzae could thus have remained undetected,. but
this seems'unlikely.
247.
APPENDIX I : Continued.
The four species of Histeridae recorded are of some interest. Various
species of Caroinops are often found in stored products (including records
from Nigeria - Cornes, (1973)) and Teretrius spp. are recorded as predators
of wood-boring beetles (Aitken, 1975). These records from maize cribs,
however, appear to be unusual. The last published record for Diplostix
mayeti (as Carcinops mayeti) from stored produce appears to be from 1899
(Hinton, 1945; Halstead, 1969).
Minthea rugicollis (Lyctidae) is found throughout the world attacking -
especially wood and bamboo (Aitken, 1975). Maize seems to be an unusual
substrate for it and it may have moved to the maize from the structure of
the crib.
Typhaea stercorea and Litargus balteatus (Mycetophagidae) are both
well known from stored products in poor conditions but the frequent
occurrence of a second Litargus sp. is of interest. The sp'ecies has not
yet been determined and the name 'varius' used in the text (S6.4.2)
appears to be invalid (Halstead,' p.c.).,
Carpophilus species are frequently recorded in stored products,
especially when these are damp or mouldy. C. binotatus, however, does
not seem to have been recorded from this environment before although it
was frequently recorded (in very small numbers) from the cribs. C.
zeaphiZus was described from maize in Uganda (Dobson 1969) and has been
found in cribs ih Kenya and Ethiopia (Haines, 1974). Its occurrence in
the experimental cribs considerably extends the known distribution and
adds to the evidence that it may be widespread in this habitat.
248.
APPENDIX I : Continued.
Hypothenemus spp. (Scolytidae) have been recorded from various other
stored products and H. hampei is a pest of coffee. H. obscurus was quite
common in the maize cribs and there appeared to be at least one other
species present at times. The commoner Silvanidae have already been
discussed (S6.4.2). The remaining species were only found occasionally
and are probably 'accidentals1 from natural habitats,
Alphitobius viator (Tenebrionidae) has on a few occasions been
recorded in cargoes of African produce and has been -found in a maize-
store in Ghana (Green, 1980). The other two Alphitobius species are more
familiar, being cosmopolitan pests of cereal products in poor condition.
Gonooephalum simplex is common in fields of cereals but rarely recorded
from stores. Though found only occasionally in the cribs a considerable
infestation was observed on a batch of rice at the study site that had
b,een in store for a long period. Three of the Palorus species (P. sub- —
depressus3 P. fioioola and P. cerylonoides) are familiar from stored
products, although P. cerylonoides is mainly an Oriental species (Halstead,
1967). • Palorus bobiriensis3 P. carinicollis and P. crampeli have all been
found in West Africa in natural habitats but have not previously been
recorded from stored products (Halstead, pers. comm.). Tribolium anaphe
is widespread in Africa and has previously been recorded in small numbers
in stores in Nigeria (Howe, 1952), but T, semicostata (= T. giganteum)
has not been found in stored products before (Halstead, pers. comm.).
Lepidoptera
.All the Lepidoptera recorded are well known from stored maize or from
the field crop although, as mentioned in the text, the abundance Pyroderces
249.
APPENDIX I : Continued.
sp. at the time of harvest was unusual.
Diptera
Among the Diptera, Medetera sp. (Dolichopodidae) does not seem to
have been found previously in stores (Dyte, pers. comm.) but other
predatory flies, especially Scenopinus fenestrates (Scenopinidae), are
commonly found in warehouses (Hinton & Corbet, 1975).
Hymenoptera
The commonly occurring Hymenoptera have already been discussed
(S6.4.5). The Bethylidae and Pteromalidae recorded here are all well
known from stored products as is Braoon hebetor. The incidence of more
'marginal1 species is, however, difficult to assess in the absence of a
convenient published collection of records. Bruchocida Vuilleti, has
previously been recorded from stored cowpeas and soya beans in Nigeria
(Cornes, 1973) although not, apparently from maize. Aphanogmus sp.
(Ceraphronidae) and Gryon and Telenomus spp. (Scelionidae) were all common
or abundant at times in the cribsVbut do not seem to be mentioned in the
recent stored products literature; the association of Telenomus sp. with
eggs of Dieuohes (Het., Lygaeidae) has already been noted ( 6.4.5).
Allophrys sp. (Ichneumonidae), which was recorded several times in small
numbers, appears to belong to an underscribed species which has been
collected from other localities in Africa (Gauld, pers. comm.); other
Tersilochinae are known to be parasites of coleoptera larvae, especially
Curculionidae and Bruchidae.
Material of all the species listed above has been deposited at the
Tropical Stored Products Centre (Slough) or, in the ca e of less common
species, at the British Museum (Natural History)".
250.
APPENDIX II : Collated Data - Succession Studies.
Data given are numbers of insects collected on samples of
cobs from the centre of cribs on successive occasions during the
storage season. For details see Chapter 6.
Data on insect numbers presented here have been corrected
arithmetically to standard sample weights as indicated, using
the sample data presented at the head of each column. Shelling
indices were estimated from the initial and final observed values
and assuming a linear change with time.
Cribs were as follows:
location Wet Season
A B
C
D
E F.
Dry Season . L M
Ibadan
Ibadan
Ilora
'Ibadan
treatment
untreated
fumigated
fumigated
untreated
starting date
16th Aug. 1978
23rd Aug. 1978
8th Jan. 1979.
'Time Scale' on data sheets is in days, for the wet season cribs (A - F) and at dry season cribs (L - M).
starting at 8th January
1st August 1978 = 0 1979 = 0 for the
251.
S E Q U E N T IflL S A M P L E S C r i b A IITfi 1 9 7 8 Wet S e a s o n
N u m b e r o f i n s e c t s c o r r e c t e d t o 1 0 0 0 g r a m s at 13 m o i s t u r e c o n t e n t
• S a m p 1e T i m e s c a l e S a m p 1 e w e l g h t M o i s t u r e c o n t e n t C o r e m o i s t u r e c o n t e n t E s t i m a t e d s h e l l i n g
1 2 3 4 5 6 7 8 9 10 3 0 4 4 5 8 7 2 180 1 2 8 176 2 0 4 2 3 3 2 9 2
5 3 0 0 3 1 7 5 2 6 6 4 2 6 3 8 2 5 4 2 2 2 1 9 1 6 6 8 1 3 5 8 1 3 4 7 1 2 6 5 1 9 . 5 1 7 . 0 1 7 . 0 1 6 . 5 1 5 . 9 1 4 . 1 1 2 . 0 1 2 . 0 1 2 . 2 1 3 . 9 2 1 . 1 1 7 . 2 1 7 . 6 1 7 . 3 16.4 1 1 . 8 1 0 . 9 9 . 2 1 2 . O 1 2 . 3
8 6 8 6 8 6 8 5 8 5 8 3 8 2 8 1 8 1 <56
L a s i o d e r m a s e r ^ i c o r n t 0 0 R r a e c e r u s f a s c i c u l - i n s +• 0 H e t e r o b o s t r y c h u s b r u n n e u s 0 0 i n d e t . C o r y 1 o p h i d a e . 1 0 C . p u s l 1 1 u s 0 1 P l a c o n o t u s p o l i t i s s i m u s 0 . 0 i n d e t . C u c u j i d a e t o t a l C u c u j i d a e 0 1 S i t o p h i 1 u s s p . 8 L . ? v a r i u s 1 + T y p h a e a s t e r c o r e a 0 0 C . d i m i d i a t u s 11 2 2 C . f u m a t u s 2 1 17 C . m a c u l a t u s 0 • C . pi 1 o s e 1 1 u s 0 0 Cf z e a p h i 1 u s + ^ 0 i n d e t . C a r p o p h i l u s
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2 5 9 .
APPENDIX III : Collated Data - Distribution Studies
Data presented are as follows
Preliminary Distribution Trial (Sitopilus and Carpophilus adults only)
Long-Term Distribution Trial
sample 1 8/9/78 adults sample 2 6/10/78 sample 3 22/11/78 sample 4 11/4 /79
(cribs were loaded 25/8/78)
samples 1 & 2 samples 3 5 4
it ii--ii
emergences
it
Short-Term Distribution Trial (dates as indicated on"individual sheets)
adults emergences
260
261 262 263 264
265 266
266-284 285-287
Details of the sampling programme are given in Chapter 6.
F:R EL I.N I.N ftR V. D-I S.T ft J B U T.I 0 H IR I H L 260.
Adult counts: D it t a. c o r r ~ •; T. •=• d t, n u n. b .= r o f i n s
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LONG-TERN DISTRI BUT I OH TRIhL
Sample 1 - emergences from lOOg in 1 week (.Figures are means of three samples; » denotes mean <.5>
position 1 2 3 4 5 6
Sitophilus sp. 2 other minor pest spp. 2 parasitoids 0
3 4 9 4 3 * * 2 2 1 u 0 * 0 0.
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•f. i tophi I us sp. other hi i nor p« p at" as 11 o i ds
pos11 i on
9 10 11 12
Sitophilus sp. 3 2 4 3 6 5 other minor pest spp. 2 1 * 1 0 2 par ii 11 o i ds 0 0 0 * 0 1
13 14 15 16 1? 18
3 2 2 2 5 3
s p p . 1 * 2 1 3 *
0 0 0 1 1 1
19 20 21 22 23 24
Sitophilus sp. 1 3 1 . 1 1 1
3 other minor pest spp. • * • *
1 1 »
paras11 o i ds 0 * 0 0 0 2
L O H G - T E R M D I S T R I B U T I O N T R I A L
S a m p l e 2 - e m e r g e n c e s f r o m 1 0 0 g in 1 w e e k ( F i g u r e s a r e m e a n s o f t h r e e s a m p l e s ; * d e n o t e s in e a n < . 5 )
p o s i t i o n 1 2 3 4 .5 6
S i t o p h i l u s . s p . 3 4 4 4 7 5 o t h e r M i n o r p e s t s p p . 4 8 2 * * 1 p a r a s i i o i d s 0 0 0 * 0 0
p o s » t i o n 7 8 9 10 11 12
S i t o p h i l u s s p . 16 8 1 0 8 7 5 o t h e r m i n o r p e s t s p p . 3 1 3 2 5 2 p a r a s i t o i d s 0 * 0 0 0 0
p o s i t i o n 13 14 15 16 17 18
S i t o p h i l u s s p . 5 6 3 9 7 3 o t h e r m i n o r p e s t s p p . 3 3 2 7 2 2 p a r a s i t o i d s 0 0 0 0 0 0
p o s i t i o n 19 2 0 2 1 2 2 2 3 2 4
8 • 1 0
S i t o p h i 1 u s s p . 6 10 3 3 7 o t h e r m i n o r p e s t s p p . 2 1 I 2 2
p a r a s i t o i d s * 0 0 * 0
ro Ln
L O N G - T E R M D I S T R I B U T I O N T R I A L
S a m p l e 3 - e m e r g e n c e s f r o m l O O g in 1 w e e k
( F i g u r e s a r e m e a n s o f t h r e e s a m p l e s ; * d e n o t e s m e an < . 5 )
p o s i t i o n 1 2 3 4 5 6
Si t o p h i l u s s p . 14 14 ' 7 16 12 17
C a t h a r t u s q u a d r i c o l l i s * 1 1 1 * 1 .
S i t o t r o g a c e r e a l e l l a 13 14 13 12 12 5 o t h e r m i n o r p e s t s p p . 0' 2 1 2 1 1 p a r a s i t o i d s 1 2 # * * 0
p o s i t i o n 7 • 8 9 10 11 12
S i t o p h i 1 u s s p . 16 19 2 5 18 19 18
C a t h a r t u s q u a d r i c o l l i s * 0 0 2 0 1 S i t o t r o g a c e r e a l e l l a 8 9 4 10 18 8 o t h e r m i n o r p e s t s p p . ' 2 • 1 2 2 2 2 p a r a s i t o i d s 1 .2 1 2 .1 0
p o s 1 1 i o n 13 14 15 16 17 18
S i » o p h i 1 u s s p . 1 1 13 13 12 1 1
C a t h a r t u s q u a d r i c o l l i s 1 * 0 0 * 1
S i t o t r o g a c e r e a l e l l a 5 1 1 1 11 6 4 o t h e r m i n o r p e s t s p p . 0 2 1 4 * *
p a r as i t o i d s • 2 • 2 * 0 *
p o s i t i o n 19, 2 0 21 2 2 • 2 3 2 4
S i t o p h i l u s s p . 17 2 7 . 19 15 17 19
C a t h a r t u s q u a d r i c o l l i s 1 1 1 * 0 1
S i t o t r o g a c e r e a l e l l a 6 5 6 12 lu 5 o t h e r m i n o r p e s t s p p . 2 2 1 * 3 3
p a r a s i t o j d s .* 2 ' 0 2 0
L O N G - T E R M D I S T R I B U T I O N T R I A L
S a m p l e 4 - e m e r g e n c e s f r o m l O 0 g in 1 w e e k
( F i g u r e s a r e m e a n s o f t h r e e s a m p l e s ; * d e n o t e s m e a n < . 5 >
p o s i t i o n 1 2 3 4 5 6
C r y p t o l e s t e s s p p . 12 2 4 9 3 10
S i t o p h i 1 u s s p . 13 24 13 2 0 17 18
G n a t o c e r u s m a x i l l o s u s 6 4 8 10 8 3
C h o e t o s p i l a e l e g a n s 2 1 16 9 2 5 15 7
o t h e r m i n o r p e s t s p p . 4 1 3 6 2 5
p a r a s i t o i d s 3 3 1 5 1 •
p o s i t i o n 7 8 9 10 11 12
C r y p t o l e s t e s s p p ; 18 11 6 6 8 2
S i t o p h i l u s s p . 9 14 16 12 2 5 10
G n a t o c e r u s m a x i l l o s u s 6 4 5 6 6 3
C h o e t o s p i l a e l e g a n s 2 5 14 2 14 15 3
o t h e r m i n o r p e s t s p p . 4 4 4 4 5 2
p a r a s i t o i d s 4 2 1 1 2 #
p o s i t i o n 13 14 15 16 17 18
C r y p t o l e s t e s s p p . 15 13 7 14 10 7
Si t o p h i 1 u s s p . 14 17 3 0 5 10 2 3
G n a t o c e r u s m a x i l l o s u s 5 5 7 7 4 6
C h o e t o s p i l a e l e g a n s 19 18 4 2 5 13 8
o t h e r m i n o r p e s t s p p . 3 3 3 7 3 4
p a r a s i t o i d s 1 1 1 1 0 1
30s i t i o n 19 2 0 2 1 2 2 cl i 24
C r y p t o l e s t e s s p p . 11 7 5 8 8 3
S i t o p h i l u s s p . 16 1 1 12 13 10 16
G n a t o c e r u s m a x i l l o s u s 8 5 5 5 7 3
C h o e t o s p i l a e l e g a n s 19 18 6 16 19 Cl o t h e r m i n o r p e s t s p p . 4 2 5 3 3 b
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1 2 0 0 2 4 1 0 1 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 12
5 9 7 0 9 3 9 5 9 13 2 1 0 0 0 1 0 0 1 6 0 0 0 0 1 0 0 0
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2 6 2 0 18 6 9 8 4 3 18 3 4 0 0 0 0 0 1 . 0 0 0 0 0 0 0 0 0 0 0 0 1 1 11 3 0 0 0 0 0 0 1 0 . 0 0 '0 0 0 0 1 0 0 0 '0 0 0 1 10 8 2 0 6 2 2 7 3 7 2 3 18 9 0 0 0 0 1 0 0 0 0 0 0 0 0 • 0 0 0 0 0 0 0 9 1 • "1 .1 0 0 0 0 1 2 0 0 0 0 0 0 0 0 0 0
. 0 0 • 0 0 0 0 0 0-' 0 0 0 0 0 0 0 0 18 2 5 10 3 3 0 11 2 1 .,8 16 1 1 1 13 5 2 4 3 0 . 0 0 9 4 . 0 0 1 1 1 2 4 1 2 0 1 0 1 ' 5 0 1 2 . 0 0 " ' 0 0 1 1 0 0 1 0 0 0 0 0 0 0 0 0 0 ' 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 2 0 4 • 3 2 0 •0 8 1 1 4 4 2 .0 0 0 1 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 1 1 1 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 ' 0 5 5 5 • 3 5 6 5 0 2 11 5 5 11 • 12 •A 8
• 0 0 0 • 0 0 0 0 0 0 1 0 0 0 0 0 0 1 0 0 6 0- 1 0 • 4 0 0 0 0 1 ' 0 0 .1 1 0 .0 2 '3 2 c. • .3
L a s i o d e r m a s e r r i c o r n e ftraecerus f a s c i c u l a t u s D i n o d e r u s m i n u t u s H e t e r o b o s t r y c h u s b r u n n e u s M i c r o p r i u s c o n f u s u s P s e u d o b o t h r i d e r e s c o n r a d . i n d e t . C o r y l o p h i d a e C r y p t o l e s t e s s p p . P l a c o n o t u s s p p . P l a n o l e s t e s c o r n u t u s S i t o p h i 1 u s s p . i n d e t . H i s t e r i d a e L y c t u s n r a f r i c a n u s L i t a r g u s b a l t e a t u s L . v w a r i u s ' T y p h a e a s t e r c o r e a B r a c h y p e p l u s ? g a b o n e n s i s B . pi 1 o s e l 1 u s C . d i m i d i at u s C . f u m a t u s C . h e m i p t e r u s C . o b s o l e t u s L a s i o d a c t y 1 u s s p . H y p o t h e n e m u s s p . C a t h a r t u s q u a d r i c o l l i s M o n a n u s c o n c i n n u l u s O r y z a e p h i l u s m e r c a t o r S i l v a n u s i n a r m a t u s G n a t o c e r u s m a x i l l o s u s P a l e m b u s i v o i r e n s i s P . o c u l a r i s P a l o r u s b o b i r i e n s i s P . c a r i n i c o 1 1 i s P . c e r y l o n o i d e s P . fi c i c o l a P . s u b d e p r e s s u s P a l o r u s c r a m p e 1 i S i t o p h a g u s h o i o l e p t o i d e s T r i b o l i u m c a s t a n e u m T . c o n f u s u m T r i b o l i u m s p . P l a t y d e m a s p p . L a t h e t i c u s o r v z a e S i t o t r o g a c e r - = a l e l l a C a r d i a s t e t h u s s p . ". C a r d i a s t e t h u s s p . n y m p h s L y c t o c o r i s c o c h i c i L . c o c h i c i n y m p h s X y l o c o r i i s a f e r X . a f e r n y m p h s
C . m u s i v a n y m p h s
P . b i a n n u l i p e s n y m p h s S c o l o p o i d e s d i v a r e t i S . d i v a r e t i n y m p h s D i e u c h e s a r m a t i p e s D . a r m a t i p e s n y m p h s M i z a l d u s s p . M i z a l d u s s p . n y m p h s i n d e t . H e t e r o p t e r a i n d e t . B e t h y l i d a e Z e t e t i c o n t u s l a e v i g a t u s E u p e l m u s u r o z o n u s
finisopteromalus c a l a n d r a e C e r o c e p h a l a d i n o d e r i C h o e t o s p i l a el'egans H a b r o c y t u s c e r e a l e l l a e M e s o p o l o b u s s p . i n d e t ; " S c e 1 i o n i d a e i n d e t L a b i i d a e a d u l t s ' n d e t L a b i i d a e n y m p h s
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2 2 2 3 24 , 2 5 2 6 2 7 2 8 2 9 3 0 31 3 2 2 8 . 8 2 8 . 9 2 9 . 0 ' 2 9 . 7 2 3 . 0 2 8 . 3 2 8 . 0 3 1 . 0 2 8 . 3 2 8 . 3 2 8 . 7 1 6 . 6 l b . 1 1 6 . 5 1 6 . 5 1 7 . 0 1 7 . 6 1 7 . 1 1 6 . 6 1 6 . 8 1 6 . 4 1 5 . 3 3 1 6 4 3 0 3 2 3 3 30 3 6 7 3 5 9 3 1 2 4 0 5 352 3 9 0 4 5 3 1 1 6 1 1 4 1 0 5 1 3 1 124 1 3 7 104 1 13 123 1 1 6 1 13
L a s i o d e r m a s e r r i c o r n e 0 2 firae-erus f a s c i c u l a t u s 1 1 Din: .-r-us minutus 0 0 H e t e r s t r y c h u s b r u n n e u s 0 0 M i c r o p r u s c o n f u s u s 0 0 P s e u d o b : t h r i d e r e s c o n r a d . 0 0 i n d e t . C o r y ? o p h i d a e 1 0 C r y p t o l e s t e s s p p . 2 3 3 3 P l a c o n o t u s s p p . 0 0 P l a n o l e s t e s c o r n u t u s 0 0 Si t o p h i l u s s p . 2 9 5 3 6 2 i n d e t . H i s t e r i d a e 0 0 L y c t u s n r a f r i c a n u s 0 0 L i t a r g u s b a l t e a t u s 3 0 L . x v a r i u s ' 0 0 T y p h a e a s t e r c o r e a 0 1 B r a c h y p e p l u s ? g a b o n e n s i s 0 0 B . p i 1 o s e l 1 u s 0 0 C . d i m i d i a t u s 9 1 4 7 C . f u m a t u s 1 1 C . h e m i p t e r u s 0 0 C . o b s o l e t u s 0 0 L a s i o d a c t y 1 u s s p . 0 0 H y p o t h e n e m u s s p . 0 0 C a t h a r t u s q u a d r i c o l l i s 2 1 M o n a n u s c o n c i n n u l u s 1 1 O r y z a e p h i l u s m e r c a t o r 0 0 S i l v a n u s i n a r m a t u s 0 0 G n a t o c e r u s m a x i l l o s u s 2 3 . 5 7 P a l e m b u s i v o i r e n s i s 0 •f 0 P . o c u l a r i s ' 0 0 P a l o r u s b o b i r i e n s i s 0 1 P . c a r i n i c o l l i s 1 0 P . c e r y l o n o i d e s 0 • 0 P . f 1 c i c o l a 4 0 Pi s u b d e p r e s s u s 5 3 8 3 P a l o r u s c r a m p e l i 0 -0 S i t o p h a g u s h o i o l e p t o i d e s 0 0 T r i b o l i u m c a s t a n e u m 1 5 T . c o n f u s u m 0 , 0 T r i b o l . i u m s p . 0 1 0 P I a t y d e m a s p p . '0 0 L a t h e t i c u s o r y z a e 0 '0 S i t o t r o g a c e r e a l e l l a 14 8 C a r d i a s t e t h u s s p . 2 . 0 C a r d i a s t e t h u s ' s p . n y m p h s 2 0 L y c t o c o r i g c o c h i c i 5 8 L . c o c h i c i n y m p h s 0 1 r < y l : c o r i s a f e r 0 0 X . a f e r n y m p h s 0 0 C . m u s i v i n y m p h s 0 0 P . b i a n n u l i p e s n y m p h s 0 0 S c o l o p o i d e s d i v a r e t i 0 1 S . d i v a r e t i ( n y m p h s 0 0 D i e u c h e s a r m a t i p e s 0 1 D . a r m a t i p e s n y m p h s 0 0 M i z a l d u s s p . 1 0 M i z a l d u s s p . n y m e n s 1 0 i n d e t . H e t e r o p t e r a 0 0 • i n d e t . B e t h y l i d a e 0 1 Z e t e t i c o n t u s l a e v i g a t u s 0 0 E u p e l m u s u r o z o n u s 1 0 flnisopteromalus c a l a n d r < J " 0 C e r o c e p h a l a d i n o d e r i
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L a s i o d e r m a s e r r i c o r n e 0 1 1 2 1 0 firaecerus f a s c i c u l a t u s 0 0 0 0 2 0 D i n o d e r u s m i n u t u s 0 0 0 0 1 2 i n d e t . B o s t r i c h i d a e 0 0 0 0 0 0 i n d e t . C a r a b i d a e 0 0 0 0 0 0 i n d e t . C o r y l o p h i d a e 0 2 1 2 5 0 M i c r o p r i u s c o n f u s u s 0 0 0 0 0 0 C r y p t o l e s t e s s p p . 7 9 4 7 12 10 P l a c o n o t u s s p p . 0 0 0 0 0 0 P l a n o l e s t e s c o r n u t u s 1 0 0 0 0 0 S i t o p h i 1 u s s p . 5 6 3 3 3 5 2 2 6 2 3 4 4 5 6 4 4 1 i n d e t . H i s t e r i d a e 0 0 0 0 0 0 L y c t u s ? a f r i c a n u s 0 0 0 0 0 0 H i n t h e a r u g i c o l l i s 0 0 0 0 0 0 L i t a r g u s b a l t e a t u s -4 3 1 1 12 3 T y p h a e a s t e r c o r e a 1 0 1 0 1 1 B . ? p i 1 o s e l 1 u s 0 0 a 0 0 0 C a r p o p h i l u s b i n o t a t u s 0 0 0 0 0 0 £ . d i m i d i at u s 5 2 3 4 3 3 4 3 6 7 4 9 C . h e m i p t e r u s 0 0 0 0 0 0 C . o b s o l e t u s 0 1 0 0 1 0 L a s i o d a c t y l u s s p . 0 0 0 0 0 0 C a t h a r t u s q u a d r i c o l l i s 0 0 0 0 5 0 M o n a n u s c o n c i n n u l u s 1 0 - 0 0 6 4 O r y z a e p h i l u s m e r c a t o r 0 0 0 0 0 0 G n a t o c e r u s m a x i l l o s u s 11 4 12 8 2 0 2 7 P a l ' e m b u s i v o i r e n s i s — 0 0 0 0 0 0 P a l o r u s b o b i r i e n s i s 1 0 0 0 e 0 P . c a r i n i c o l 1i s 1 0 0 0 1 0 P . c e r y l o n o i d e s 0 0 0 0 0 0 P . f i c i c o l a 0 0 0 1 2 4 P . s u b d e p r e s s u s 3 3 2 2 16 2 2 P . c r a m p e M 0 0 0 0 ' 0 2 S i t o p h a g u s h o i o l e p t o i d e s 0 0 0 0 0 0 T r i b o l i u m c o n f u s u m 0 0 0 0 0 0 T r i b o l i u m c a s t a n e u m • 0 1 1 0 0 1 i n d e t . C o l e o p t e r a 0 • 0 0 0 0 0 S i t o t ' r o g a c e r e a l e l l a 3 6 5 3 2 1 3 1 4 5 2 5 C a r d i a s t e t h u s sp-. 16 3 1 2 18 8 C a r d i a s t e t h u s s p . n y m p h s " 4 0 • 0 1 3 3 L y c t o r i s . c o c h i c i 0* 2 0 2 7 2 L . c o c h i c i n y m p h s 0 0 0 0 1 0 X y l o c o r i s a f e r ' 0 1 1 1 • 0 1 C e t h e r a m u s i v a • 0 0 1 0 0 0 C e t h e r a m u s i v a n y m p h s 0 0 0 0 0 0 i n d e t . E m e s i n a e n y m p h s 0 0 - 0 0 • 0 0 S c o l o p o i d e s d i v a r e t i 0 0 0 0 0 0 S . d i v a r e t i n y m p h s 0 0 0 0 0 0 D i e u c h e s a r m a t i p e s 0 0 0 0 0 0 D i e u c h e s s p . nympfts 0 1 4 0 0 0 M i z a l d u s s p . 1 1 0 2 1 2 M i z a l d u s s p . n y m p h s 0 0 0 0 0 0 i n d e t . H e t e r o p t e r a 0 0 0 0 0 1 B r a c o n h e b e t o r 0 0 0 0 0 0 i n d e t . C h a l c i d i d a e 0 0 0 0 0 0 i n d e t . B e t h y 1 i d a e 0 0 0 0 0 0 Z e t e t i c o n t u s l a e v i g a t u s 0 0 0 1 0 0 E u p e l m u s u r o z o n u s 2 0 0 0 0 0 flnisopteromalus c a l a n d r a e 0 0 0 0 . 0 0 C e r o c e p h a l a d i n o d e r i I 2 1 I 0 1 C h o e t o s p i l a ' e l e g a n s 5 9 . 10 1 6 8 H a b r o c y t u s c e r e a l e l l a e 0 0 0 0 0 0 M e s o p o . l o b u s s p . 0 0 1 0 0 0 i n d e t . S e e l i o n i d a e 1 0 - 0 • 2 1 1 indet" L a b i i d ' a e a d u l t s 0 2 0 1 0 0 i n d e t L a b i i d a e nymp.bs 0 0 1 1 ' 2 0
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2 2 2 3 24 2 5 26 27 2 8 2 9 3 0 3 1 3 2 2 6 . 1 2 6 . 3 2 6 . 9 2 6 . 8 2 5 . 9 2 6 . 0 2 6 . 0 2 6 . 0 2 5 . 9 2 6 . 2 2 6 . 2 1 7 . 7 1 7 . 3 1 7 . 4 1 7 . 2 1 7 . 4 1 7 . 8 1 7 . 6 1 7 . 3 1 6 . 5 1 5 . 9 1 6 . 2 3 1 5 2 9 4 4 1 2 3 5 4 3 5 6 3 1 8 3 2 9 3 5 3 3 2 3 3 6 5 3 4 7 1 1 2 1 3 9 109 1 1 0 144 1 2 2 143 136 1 2 9 108 104
L a s i o d e r m a s e r r i c o r n e 2 flraecerus f a s c i c u l a t u s 2 D i n o d e r u s m i n u t u s 0 i n d e t . B o s t r i c h i d a e 0 i n d e t . C a r a b i d a e 0 i n d e t . C o r y l o p h i d a e 0 M i c r o p r i u s c o n f u s u s 0 C r y p t o l e s t e s s p p . 15 P l a c o n o t u s s p p . 0 P l a n o l e s t e s c o r n u t u s 0 S i t o p h i 1 u s s p . 3 2 3 i n d e t . H i s t e r i d a e 0 L y c t u s ? a f r i c a n u s 0 M i n t h e a r u g i c o l l i s . 0 L i t a r g u s b a l t e a t u s 0 T y p h a e a s t e r c o r e a 0 B . ? p i 1 o s e l 1 u s 0 C a r p o p h i l u s b i n o t a t u s 0 C . d i m i d i at u s 6 7 C . h e m i p t e r u s 0 C,. o b s o l e t u s 0 L a s i o d a c t y 1 u s s p . 0 C a t h a r t u s q u a d r i c o l l i s 0 M o n a n u s c o n c i n n u l u s 2 O r y z a e p h i l u s m e r c a t o r 0 G n a t o c e r u s m a x i l l o s u s 2 9 P a l e m b u s i v o i r e n s i s 0 P a l o r u s b o b i r i e n s i s 0 P . c a r i n i c o 1 1 i s - 0 P . c e r y l o n o i d e s 0 P . f i c i c o l a 0 P . s u b d e p r e s s u s 3 4 P . c r a m p e l i 0 S i t o p h a g u s h o 1 o 1 e p t o i d e s 0 T r i b o l i u m c o n f u s u m 0 T r i b o l i u m c a s t a n e u m 1 i n d e t . C o l e o p t e r a 0 S i t o - t r o g a c e r e a l e l l a 2 3 C a r d i a s t e t h u s s p . 2 C a r d i a s t e t h u s s p . n y m p h s 0 L y c t o r i s c o c h i c i 6 L.- cocljici n y m p h s 0 X y l o c o r i s a f e r 0 C e t h e r a m u s i v a 0 C e t h ' e r a m u s i v a n y m p h s 0 i n d e t . E m e s i n a e n y m p h s 0 S c o l o p o i d e s d i v a r e t i 0 S . d i v a r e t i n y m p h s - 0 D i e u c h e s a r m a t i p e s 0 D . i e u c h e s s p . n y m p h s 0 M i z a l d u s s p . 3 M i z a l d u s s p . n y m p h s 0 i n d e t . H e t e r o p t e r a 0 B r a c o n h e b e t o r 0 i n d e t . C h a l c i d i d a e 0 i n d e t . B e t h y 1 i d a e 1 Z e t e t i c o n t u s l a e v i g a t u s 0 E u p e l m u s u r o z o n u s 0 flnisopteromalus c a l a n d r a e 0 C e r o c e p h a l a d i n o d e r i 5 C h o e t o s p i l a e l e g a n s 7 H a b r o c y t u s c e r e a l e l l a e 0 M e s o p o l o b u s s p . . 1 i n d e t . S c e l i o n i d a e 0 i n d e t L a b i i d a e a d u l t s 0 i n d e t L a b i i d a e n y m p h s 1
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. T y p h a e a s t e r c o r e a 0 0 0 2 B r a c h y p e p l u s p i l o s e l l u s 0 0 0 0 C a r p o p h i l u s d i m i d i a t u s 2 5 3 2 7 6 1 C . f u m a t u s 0 1 0 2 ,C. o b s o l e t u s 0 0 0 0 L a s i o d a c t y 1 u s s p . 0 0 0 0 H y p o t h e n e m u s s p . 0 0 0 0 C a t h a r t u s q u a d r i c o l l i s 0 1 0 0 M o n a n u s c o n c i n n u l u s 1 0 0 0 O r y z a e p h i l u s m e r c a t o r 0 0 0 0 i n d e t . S i 1 v a n i d a e 0 0 0 0 - i n d e t . S t a p h y l i n i d a e 0 0 0 0 G n a t o c e r u s roaxillosus- 4 0 18 2 2 7 1 P a l e m b u s i v o i r e n s i s 0 0 0 0 P a l o r u s b o b i r i e n s i s 0 0 0 0 P . c a r i n i c o l l i s 0 0 0 0 P . f i c i c o l a 0 0 0 3 P . ^ s u b d e p r e s s u s 3 7 18 19 34 S i t o p h a g u s h o i o l e p t o i d e s 2 4 0 2 T r i b o l i u m c a s t a n e u m 4 7 2 0 T . c o n f u s u m 0 0 0 0 P y r o d e r c e s s p . 0 0 0 0 S i t o t r o g a c e r e a l e l l a 7 11 13 10 E p h e s t i a c a u t e l l a l a r u a e 0 0 0 0 C a r d i ^ s t e t h u s s p . 2 0 0 0 L y d j t o c o r i s c o h i c i 0 1 • 0 1 X y l o c o r i s a f e r 0 0 0 5 i n d e t . flnthocorid n y m p h s ' 0 0 0 . 1 C e t h e r a m u s i v a 0 0 0 0 C e t h e r a m u s i u a n y m p h s 0 0 0 1 P e r e g r i n a t o r M a n n u l i p e s 0 1 0 0 P e r e g r i n a t o r b i . n y m p h s 0 2 0 0 S c o l o f J o i d e s d i v a r e t i 0 0 0 0 3 . d i v a r e t i n y m p h s 0 0 0 0 D i e u c h e s a r m a t i p e s 0 1 2 1 D i e u c h e s s p . n y m p h s 0 0 3 • 5 M i z a l d u s s p . 1 1 1 1 M i z a l d u s s p . n y m p h s 5 1 2 2 i n d e t . H e t e r o p t e r a 0 0 0 1 B r a c o n h e b e t o r 0 0 0 1 i n d e t . B r a c o n i d a e 0 0 0 0 i n d e t . C h a l c i d i d a e 0 0 0 0 i n d e t . B e t h y l i d a e 1 0 1 0 Z e t e t i c o n t u s l a e v i g a t u s 0 0 0 2 E u p e l m u s u r o z o n u s 0 0 0 0 finisopteromalus c a l a n d r a e 0 0 0 0 C e r o c e p h a l a d i n o d e r i 3 3 2 4 C h o e t o s p i l a e l e g a n s 13 9 2 4 H a b r o c y t u s c e r e a l e l l a e 0 0 . 0 0 M e s o p o l o b u s s p . 0 . . 1 0 1 i n d e t . S c e l i o n i d a e 1 0 0 5 i n d e t L a b i i d a e a d u l t s 1 0 0 1 i n d e t L a b i i d a e n y m p h s 0 0 0 1
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L a s i o d e r m a s e r r i c o r n e 0 0 0 0 0 3 0 0 0 0 0 0 0 0 0 0. . firaecerus f a s c i c u l a t u s 0 0 0 0 0 0 0 1 3 0 1 1 0 1 0 0 D i n o d e r u s m i n u t u s 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 i n d e t . C o r y l o p h i d a e 4 0 0 3 2 2 1 12 0 3 3 3 r 4 0 1 0 M i c r o p r i u s c o n f u s u s 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 C r y p t o l e s t e s s p p . 6 2 9 2 7 18 3 7 24 2 8 14 9 1 1 17 12 10 8 13 P I a c o n o t u s s p p . 0 0 0 0 0 0 0 2 0 0 0 1 0 0 0 0 C u c u j i n u s s p . 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 S i t o p h i 1 u s s p . 2 5 5 2 0 8 1 4 9 134 2 7 7 174 8 5 1 0 9 1 9 9 4 0 5 1 6 8 1 2 2 166 2 7 9 164 6 0 i n d e t . H i s t e r i d a e 0 0 0 0 1 0 8 0 0 0 0 0 9 0 0 0 L y c t u s ? a f r i c a n u s 1 0 1 0 1 0 0 0 0 0 0 0 0 0 0 0 L i t a r g u s b a l t e a t u s 4 0 1 0 3 3 0 9 8 1 7 5 4 2 1 0 T y p h a e a s t e r c o r e a 0 - 0 8 0 2 2 0 1 0 0 1 1 0 0 0 8— B . p i 1 o s e 1 1 u s 0 0 8 0 0 0 0 0 0 0 0 0 0 0 0 0 C . d i m i d i at u s 3 5 9 19 2 1 6 5 14 16 2 1 24 2 6 2 1 2 1 2 6 2 3 12 14 C . f u m a t u s 2 0 2 0 13 0 2 0 2 2 4 4 2 1 3 2 4 7 0 0 4 C . h e m i p t e r u s 0 0 8 1 0 0 0 0 0 0 0 0 0 0 '0 0 L a s i o d a c t y 1 u s s p . 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 H y p o t h e n e m u s s p . \ 0 0 0 0 1 . 0 0 0 0 1 0 0 0 0 0 0 C a t h a r t u s q u a d r i c o l i i s 0 0 0 0 1 0 0 1 0 1 0 3 0 0 0 0 M o n a n u s c o n c i n n u l u s 6 1 1 0 1 0 1 4 1 2 2 1 1 1 0 0 O r y r a e p h i 1 u s m e r c a t o r 0 0 0 0 0 0 0 0 0 0 0 0 0 0" 0 0 C o e n o n i c a s p . 0 0 0 0 0 0 0 I 0 0 0 0 0 0 0 0 C o p r o p o r u s s p . 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 G n a t o c e r u s m a x i l l o s u s 2 5 13 9 10 4 6 3 5 2 6 ' 17 5 1 15 11 12 14 2 7 1 9 P a l o r u s b o b i r i e n s i s 0 0 0 0 5 2 0 2 0 0 0 1 0 2 1 0 P . c a r i n i c o l l i s 0 0 0 0 2 0 0 0 0 0 0 0 0 0 0 0 P . c e r y l o n o i d e s 0 0 0 1 * 0 0 0 0 ' 0 0 0 0 0 0 0 0 P . f i c i c o l a 3 1 0 3 12 1 7 5 5 1 3 6 0 0 1 0 P . s u b d e p r e s s u s 18 14 14 11 3 1 4 0 2 9 2 1 4 9 3 3 3 5 2 1 2 6 6 3 36 3 8 _ P . c r a m p e 1 i 1 0 0 . 0 1 0 0 1 - 0 0 0 0 0 0 0 0* S i t o p h a g u s h o i o l e p t o i d e s 0 0 0 ' 0 0 0 0 . 0 0 0 » 0 0 0 0 0 0 T r i b o l i u m c a s t a n e u m 2 1 0 0 2 0 1 1' 1 - 0 1 0 0 3 0 0 T . c o n f u s u m 0 0 0 0 0 1 0 0 0 0 0 0' 1 0 0 0 P l a t y d e m a s p . 0 0 0 . 0 0 0 1 0 0 0 0 0 " 0- 0 0 0 i n d e t . C o l e o p t e r a 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 P y r o d e r c e s s p . 0 0 0 . 0 0 • 0 0 0 0 0 0 . 0. 0 0 0 0 S i t o t r o g a c e r e a T e l l a 10 3 4 4 9 2 4 2 4 7 4 • 3 . ? I" 3 3 C a r d i a s t e t h u s s p . 19 2 0 1 10 5 4 2 12 0 3 1 9 * 0 0 1 C a r d i a s t e t h u s s p . - n y m p h s 15 1 2 . 1 7 ' 1 1 2 2 0 3 0 • 4 3 0 0 L y c t o c o r i s c o c h i c i 3 1 0 2 3 3 4 3 3 . 2 4 l' 4 0 2 0 L i c o c h i c i n y m p h s 4 3 1 9 7 3 5 8 6 v -3 13 4 0 3 3 4 X y l o c o r i s a f e r 0* 1 0 0 0 0 0 0 0 0 0 0 0 0 0 n
X . a f e r n y m p h s 0 0 0 0 • 0 0 ' 0 0 • 0 0 . 0 0 0 0 0 0 " C e t h e r a m u s i v a 0 0 0 2 0 0 . 0 0 0 0 0 0 0 0 0 1 . i n d e t . E m e s i n a e 0 0 0 0 0 0 0 0 0 ' 0 .0 - 0 - 0 8 0 0 C . m u s i v a n y m p h s 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 P . b i a n n u l i p e s n y m p h s 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 S c o l o p o i d e s d i v a r e t i 5 0 0 6 1 0 3 1 4 e. 1 •y 3 3 1 2 3 S . d i v a r e t i n y m p h s 3 0 0 5 3 0 0 1 0 1 0 2 0 1 0 0 D i e u c h e s a r m . n y m p h s 0 0 0 0 0 1 • 0 ' 0 0 0 0 0 0 0 0 0 M i z a l d u s s p . 6 0 1 2 3 0 0 0 2 0 0 0 1 0 1 1 M i z a l d u s s p . n y m p h s 2 0 0 3 1 0 0 1 0 1 2 1 2 0 £ 0 i n d e t . B e t h y l i d a e 0 0 . 0 0 0 0 0 I 0 0 0 0 0 0 0 0 flnisopteromalus c a l a n d r a e 0 0 0 0 0 0 0 0 0 0 0 0 0 0 2 0 C e r o c e p h a l a d i n o d e r i 6 7 4 1 3 2 0 1 4 8 ".1 1 2 ^ 0 3
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S i t o p h i 1 u s s p . 3 4 9 3 6 0 2 5 4 166 2 4 4 2 5 1 1 9 6 186 1 7 2 1 9 1 164 1 2 3 2 6 8 184 114 3 0 i n d e t . H i s t e r i d a e 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 L y c t u s n r . a f r i c a n u s 1 0 0 0 0 0 0 . 0 0 0 0 0 0 1 0 0 M i n t h e a r u g i c o l l i s 0 0 0 0 0 0 0 0 • 0 0 0 0 0 0 0 0 L i t a r g u s b a l t e a t u s 5 2 0 0 5 7 9 2 1 2 0 4 2 1 0 2 T y p h a e a s t e r c o r e a 0 0 0 0 1 1 0 2 0 0 0 0 0 0 0 1 B . p i 1 o s e 1 1 u s 0 0 1 0 0 0 0 0 0 0 0 . 0 — 0 0 6 - 0 C . d i m i d i a t u s 3 4 2 8 3 0 14 2 5 2 6 3 9 2 2 17 11 2 1 8 2 3 6 11 14 C . f u m a t u s 15 .18 13 4 9 5 13 1 1 1 0 5 6 2 0 4 C . h e m i p t e r u s 1 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 L a s i o d a c t y l u s s p . 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 H y p o t h e n e m u s s p . 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 C a t h a r t u s q u a d r i c o l l i s 0 0 0 0 0 0 2 0 0 0 0 0 1 0 0 0 M o n a n u s c o n c i n n u l u s 0 0 0 2 4 2 0 1 3 0 1 2 1 4 d 0 C r y z a e p h i l u s m e r c a t o r 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 S i 1 v a n o p r u s s p . 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 C o e n o n i c a s p . 0 0 0 0 0 0 0 a 0 0 0 0 0 0 0 0 G n a t o c e r u s m a x i l l o s u s 15 2 0 2 0 5 2 8 2 5 15 3 9 16 11 2 1 • 6 19 8 9 6 P a l o r u s b o b i r v i e h s i s 0 0 0 1 0 0 0 0 0 0 0 0 2 0 0 0 P . c e r y l o n o i d e s 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 P . f i c i c o 1 a 2 0 0 1 0 1 4 6 4 6 7 2 2 0 o 0 P . s u b d e p r e s s u s 5 4 8 8 16 3 1 5 1 3 1 2 4 3 8 2 0 17 2 0 2 0 36 15 P . c r a m p e l i 0 0 0 0 4 0 0 0 0 0 0 0 0 0 0 0 T r i b o l i u m c a s t a n e u m 0 0 1 2 1 1 0 2 0 0 3' 0 0 2 0 3 T . c o n f u s u m . * 0 0 0 0 1 2 0 0 . 0 0 0 0 " 0 0 0 0 S i t o t r o g a c e r e a l e l l a 15 , 10 9 15 7 8 7 0 6 • 7 6 5 10 3 10 13 C a r d i a s t e t h u s s p . " 17 5 3 5 2 2 11 1 2 7 1 2 1 ,2 0 1 4 C a r d i e s t e t h u s s p . n y m p h s 7 0 0 1 5 3 O 3 4 3 0 3 1 0 0 ' 0 L y c t o c o r i s c o c h i c i 1 0 1 • 0 1 0 0 1 0 1 2 1 2 ? 0 1 L . c o c h i c i n y m p h s ' 1 1 3 2 2 5 5 5 5 4 0 1 0 5' 1 1 X y l o c o r i s a f e r 0 0 1 0 0 0 0 1 0 0 0 0 0 0 0 2 C e t h e r a m u s i v a 0 0 ' 0 0 0 0 0 0 0 0 0 0 n. 0 0 0 0 C . m u s i v a n y m p h s 0 0 0 0 • 0 0 0 0 2 0 1 0 ' 0 0 0 1 P . b i a n n u l i p e s n y m p h s ef 0 0 .' 0 1 0 0 0 0 0 .0 0 0 0 . 1 0 S c o l o p o i d e s d i v a r e t i 0 0 1 0 0 3 0 • 8 0 1 1 1 3. 0 1. 0 S . d i v a r e t i n y m p h s - 0 0 0 0 0 1 0 3 1 x 0 0 0 1 0 0 0 D i e u c h e s a r m . n y m p h s 0 0 1 0 0 0 0 0 1 1 0 0 • 0 ' 0 • 0 0 M i z a l d u s s p . l 0 0 0 0 .0 0- 1 1 0 1 • 0 . 0 0 0 1 M i z a l d u s s p . n y m p h s 0 2 0 0 1 3 0 0 6 ' 3 n 2 0 1 1 i n d e t . C h a l c i d i d a e • 0 0 0 0 0 0 0 0 0 0 0 0 0 , 0 0 0 i n d e t . B e t h y 1 i d a e 0 0 0 0 0 0 2 0 1 0 0 1 1 0 1 2 l e t e t i c o n t u s l a e u i g a t u s 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 E u p e l m u s u r o z o n u s 0 0 0 0 0 0 • 0 0 0 0 0 0 0 1 0 1 C e r o c e p h a l a d i n o d e r i 3 5 1 2 4 3 4 4 4 2 3 2 1 5 6 C h o e t o s p i l a e l e g a n s '6 7 3 6 9 1 6 2 4 5 5 7 10 e 6 5 H a b r o c y t u s c e r e a l e l la* 0 0 0 0 0 0 0 0 0 0 0 0 0 o 1 0 M e s o p o l o b u s s p . 0 0 0 0 0 0 0 0 1 1 0 0 0 o 0 0 i n d e t . S e e l i o n i d a e 0 0 0 1 1 0 0 0 0 0 1 0 0 I 0 1 i n d e t L a b i i d a e a d u l t s 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 i n d e t L a b i i d a e n y m p h s 0 ' 0 I 0 1 C. 0 0 0 0 0 0 1 0 0 0 V-": - " t;
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L a s i o d e r m a s e r r i c o r n e 1 8 8 0 0 0 0 8 0 0 0 0 0 0 0 flraecerus f a s c i c u l a t u s 1 8 8 0 0 8 0 0 0 0 0 0 1 0 0 0 D i n o d e r u s m i n u t O s 8 8 8 0 1 8 0 0 0 0 0 0 0 0 2 0 i n d e t . C o r y l o p h i d a e 8 8 8 0 1 8 0 4 0 0 4 1 • 0 0 C r y p t o l e s t e s s p p , 2 7 16 19 6 2 9 3 8 14 3 9 9 5 11 3 5 9 16 5 P l a c o n o t u s s p p . 8 8 8 0 8 8 0 0 0 0 0 0 0 0 0 0 Si t o p h i 1 u s s p . 2 5 9 2 4 6 2 3 7 2 2 8 1 8 1 2 9 0 1 2 7 2 3 3 3 2 5 3 5 6 3 4 9 2 2 7 4 1 6 3 5 7 2 1 0 2 9 3 i n d e t . H i s t e r i d a e 8 8 8 0 ' 8 8 0 0 0 0 0 0 0 0 0 0 L y c t u s n r . a f r i c a n u s 8 8 8 0 1 8 8 0 0 0 0 0 0 0 0 0 L i t a r g u s b a l t e a t u s 8 8 1 3 5 1 4 0 1 2 1 1 2 0 4 0 L . s u a r i u s ' 8 8 8 0 8 8 8 0 0 1 0 0 0 0 0 0 T y p h a e a s t e r c o r e a .1 8 8 0 8 1 8 2 1 0 0 0 0 0 7 0 B . p i 1 o s e 1 1 u s 8 8 1 1 8 8 8 0 0 0 0 0 0 0 S " 0~ C . d i m i d i at u s 19 13 2 3 15 3 8 4 4 9 3 8 44 2 6 3 9 4 3 4 7 2 3 54 14 C . f u m a t u s 1 8 1 8 8 2 8 2 0 * 8 1 0 1 0 1 0 C . h e m i p t e r u s 8 8 8 0 8 8 8 0 0 0 0 1 0 0 2 0 C . o b s o l e t u s 8 8 8 0 8 8 8 0 0 0 0 1 0 0 0 0 L a s i o d a c t y 1 u s s p . 8 8 8 0 0 1 8 0 0 0 0 2 0 0 0 0 i n d e t . S c o l y t i d a e 8 8 8 0 0 8 8 0 0 0 8 0 • 0 0 0 0 H y p o t h e n e m u s s p . 8 8 8 0 1 1 8 0 0 0 0 0 0 0 0 0 C a t h a r t u s q u a d r i c o l l i s 8 1 8 1 0 0 0 0 0 1 0 0 0 8 0 0 M o n a n u s c o n c i n n u l u s 8 8 2 2 1 1 0 1 1 0 2 0 0 1 7 0 i n d e t . S t a p h y 1 i n i d a e 8 8 8 0 0 8 0 0 8 0 0 0 0 0 2 3 C n a t o c e r u s m a x i l l o s u s 9 19 8 9 9 15 12 14 2 4 10 34 7 16 8 .17 13 P a l o r u s l > o b i r i e n s i s 8 8 * 8 1 0 8 0 1 0 1 1 0 1 0' 0 0 P . c a r i n i c o l l i s 8 8 8 0 2 1 0 0 2 0 0 1 0 0 0 0 P . c e r y l o n o i d e s 8 8 8 0 0 0 0 8 0 0 0 0 0 0 0 0 P . f i c i c o l a 8 8 8 1 3 5 3 14 8 2 1 • 3 0 0 0 '0 P . s u b d e p r . e s s u s 3 3 5 8 5 1 6 0 4 0 6 7 6 4 6 7 4 9 4 0 4 7 4 0 4 4 7 5 1 0 2 5 3 S i t o p h a g u s h o i o ^ e p t o i d e s 8 8 8 0 0 8 0 8 0 0 0 0 0 . 1 0 0 -T r i b o l i u m c a s t a n e u m 2 2 3 0 5 3 2 2 5 7 { 0 3 1- 1 4 P * l a t y d e m a s p . 8 8 8 0 0 8 0 8 0 0 ' 0 1 0 0 • 0. 0 i n d e t . C o l e o p t e r a • 8 8 0 0 0 8 .0 8 0 0 0 0 0 0 0 O P y r o d e r c e s s p . 8' 8 8 0 1 8 8 0 0 0 0 0 0 0 0 0 S i t o t r o g a c e r e a l e l l a 18 4 9 3 9 7 8 5 O 4 . 4 0 3 3 7 C a r d i a s t e t h u s s p . 2 8 8 0 6 8 2 4 7 1 2 2 6 1 0 " C a r d i a s t e t h u s .sp. n y m p h s 2 8 8 0 1. 1 8 4 2 0 • 0 0 2 0 1 1 L y c t o c o r i s c o c h i x i 8 1 8 0 0 " 8 8 0 0 ' 2 1 0 0 0 8 L . c o c h i c i n y m p h s 8 2 2 1 0 1 1 0 1 2 5 1 0 2 4 1 X y l o c o r i s afer. 8 8 8 0 0 • 8 8 I 0 0 . 0 1. 0 0 1 0 X . a f e r n y m p h s 8 8 8 0 0 0 8 1 0 0 0 1 0 0 1 0 P e r e g r i n a t o r b i a n n u l i . p e s 8 8 8 0 0 1 . 8 0 0 > 0 0 0 0 1 1 0 P . b i a n n u l i p e s n y m p h s ' 8 8 8 0 0 0 1 0 1 . 0 0 0 0 0 0 0 C . m u s i v a n y m p h s 8 8 1 0 0 0 8 0 1 0 0 . ' 1 0 0 1 0' S c o l o p o d e s d i v a r e t i ' 3 8 1 X 3 4 7 3 0 0 2 0 " 0 4 0 " S . d i u a r e t i n y m p h s 1 1 8 1 0 8 8 4 . 1 0 . 2 3 1 0 0 0 D i e u c h e s a r m a t i p e s 8 8 8 0 0 8 8 1 0 0 0 0 0 0 0 0 D . a r m a t i p e s n y m p h s 1 8 8 0 0 8 0 1 1 0 0 .0 0 0 >J 2 M i z a l d u s s p . 1 1 2 3 1 1 1 2 1 2 0 1 0 1 2 1 M i z a l d u s s p . n y m p h s 4 1 4 0 3 2 2 0 1 0 1 0 0 10 1 i n d e t . C h a l c i d i d a e 8 8 8 0 0 0 8 0 0 0 1 0 0 0 0 0 i n d e t . B e t h y l i d a e 1 8 8 0 1 1 2 0 1 0 1 0 0 0 4 1 E u p e l m u s u r o z o n u s 8 e 8 0 0 0 8 0 0 0 0 0 0 0 0 0 C e r o c e p h a l a d i n o d e r i 4 7 3 0 4 10 6 1 5 • 6 4 1 5 4 ~> 4 C h o e t o s p i l a e l e g a n s 9 13 3 2 7s
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L a s i o d e r m a s e r r i c o r n e 2 0 1 0 0 flraecerus f a s c i c u l a t u s 1 0 0 0 0 B o s t r y c h o p l i t e s c o r n u t u s 0 0 0 0 0 D i n o d e r u s m i n u t u s 0 0 0 0 0 H e t e r o b o s t r y c h u s b r u n n e u s 0 0 0 0 0 N e c r o b i a r u f i p e s 0 0 0 0 0 M i c r o p r i u s c o n f u s u s 0 0 0 0 0 P s e u d o b o t h r i d e r e s c o n r a d . 0 0 0 0 0 i n d e t . C o r y l o p h i d a e 1 1 0 0 0 C r y p t o l e s t e s s p p . 42 7 11 22 10 P l a c o n o t u s s p p . 0 0 0 0 0 Si t o p h i 1 us s p . 172 120 106 76 105 i n d e t . H i s t e r i d a e 0 0 0 0 0 L y c t u s ? a f r i c a n u s 0 0 0 0 0 M i n t h e a r u g i c o l l i s 0 0 0 0 0 L i t a r g u s b a l t e a t u s 7 2 1 5 3 T y p h a e a s t e r c o r e a 0 0 0 0 0 B . pi 1ose11 us 0 0 0 0 1 C . di m i di at us 19 7 6 12 17 C . f u m a t u s 37 7 3 2 3 L a s i o d a c t y 1 us s p . 0 0 0 0 0 H y p o t h e n e m u s s p . 0 0 0 0 0 C a t h a r t u s q u a d r i c o l l i s 0 0 1 0 0 M o n a n u s c o n c i n n u l u s 1 1 1 1 1 G n a t o c e r u s m & x i l l o s u s 20 6 12 9 36 Pal e m b u s i v o i r e n s i s 0 ' 0 0 0 1 P a l o r u s b o b i r i e n s i s 1 0 0 1 0 P . c a r i n i c o l l i s 0 0 1 0 1 P . f i c i c o l a 3 1 0 1 6 P . s u b d e p r e s s u s 2 5 10 17 15 34 P . c r a m p e 1 i 0 0 0 0 0 Sitophaflus hoi o l e p t o i d e s 0 a 0 0 0 T r i b o l i u m c a s t a n e u m 0 I 1 0 2 T., c o n f u s u m 0 0 0 0 0 S t o m y l u s s p . 0 0 0 0 . 0 L a t h e t i c u s o r y z a e 0 o 0 0 0 i n d e t . C o l e o p t e r a • '0 0 0 0 0 S i t o t r o g a c e r e a l e l l a 8 7 15 5 7 C a r d i a s t e t h u s s p . 14" 1 1 - 1 7 C a r d i a s t e t h u s s p . n y m p h s 9 0 0 0 0 L y c t o c o r i s c o c h i c i 6 -3 3 . 3 5 L . c o c h i c i n y m p h s 2 2 1 9 2 X y l o c o r i s a f e r 0 0 0 0 0 X . a f e r n y m p h s 0 0 0 0 0 P . b i a n n u l i p e s n y m p h s 0 0 0 0 0 S c o l o p o i d e s d i v a r e t i 3 1 0 0 6 S . d i v a r e t i n y m p h s 3 1 0 0 2 i n d e t . flnthocoridae 0 0 0 1 2 i n d e t . flnthoc. n y m p h s 0 0 0 1 0 D . ? a r m a t i p e s n y m p h s 0 0 0 0 0 M i z a l d u s s p . 2 0 0 0 1 M i z a l d u s s p . 0 0 0 0 1 i n d e t . B e t h y l i dae 0 0 0 0 0 E u p e l m u s u r o z o n u s 0 0 0 0 0 finisopteromalus c a l a n d r a e 1 0 0 0 0 C e r o c e p h a l a d i n o d e r i 3 1 2 2 1 C h o e r o s p i l a e l e g a n s 1 3 4 1 1 H a b r o c y t u s c e r e a l e l l a e 0 0 0 0 0 M e s o p o l o b u s s p . 0 0 0 0 0 i n d e t . S c e l i o n i d a e ' 0 0 0 0 0 indet L a b i i d a e 0 0 0 1 0 indet L a b i i d a e n y m p h s 0 0 • 0 1 0
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L a s i o d e r m a s e r r i c o r n e . 0 0 1 0 0 1 0 0 0 0 1 0 0 1 0 0 flraecerus f a s c i c u l a t u s 0 0 0 0 2 0 0 1 1 1 1 2 1 0 1 0 B o s t r y c h o p 1 i t e s c o r n u t u s 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 D i n o d e r u s m i n u t u s 0 0 0 0 0 1 0 0 0 0 0 2 0 0 0 0 irfdet. C o r y l o p h i d a e 2 0 0 2 3 0 0 0 o 0 2 1 0 0 0 2 C r y p t o l e s t e s s p p . 2 9 12 22 54 39 31 26 50 29 9 27 10 10 9 20 26 P l a c o n o t u s s p p . 0 0 0 0 1 0 0 0 0. 0 0 0 0 0 0 0 P s e u d o s t e n o t r u p i s s p . 0 0 0 . 0 0 0 0 0 0 0 0 0 0 0 0 0 Si t o p h i 1 us s p . 2 1 1 167 180 142 2 0 2 172 153 77 324 283 267 159~ 305 2 7 1 191- 1 l"o' i n d e t . H i s t e r i d a e 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 L i t a r g u s b a l t e a t u s 1 0 0 0 4 3 1 8 1 0 0 3 2 0 1 0 L . w a r i u s ' 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 T y p h a e a s t e r c o r e a 0 0 0 0 1 0 0 2 1 0 0 0 0 0 0 0 B r a c h y p e p l u s ? g a b o n e n s i s 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 C . di mi di a t u s . 15 8 2 6 2 2 2 1 20 24 39 1 1 39 17 20 8 15- 9 C . f u m a t u s 1 0 0 0 0 1 1 0 5 1 2 0 0 0 1 0 H y p o t h e n e m u s s p . 0 0 0 0 1 1 0 0 0 0 0 0 0 0 0 0 C a t h a r t u s q u a d r i c o l l i s 0 0 0 0 0 0 0 0 0 2 0 0 0 0 0 0 M o n a n u s ? c o n c i n n u l u s 1 0 0 0 2 0 0 0 5 2 5 2 2 0 0 0 O r y z a e p h i l u s m e r c a t o r 0 0 0 0 0 0 0 1 0 2 0 0 0 0 0 0 G n a t o c e r u s m a x i l l o s u s 17 6 26 15 34 2 1 13 12 32 19 19 9 6 15 20 9 P a l o r u s b o b i r i e n s i s 0 0 0 0 0 1 1 0 1 0 0 0 0 0 1 0 P . c a r i n i c o l l i s 0 0 0 0 0 0 0 0 2 0 0 0 0 0 0 0 P . e e r y l o n o i d e s 0 0 0 0 0 0 1 0 0 0 0 0 0 0- 0 0 P . H e icol a 1 0 0 0 11 3 2 3 25 2 2 3 4 0 0 0 P . s u b d e p r e s s u s . . JL08 24 7 9 6 1 _ 90 137 89 6 2 199 202 140 54 183 100 . 146 _ 102 S i t o p h a g u s h o i o l e p t o i d e s 1 0 0 0 0 0 0 0 • 1 0 0 0 • 0 0 0 0 T r i b o l i u m c a s t a n e u m 1 1 4 1 0 1 0 4 15 1 3 2 3 1 4 1 P l a t y d e m a s p . 0- 0 0 . 0 0 0 0 0 .0 0 0 0 0 0 0 0 S i t o t r o g a c e r e a l e l l a 8 2 8 5 5 5 3 3 5 6 4 1 2 4 1 4 C a r d i a s t e t h u s s p . 11 1 0 . 2 16 3 2 0 13 1 3 0 5 0 0 0 C a r d i a s t e t h u s s p . n y m p h s - 5 1 0 1 2 0 0 0 1 2 " 2 0 3 0 0 0 L y c t o c o r i s c o c h i c i 1 • 2 1 0 1 0 1 0 1 4 0 1 •0 2 • 2 3 L . c o c h i c i n y m p h s •v 1 1 2 1 1 0 0 . 0 2 3 0 '4 0 O . 0 0 X y l o c o r i s a f e r 0 0 0 1 0 0 0 e 0 0 2 0 0 0 0 1 X . a f e r n y m p h s 0 0 0 1 0 1 0 0- 0 0 0 2 . 0 . 0 0 3 P e r e g r i n a t o r b i a n n u l i p e s 0 ' 0 0 0 0 0 0 0 0 0 1 0 0 ' 0 0 1 P . b i a n n u l i p e s n y m p h s .0 . 0 0. 0 0 0 0 0 - 0 ' 0 0 o" 0 1 0 1 S c o l o p o i d e s d i v a r e t i 6 1 3 4 9 8 6 7 19 9 * 15 .9 3 0 3 4 S . d i v a r e t i n y m p h s 12 0 0 5 12 4 10 28 15 2 14 5 2 5 1 (
i n d e t . finthocoridae 1 ' 0 0 0 . 0 0 0 1 0 0 0 0 0 ' 0 0 • 0 i n d e t . flnthoc. n y m p h 0 0 0 0 0 0 0 2 0 0 0 . 0 0 0 0 0 D . ? a r m a t i p e s n y m p h s 0 0 0 0 0 0 0 0 0 0 3 1 0 0 0 2 M i z a l d u s s p . 2 2 1 2 1 2 3 0 2 1 0 0 0 1. J •p M i z a l d u s s p . n y m p h s 1 1 4 5 2 0 3 0 0 5 0 0 4 3 1 1 i n d e t . B e t h y l i d a e 0 0 0 1 0 0 1 1 0 0 0 0 0 2 0 flnisopteromalus c a l a n d r a e 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 C e r o c e p h a l a d i n o d e r i 3 7 3 3 3 6 7 4 8 8 3 6 7 9 8 1 C h o e t o s p i l a e l e g a n s 6 1 6 2 10 8 7 5 10 7 10 7 18 11 11 3 H a b r o c y t u s c e r e a l e l l a e 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 M e s o p o l o b u s s p . 0 1 0 0 0 • 0 0 0 0 0 0 0 0 0 0 0 i n d e t . S c e l i o n i d a e 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 indet L a b i i d a e a d u l t s 0 0 0 1 0 0 1 0 0 • 0 0 0 0 0 0 0
D I S T R I B U T I O N T R I A L IV C r i b 2 s a m p l e 3 e m e r g e n c e s from lO0g g r a i n / one week
S a m p l e 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 g r a i n m . c . / X 1 7 . 5 16.6 15.9 16.6 1 7 . 1 16.7 16. 1 16.6 16.2 16.6 15. 7 16.8 16.5 16.0 15.9 15.3
L a s i o d e r m a s e r r i c o r n e 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 A r a e c e r u s f a s c i c u l a t u s 0 0 0 0 0 0 0 0 2 0 0 0 1 0 0 0 D i n o d e r u s m i n u t u s 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 R h y z o p e r t h a d o m i n i c a 0 0 8 0 0 0 0 0 0 0 0 0 0 0 0 0 C r y p t o l e s t e s s p p . 6 0 2 9 9 5 4 4 10 4 6 2 4 2 5 4 Si t o p h i l u s s p . 9 3 3 6 11 7 6 9 15 3 8 13 Q 5 3 8 L y c t u s ? a f r i c a n u s 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 L i t a r g u s b a l t e a t u s 0 0 0 0 0 1 0 0 8 0 8 0 <0 0 0 0 C . ditnidiatus 4 1 8 1 2 0 0 2 0 0 0 0 0 0 0 0 C . pi 1 o s e l l u s 0 0 0 0 0 1 1 0 0 0 0 0 1 0 8 0 H y p o t h e n e m u s s p . 0 0 0 0 2 1 1 0 0 0 0 0 1 0 0 0 M o n a n u s ? c o n c i n n u l u s 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 i n d e t . S i l u a n i d a e 0 0 0 0 0 0 0 0 0 0 0 0 0 0 8 0 G n a t o c e r u s m a x i l l o s u s ' 12 1 1 3 3 5 11 13 2 1 1 1 1 1 4 7- 9 1 6 6 P a l o r u s b o b i r i e n s i s 0 0 0 0 0 0 0 0 0 1 0 0 0 0 1 8 P . f i c i c o l a 0 0 8 0 0 0 2 2 0 1 0 0 0 0 0 8 P". s u b d e p r e s s u s 0 0 8 0 9 4 0 0 3 3 0 2 0 4 2 " "0— T r i b o l i u m c a s t a n e u m 0 0 0 0 0 0 0 0 0 1 0 0 0 0 1 0 S i t o t r o g a c e r e a l e l l a 1 0 1 2 2 5 3 1 1 1 4 2 7 5 2 2 R h a b d e p y r i s zeae 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 i n d e t . Bet hy1i dae 0 0 8 1 0 0 0 0 0 0 1 0 0 0 0 8 C e r o c e p h a l a d i n o d e r i 2 0 4 1 3 2 3 2- 1 3 3 1 0 2 2 1 C h o e t o s p i l a e l e g a n s 5 3 8 1 11 «j 8 3 6 4 2 2 7 7 3 0 M e s o p o l o b u s s p . 0 0 1 0 0 1 0 0 0 0 1 0 0 0 1 0 . i n d e t . S c e l i o n i d a e 0 0 8 0 0 1 8 2 0 0 0 0 0 0 0 0
S a m p 1e 17 18 19 20 2 1 2 2 23 24 2 5 26 - 27 ' 23 29 -30 31* 32 ~ g r a i n ID. c . s X 15.6 15.2 15. 5 15.4 16.2 15.6 15.4 15.2 16.0 17. 1 16. 3 16.2 15.6 15.2 15.0 J 5 . 3
L a s i o d e r m a s e r r i c o r n e 0 0 0 0 0 0 1 0
J
! 0 ' 0 0 0 0 0 0 8 A r a e c e r u s f a s c i c u l a t u s 0 0 0 0 0 0 8 0 i 0 •0 0 0 0 0 0 8 D i n o d e r u s m i n u t u s . 0 0 0 0 0 " 0 8 R ! ' 0 0 0 0 0 0 0 0 R h y z o p e r t h a d o m i n i c a 0 0 0 0 0 0 8 I 0 0 0 0 0 0 ••0 0 C r y p t b l e s t as spp.' 4 2 5 6' 1 12 12 6 S 7 3 4 5 0 3 & -
Si t o p h i 1 us s p . 10 4 7 5 10 10 12 9 ! 9 8 14- 14 10 . 8 3 7 L y c t u s ''africanus 0 0 0 0 0 0 8 0 ! '0 1 0 0 0 0 0 0 L i t a r g u s ba.lt eat us 0 0 0 0 ' 0 0 8 0 ! 0 0 '0 0 1 0 • 0 0 ~ C . di m i d i at us 0 0 0 0 0 3 0 0 3 1 0 1 2 2 0 C . pi 1 o s e l 1 us 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 H y p o t h e n e m u s s p . 1 0 0 0 0 0 0 8 1 0 ' 0 0 0 0 0 0 0 M o n a n u s ? c o n c i n n u l u s 0 0 0 0 0 0 0 0 0 0 2 1 0 0 0 1 i n d e t . S i l v a n i d a e 0 0 0 0 0 1 0 0 0 0 0 - 0. 0 0 0 0 G n a t o c e r u s m a x i l l o s u s 9 9 15 6 5 5 4 0 8 7 2 4 10 6 4 6 P a l o r u s b o b i r i e n s i s 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 9 P . f i c i c o l a 0 0 8 0 0 2 0 1 0 0 0 2 0 0 0
3 0
P . s u b d e p r e s s u s 0 2 2 1 3 6 2 1 * 5 1 3 3 4 e. 0 3
T r i b o l i u m c a s t a n e u m 0 0 0 0 0 1 1 0 1 0 1 1 0 1 0 • 0 S i t o t r o g a c e r e a l e l l a 1 1 0 1 3 1 2 1 5 3 0 1 • 3 0 0 R h a b d e p y r i s zeae 0 0 0 0 0 0 0 0 0 0 0 0 1 2 0 0 i n d e t . B e t h y 1 i dae 0 0 0 0 0 0 0 8 0 0 0 0 0 0- 0 0 C e r o c e p h a l a d i n o d e r i 3 3 2 1 2 7 5 . 1 2 1 0 2 4 1 ,1 C h o e t o s p i l a e l e g a n s 4 6 2 3 4 2 5 3 6 6 0 9 9 5 1 6 M e s o p o l o b u s s p . 0 0 2 0 0 0 0 8 0 0 •0 0 0 0 . 0 0. i n d e t . S c e l i o n i d a e 0 ' 0 0 0 0 0 0 1 0 0 0 0 ' 0 0" 0 0
D I S T R I B U T I O N T R I A L IV C r i b 3 s a m p l e 2 e m e r g e n c e s f r o m 100g g r a i n / one week
S a m p 1e g r a i n m.c / 'A 16.
2 3 4 5 6 7 8 17.9 18.8 2 0 . 8 18.5 17.5 17.9 2 0 . 2
9 10 11 12 13 14 15 16 17.0 17.2 13.2 18.9 17.0 16.4 17.4 17.3
L a s i o d e r m a s e r r i c o r n e 0 0 0 0 0 0 0 0 0 1 0 0 0 8 0 0 flraecerus f a s c i c u l a t u s 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 D i n o d e r u s m i n u t u s 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 C r y p t o l e s t e s s p p . 2 3 3 5 2 5 6 1 4 4 3 3 0 0 1 2 Si t o p h i 1 us s p . 17 5 4 18 10 9 13 9 9 11 4 10 13'' 9 10 8 C . d i m i d i a t u s 1 1 0 0 0 1 1 0 4 0 0 0 0 0 0 0 C . pi 1osel1 us 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 H y p o t h e n e m u s s p . 0 0 0 1 0 0 1 0 0 0 0 0 0 0 0 0 M o n a n u s ? c o n e i n n u l u s 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 G n a t o c e r u s m a x i l l o s u s • 3 4 7 3 9 5 7 3 9 9 4 e. 6 14 5 4 P . fi c i col a 0 0 0 0 0 0 0 0 1 1 0 0 2 0 0 0 0 P . s u b d e p r e s s u s 0 0 0 0 0 0 1 2 0 6 3 1 1 1 0 0 T r i b o l i u m c a s t a n e u m 0 0 0 0 0 0 0 0 0 1 £ _0 0 0 0 0 S i t o t r o g a c e r e a l e l l a 4 3 1 1 3 1 0 4 5 2 0 1 7 2 2 2 i n d e t . C h a l c i d i d a e 0 0 9 0 0 0 0 0 0 0 0 1 •0 0 0 0 i n d e t . B e t h y l i d a e 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 E u p e l m u s u r o z o n u s 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 flni sopt e r o m a l us c a l a n d r a e 0 0 0 0 0 0 0 0 • 0 0 0 0 0 1 0 0 C e r o c e p h a l a d i n o d e r i 2 2 0 1 2 0 . 2 1 4 2 1 4 2 4 1 0 C h o e t o s p i l a e l e g a n s 4 2 1 1 5 5 2 3 9 5 3 4 2 2 3 2 M e s o p o l o b u s s p . 0 1 0 0 0 0 0 0 0 0 0 0 0 2 0 0
S a m p 1e g r a i n m . c . /
17 18 19 20 21 22 23 24 16.7 16.4 17.4 18.0 16.9 16.5 16.7 17.9
25 26 27 23 17.0 1 7 . 1 1S.1 18.6
29 30 31 32 17.0 16.3 18.4 16.1
L a s i o d e r m a s e r r i c o r n e 0 0 0 . 0 0 • 0 0 0 0 0 • 0 0 0 • 0 ' 0 0 flraecerus f a s c i c u l a t u s 0 0 0 ' 0 • 0 0 0 1 0 0 0 ' 0 . 0' • 0 0 0 D i n o d e r u s m i n u t u s 0 0 0 0 0 0 0 0 0 •0 . 0 0 0 O 0 0 C r y p t o l e s t e s s p p . 4 * . 0 5 3 6 4 12 6 3 " 5 3 3 0 0 0 0 Si t o p h i 1 us s p . 19 9 10 20 10 11 12 18 12 13 12 23 12 6 14 C . d i m i d i a t u s 1 0 0 0 0 1 1 1 1 0 0 0 1 0 " 0 0 C . pi 1 o s e 1 1 us 0 0 0 0 0 0 1 0 0 0 . 0 0 0 1 1 1
0 H y p o t h e n e m u s s p . 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0
M o n a n u s ? c o n c i n n u l u s 0 0 0 0 0 0 0 0 0 0 • 1 0 8 0 0 0 G n a t o c e r u s m a x i l l o s u s 13 13 18 5 6 15 9 10 1 6 4 6 10 8 6 P . f i c i c o l a 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 P . s u b d e p r e s s u s 2 6 7 1 5 2 4 .8 2 4 - 5 6 1 2 6 4 T r i b o l i u m c a s t a n e u m 0 2 2 0 0 0 2 0 0 0 0 0 0 1 0 1 S i t o t r o g a c e r e a l e l l a 7 2 1 1 1 0 0 2 0 2 7 3 1 5 0 0 i n d e t . C h a l c i d i d a e 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 i n d e t . B e t h y l i d a e 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 E u p e l m u s u r o z o n u s 0 0 0 0 0 0 0 0 0 1 0 . 0 0 0 0 o flnisopteromalus c a l a n d r a e . 0 0 0 0 0 0 0 0 0 0 0 • 0 0 0 0 0 C e r o c e p h a l a d i n o d e r i 4 2 • 2 2 7 2 2 1 1 4 2 5 9 2 1 5 C h o e t o s p i l a e l e g a n s 5 3 4 3 13 8 4 5' •p . 7 10 10 8 a 2 10 M e s o p o l o b u s s p . 0 1 0 0 0 0 0 1 0 1 0 0 0 1 • 0 0
D I S T R I B U T I O N T R I A L IV C r i b 3 s a m p l e 3 e m e r g e n c e s f r o m 100g g r a i n / o n e week
S a m p l e g r a i n m . c .
1 2 3 4 5 6 7 8 16.9 17.1 16.7 16.3 17.3 17.4 17.8 16.7
9 10 11 12 13 17.0 16.9 17.3 17.0 17.3
14 16.4
15 16 16.0
A r a e c e r u s f a s c i c u l a t u s 0 0 1 0 0 8 0 1 0 1 1 1 2 0 0 0 D i n o d e r u s m i n u t u s 0 0 0 0 0 8 0 0 0 0 0 0 0 0 0 1 C r y p t o l e s t e s s p p . 16 19 14 7 12 13 13 6 8 9 5 6 3 5 2 5 Si t o p h i 1 us s p . 11 4 18 12 8 6 6 14 7 6 1 1 7 16 3 4 4 L y c t u s ? a f r i c a n u s 0 0 0 0 8 8 0 0 0 0 1 0 0 0 0 0 T y p h a e a s t e r c o r e a 0 1 0 0 8 8 0 0 0 0 0 0 0 0 0 0 C . d i m i d i a t u s 1 3 0 0 1 8 0 0 0 1 0 2 1 0 0 0 C . f u m a t u s 0 0 0 0 0 8 0 0 0 0 0 1 0 0 0 0 C . p i l o s e l l u s 0 0 1 0 0 8 0 0 0 1 0 0 0 1 0 0 C . z e a p h i 1 us 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 M o n a n u s ? c o n c i n n u l u s 0 0 0 0 0 1 0 0 0 0 0 0 2 0 0 0 G n a t o c e r u s m a x i l l o s u s 4 6 2 5 8 3 4 3 5 2 3 2 2 8 2 8 P a l o r u s b o b i r i e n s i s • - 0 ...... 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 P . f i c i c o l a 1 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 P . s u b d e p r e s s u s 4 0 5 2 3 2 7 l 4 5 3 4 4 2 0 2 P . indet 0 0 0 0 . 0 0 0 0 0 0 0 0 0 0 0 0 S i t o p h a g u s h o i o l e p t o i d e s 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 T r i b o l i u m c a s t a n e u m 1 0 0 0 1 0 0 0 0 0 2 0 0 0 1 0 S i t o t r o g a c e r e a l e l l a 4 6 4 4 0 4 1 2 1 4 4 0 1 1 5 3 R h a b d e p y r i s z e a e 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 i n d e t . B e t h y l i d a e 1 0 0 0 1 0 1 0 0 0 0 0 0 0 0 0 E u p e l m u s u r o z o n u s 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 A n i s o p t e r o m a l u s c a l a n d r a e 1 0 0 0 1 0 0 0 0 '0 0 0 0 0 0 0 C e r o c e p h a l a d i n o d e r i 1 5 6 1 4 2 ' 2 2 7 1 2 5 7 r 1 0 C h o e t o s p i l a e l e g a n s 4 8 . 6* 3 7 4 3 2 i 2 8 4 8 8 9 1 5 M e s o p o l o b u s s p . 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0
S a m p l e 17 18 19 20
—ar
2 1 22 * 23 24 25 26 27 28 29 30 * 31 32 .grai n m . c . / v. 16.4 16.2 15.6 15.6 1 6 . 9 16.2 15.9 16. 1 16.5 16.5 16.6 16.6 16.7 16.3 16.0 16.2
*
A r a e c e r u s fasc i cu l.atus D i n o d e r u s m i n u t u s C r y p t o l e s t e s s p p . Si t o p h i fus s p . . L y c t u s ? a f r i c a n u s T y p h a e a s t e r c o r e a C . d i m i d i at us C . f u m a t u s C . pi 1 o s e l 1 us C . z e a p h i1 us M o n a n u s ? c o n c i n n u l u s G n a t o c e r u s m a x i l l o s u s P a l o r u s b o b i r i e n s i s P . f i c i c o l a P . s u b d e p r e s s u s P . indet
S i t o p h a g u s h o l o l e p t o i d e s T r i b o l i u m c a s t a n e u m S i t o t r o g a cere'alella R h a b d e p y r i s zeae i n d e t . B e t h y l i d a e E u p e l m u s u r o z o n u s A n i s o p t e r o m a l u s c a l a n d r a e C e r o c e p h a l a d i n o d e r i C h o e t o s p i l a e l e g a n s M e s o p o l o b u s s p .
1 0
11 16 0 0 0 0 0 0 0
15 0 0 5 0 0 0 2 0 0 0 0 2 7 0
0 0 0 0 0 0 4 3 6 £ 6 10 & 0 0
• 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 8 6" 11 6 0 0 0 0 0 0 5 5 7 0 0 0 0 0 0 0 0 0 8 5 1 0 0 0 0 1 1 0 0 0
"0 0 , 0 3 3 6.
'9 5 4 3 0 0
0 0
• 7 13 0 0
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24 0 0 6 0 0 0 5 0
• 1 0 0 4 . 3 O
0 0 6 6 0 0
. 1 0 0 0 0 5 1 1 3 0
. 0 1 3 0 0 0
,-0 2
13 0
0 i 6
11 0 0 1 0 0 0 0 6 0 0 0 0 0 0 5 0 1 0 0 3 10 0
1 0 4
19 0 0 0 0 0 0 0 8 1 3
19 0 0 0 3 0 0 0 0 2 •3 a
1 0 2 11
• 0 0 0 0 0 0 0 18 0 0
14 0 1 0 2 0. 0 0 '0 3 6 0
0 0 1
14 0 0 1 0 0 1 0 7 0 0
10 0 0 0 3 0 1 0 •0
1 1 0
5 0 0
16 •0 0 4 0 0 0 0 Q
1 0 1 1 1 0
3 0 5 11 0 0 0 0 0 0 0 8 0 0 1 1 0 0 1 3 1 1 0 0 7
12 0
0 0
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0 0 2 0 0 0
0 0 0
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2 8 8 .
APPENDIX IV : Methods for estimation of moisture contents of grain and
cores.
The method used for estimation of grain moisture content was based
on the International Organisation for Standardisation routine reference
method (I.S.O., 1979). Briefly, this specifies that a sample of cereal
(larger than 5 g) should be ground (maximum particle size specified),
weighed accurately, heated in a well-ventilated oven (minimum ventilation
specified) at 130-133°C for 2 hours (90 mins. for flours), cooled in a
desiccator and then reweighed.
The moisture content of that sample is then given by the change in
weight according to the formula:
m.c. = (m - raj x 100 o 1 m o
where mQ is the initial sample weight (after grinding) and m^ the final
weight after drying.
Samples of moisture content, higher than c. 17% are 'preconditioned1
(i.e. dried as whole grain) for 7-10 minutes before being allowed to cool
and then treated as above. The moisture content in this case is given as
x 100 m.c. = (m - m j m„ + m0 - m0 o 1 J l 5 m o
m 2
where m^ is the sample weight before preconditioning and m^ the weight
after preconditioning and before grinding.
Because of the large number of determinations required for this study
the possibility of simplifying the method was investigated, with a view
to speeding up the" handling of samples while retaining sufficient accuracy
and repeatability. Practical constraints were also imposed by the type
289 .
APPENDIX IV : Continued.
of grinders and the small size of desiccators available.
Initially the possibility of heating whole grains was considered.
However samples did not reach 'stable weight1 within a reasonable time
(Fig. IV .1). Indeed, by the end of the period, whole grain samples had
lost more weight than equivalent ground samples (which had reached stable
weight), indicating that in the former some loss of dry matter had occurred.
A suitable powered gravity-feed knife mill was not available for
routine use but one was obtained briefly for initial 'calibration1.
Results from samples ground on this mill were compared with those from a
hand-operated plate-mill and from an electric knife mill without a gravity-
feed system (similar to a domestic coffee-grinder). The hand-mill
produced samples of coarser grade than that specified in the I.S.O.
method, but was fast and convenient to use. The electric mill produced a
very'fine flour but left a small numbfer of large fragments: increasing
the grinding "time did not reduce these fragments but resulted in heating
of the sample * Samples from this mill had to be sieved before drying and
it was felt that this process might bias the results-by preferentially
selecting flour endosperm (the fragments removed being from the hard
vitelline part).
The moisture content estimates for five samples (from the same grain)
ground on each of the three types of mill and heated for 120 minutes did
not differ significantly on a single factor analysis of variance at the
5% level. Samples ground on both the hand and routine electric mills
reached stable weight in approximately the same time (Fig. IV .2), despite
the-coarser grain of the former samples. The hand mill was therefore
290 .
APPENDIX IV : Continued.
chosen for routine use as being more convenient to use.
Allowing samples to cool in a desiccator before weighing did not
discernably affect the estimate of dry weight (possibly because the top-pan
balance used was too heavily 'damped' and/or not sufficiently sensitive
to be affected by the convection currents generated by hot samples). It
was concluded that samples could be weighed immediately on completion of
the drying period (i.e. hot) without a serious loss of precision.
Other conditions specified in the I.S.O. method (regarding the oven,
measuring tins and sample density) were adhered to. Separate determin-
ations on samples from well-mixed, pooled grain usually differed by less
than 0.2% which was felt to be sufficient accuracy for the purposes of
this study.
There appears to be no generally accepted standard for the determin-
ation of the moisture contents of cores of maize cobs. An estimate of
this was, however, required'for the estimation of weight loss of whole
cobs (see Appendix V) as .there is evidence that the core moisture content
may differ considerably from the grain moisture content of the same cob,
especially at high moisture contents, early in the storage season (see Fig.
2.7b)).
A mill capable of grinding cores was not available and so the
possibility of drying whole cores or transverse sections was investigated.
•The bulk of the core consists of pith which was sufficiently porous to
allow rapid water loss. Cores from maize in the cribs (i.e. at low.
moisture content) reached their stable weight within one hour.while cores
2 9 1 .
APPENDIX IV : Continued.
from freshly harvested maize appeared to lose most of their water content
within four hours (Fig. IV .3), although there was a continuing slight
loss of weight (possibly dry matter) thereafter. Sections of cores
reached dry weight slightly faster than intact cores but the time difference
was not sufficient to warrant the extra time required to cut them up (and
label them) ..
No independent method of estimating core moisture contents was
available.. -However, the readiness with which heated cores reached a (more
or less) stable weight suggested that this method could provide an estimate
of core moisture content sufficiently accurate for the purposes of this
study.
2 9 2 .
37 *[ whole grains - high moisture content
a \
36 •
4* X GO © 35 3
0 4» O 34
33
¥
*
I *
+• ' •* • * . * : ± " l > •.. ... , x • • .
1 t . . . « t . . » t •i i i i I e 68 122 tee
Time / minutes £40
37
03 \
36
4> x u> "5 35 3
A 4> o 34
whole grains - low moisture content +.•* • •
* . ^ -.V... + ..
go ' ' ' « « • ' * ' —I— I I I 1 I I I I I 8 68 188 188 248
FIGURE I V . 1 :
Tim® a minutes
Drying curves for lOg samples of grain heated in a ventilated oven @ 130"C. (Total weights include weights of tins). The grain moisture contents (determined from ground samples were 32.4% (high) and 16.1% (low).
38 -ground samples - hand mill
37 v
4 •
I: * T * • • Jp . • « . *• *• *. •. % '•f'. s s •.'.•. s •, ». \ ', \
t l i t l i I » i i • i. i i I i I l B 60 120 160
Time / minutes 240
38 ground samples - electric mill
V +.
•+• < • > • J. •+ . . + •
i .. i i i i i i i i i 0 60 120 160
Tlmo / minutes 240 300
FIGURE IV.2 : Drying curves for lOg samples of ground maize heated in a ventilated oven @ 130*C. Initial grain moisture content was 16.1%.
294 .
whole cores - high moisture content
1 — — — 4 1 j H
-4 1 1 +
•4 1 1 1 L.
180 240 300 Time /'minutes
360 420 480
09 \
3
40 r
35
30
® 25 t. o o.
20
15
whole cores - low moisture oontent
> ^ V f
-1 1 H 1 1 I
+
8 68 128 168 248 888 Time / minutes
368 428 468
FIGURE IV.3 : Drying curves for whole cores heated in a ventilated oven at 130*C. Estimated initial moisture contents were 47+2% (high) and 11+1% (low).
295 .
APPENDIX V : Methods for estimation of dry weight loss of grain.
A method was required which would provide an indication of both the
progress of damage through the storage season and of its distribution in
the cribs, over a wide range of moisture contents. Methods.based on
weighing standard volumes or counting and weighing fixed numbers of grains
were found to require too much time or sampled material if they were to
achieve sufficient precision. The grain was inherently variable necessit-
ating heavy replication but, as discussed in Chapter 3, it was felt to be
unsatisfactory to remove from the cribs large quantities of grain which
would then have to be replaced from a different source.
The method used here involved the identification of individual cobs
which were weighed at harvest and, in some cases, at intervals during the
storage season. At the end of the season each cob was again weighed,
shelled, and the core and sieved grain weighed separately. The cores
usually^showed no visible insect damage during storage: itJwas therefore- -
assumed (see below) that they had suffered no loss of dry matter and so,
with a suitable correction for moisture content changes, their final weight
could be used to estimate, by difference from the total cob weight, the
initial weight of grain. The difference between this estimate of initial
weight (corrected to dry weight) and the final observed grain'weight
(similarly corrected) would be the estimated weight loss:
estimated initial dry weight of grain (Gdwt^):
Gdwt. = in Tot. - (Cwt _ x 100 - Cmc. ) in f in 100 - Cmcf
100
(100 - Gmc. ) m
where Tot. = initial fresh weight of whole cob. in Cwt^ = final .fresh weight of core.
Gmc. = initial grain moisture content, m
296 .
APPENDIX V : Continued.
Cmc. = initial core m.c. in Cmc^ = final core m.c.
estimated final dry weight of grain (Gdwt^)
Gdwtf = Gfwtf x 100 (100 - Gmcf)
where Gfwt^ = final fresh weight of grain.
Gmc^ = final grain moisture content.
The overall loss (dry weight basis) is then:
Loss = Gdwt. - Gdwt-m f Gdwt. in
The final grain and core moisture contents can be determined directly
but there are difficulties in estimating the initial values. In the first
trials of this method the initial figures were estimated from determinations
on separate cobs drawn at random from the same population. However, at the
high initial moisture contents (25-30%) encountered in these trials, sample
variances were often equal to or greater than the means (Fig. 2.7 a)): the
uncertainty in the estimate of dry weight of individual cobs was then of
the same order as the total weight loss that could be expected over the
storage period. Although the estimated mean dry weight loss (for a sample
of, say, 10 cobs) might well have been close to the real value, the sample
variance would reflect mainly the uncertainty in the moisture content
correction rather than the variation in damage levels.
A better estimate of the initial moisture content was provided by
taking subsamples from the cobs individually at the beginning of the trial.
APPENDIX V : Continued.
Sections could readily be cut from the cobs using secateurs, to provide a
core sample, and a ring or longitudinal file of grains shelled from the
cob, to provide a grain samples. (Loose grains along the cut edges were
stabilised with a small quantity of plastic glue). The relationship be-
tween the moisture content of such subsamples and that of the remainder of
the cob was not examined critically. The limited number of determinations
made indicated that the regression for the relationship was significant but
that, for core samples especially, variation was considerable. (This may
have been due to the uneven uptake of free water by cores exposed at the
tip by poor husk cover in the field). Given that the core accounted for
only about 16-20% of the fresh weight of a cob, it was concluded that this
system reduced the uncertainty in the moisture content correction
sufficiently for the dry weight loss to be satisfactorily estimated.
In order to test*the assumption that the core did not lose any dry
weight, bare cores (i.e: with the. grain removed) were included in one
storage trial. The cares were weighed at the beginning and end of the
season. The initial moisture content was estimated from a section (approx.
20-30% of the whole core) cut from the apex or base and the final from
the whole remainder of the core. No significant dry weight loss was
detectable over the four month storage period but the uncertainty in the
moisture content estimation (as above) was considerable and the result
cannot be regarded as conclusive.
In conclusion it would seem that the general method described here
could, if suitably refined, be used satisfactorily in a variety of
situations to estimate weight loss and especially in situations where the
initial moisture content of the maize is nearer equilibrium.
298 .
APPENDIX VI : Collated Analysis of Variance Tables.
Summary tables are given for the results of the analyses of variance which were quoted in Chapter 4v.
Long-Term Distribution Trial (adults) 299-300
Short-Term Distribution Trial (adults) * 301-303
„ ,, „ (emergences) 304
Species and sampling occasions are specified by individual tables.
Sitophilus zeamais sample 1 S o u r c e ( N a m e ) df Sums of S q u a r e s Me a n ' S q u a r e F F at i o F - P r o b
T o t a l 23 2 7 8 3 . 7 7 .121.03 A E x p o s u r e 1 . 49 . 49 . 0O4 . 9524 B E a s t - W e s t 2 3 5 1 . 4 1 1 7 5 . 7 1 1 .321 .2915 AB 2 3 8 . 15 19.07 . 143 . 3674 S a m p Ii ng E r r o r 18
-2 3 9 3 . 7 2 •1 3 2 . 98
K ^ m e ) df S u m s of S q u a r e s Mean S q u a r e F R at i o F - P r o b
T o t a l 23 1 6 2 8 8 . 2 3 7 0 8 . 1 8 A E x p o s u r e 1 1 0 6 5 . 5 4 1065.54 1. 550 .2290 B E a s t - W e s t 2 2 1 8 5 . 1 5 1 0 9 2 . 5 7 1 . 590 .2313 AB 2 6 6 6 . 8 1 3 3 3 . 4 1 . 485 .6234
S a m p l i n g E r r o r 18 1 2 3 7 0 1 7 3 "687.26
sample 3 «
S o u r c e ( N a m e ) df S u m s of S q u a r e s Mean Sfquare F Rat i o F - P r o o
T o t a l 23 1 4 9 3 2 3 . 2 3 6 4 9 2 . 3 1 A E x p o s u r e 1 2 8 6 6 6 . 3 1 2 8 6 6 6 . 3 1 4 . 958 . 0390 B E a s t - W e s t 2 8 4 2 3 . 4 8 4 2 1 1 . 7 4 .729 . 4963 AB 2 8 1 6 9 . 7 3 4 0 8 4 . 8 6 .707 . 5065 S a m p 1i ng E r r o r 18 1 0 4 0 6 3 . 7 1 5 7 8 1 . 3 2
sample 4 S o u r c e ( N a m e ) df S u m s o f S q u a r e s Mean S q u a r e F R a t i o F - P r o b
T o t a l 23 2 4 0 9 0 4 7 . 14 1 0 4 7 4 1 . 1 8 -
A E x p o s u r e 1 3 8 5 2 7 . 3 7 3 8 5 2 7 . 3 7 . 420 .5249 B E a s t - W e s t 2 2 6 7 7 0 9 . 1 9 1 3 3 8 5 4 . 6 0 1. 461 . 2 5 8 3 AB 2 4 5 3 4 5 1 . 5 3 2 2 6 7 2 5 . 7 6 2 . 474 . 1 124 S a m p l i ng E r r o r - 18 1 6 4 9 3 5 9 . 0 5 : 9 1 6 3 1 . 0 6
Cathartus quadricollis sample 1 S o u r c e ( N a m e ) df S u m s of S q u a r e s Mean S q u a r e F Rat i o F - P r o b
T o t a l 23 3 9 6 3 . 2 9 . 1 7 2 . 3 2
A E x p o s u r e 1 2 8 9 . 5 9 2 8 9 . 5 9 2.093 . 1652
B E a s t - W e s t 2 8 1 3 . 7 2 406.96 2 . 940 .0785
AB 2 3 6 9 . 2 3 1 8 4 . 61 1.334 . 2382
S a m p l i n g E r r o r 18 2 4 9 0 . 7 6 1 3 3 . 3 8
sample.2 S o u r c e ^ N a m e > df - S u m s o f S q u a r e s Mean S"quare F R a t i o F - P r o b
Total- 23 6 1 6 8 6 . 7 6 2 6 8 2 . 0 3
A E x p o s u r e 1 1 4 3 3 4 . 0 1 1 4 3 3 4 / 0 1 14.184 .0014
B E a s t - W e s t 2 2 2 3 0 3 . 5 9 111.51.79 11.035 . OO07
AB 2 6 8 5 8 . 9 3 3 4 2 9 . 4 7 3. 394 .0562
S a m p l i n g E r r o r 18 1 8 1 9 0 . 2 2 . 1 0 1 Q . 5 7
sample 3 S o u r c e ( N a m e ) df S u m s of S q u a r e s Mean S q u a r e F Rat i o F - P r o b
T o t a l 23 4 7 7 4 4 . 5 6 2 C 7 5 . 8 5 A E x p o s u r e 1 3 3 2 . 6 0 3 3 2 . 6 0 .254 .6204 B E a s t - W e s t 2 7 6 5 7 . 0 2 3 8 2 8 . 5 1 2 . 924 . 0795 AB 2 1 6 1 8 9 . 3 9 8 0 9 4 . 7 0 6 . 183 . 0090 S a m p l i n g E r r o r 18 2 3 5 6 5 . 5 5 1 3 0 9 . 2 0
sample A . _ S o u r c e ( N a m e ) . df S u m s of S q u a r e s Mean S q u a r e F Rat I o F - P r o b .
T o t a l 23 6 7 0 7 4 . 3 3 2 9 1 6 . 2 8
,- A E x p o s u r e 1 • 5 5 5 7 . 3 1 5 5 5 7 . 3 1 2 . 4 9 2 .1319
B E a s t - W e s t 2 1 9 0 8 4 . 2 9 9 5 4 2 . 1 4 4 . 2 7 S . 03O2
.AB ' 2 2 2 8 5 . 5 2 1 1 4 2 . 7 6 .512 . 6076
S a m p l i n g E r r o r 13 4 0 1 4 7 . 2 1 2 2 3 0 . 4 0
Carpophilus dimidiatus sample 1
S o u r c e ( N a m e ) df S u m s of S q u a r e s Me an S q u a r e F Rat i o F - P r o b
T o t a l A E x p o s u r e B E a s t - W e s t AB S a m p 1i ng E r r o r
23 1 2 2 18
3 3 7 . 2 2 4 . 35
8 6 . 5 2 1.16
2 4 5 ^ 2 8
14. 66 4 . 35
4 3 . 2 6 .58
1 3 . 6 2
. 3 1 9 3 . 176 . 0 4 2
. 5791
.0659
.9535
sample 2 S o u r c e ( N a m e ) df S u m s of S q u a r e s M e a n S q u a r e F R a t i o F - P r o b
Tot al fl E x p o s u r e B E a s t - W e s t AB S a m p l i ng E r r o r
23 1 2 2
18
5 6 6 9 . 6 5 3 2 . 9 8
1 4 7 5 . 5 6 2 1 . 7 6
4 1 3 9 . 3 5
2 4 6 . 5 1 3 2 . 98
7 3 7 . 7 8 10.88
2 2 9 . 9 6
. 143 3 . 2 0 8 .047
. 7093
. 0643
.9539
sample 3 S o u r c e ( N a m e ) df S u m s of S q u a r e s M e a n S q u a r e F Rat i o F - P r o b
T o t a l A ' E x p o s u r e B E a s t - W e s t A B S a m p 1i ng E r r o r
23 1 2 2 18
3 8 7 7 . 9 5 5 7 8 . 1 9 4 5 3 . 1 2 186.58
2 6 6 8 . 8 5
1 6 8 . 6 1 5 7 0 . 1 9 2 2 6 . 5 6 .93. 29 1 4 8 . 2 2
3 . 8 4 7 1. 5 2 9 . 6 2 9
. 0655
.2437
.5442
sample 4 S o u r c e ( N a m e ) df S u m s of S q u a r e s M e a n S q u a r e F R a t i o F - P r o b
T o t a l A E x p o s u r e B E a s t - W e s t A B S a m p l i ng E r r o r
23 1 2 2
18
1 2 1 3 7 5 . 1 3 3 3 6 7 5 . 3 8 1 8 7 5 6 . 6 2 1 8 9 4 6 . 2 3 6 5 9 9 6 . 9 8
5 2 7 7 . 1 8 3 3 6 7 5 . 3 0 5 3 7 3 . 3 1 5 4 7 3 . 1 1 3 6 6 6 . 5 0
9 . 185 1 . 4 6 7 1. 4 9 3
.0072
.2569
.2513
Gnatocerus maxillosus sample 3 . S o u r c e ( N a m e )
Gnatocerus maxillosus sample 3 . S o u r c e ( N a m e ) df S u m s of S q u a r e s M e a n S q u a r e F. Rat i o F - P r o b
T o t a l A E x p o s u r e . B E a s t - W e s t A B S a m p 1i ng E r r o r
23 1 2 2
18
7 6 1 . 5 7 7 . 15
7 3 . 8 1 6 2 . 8 7
6 1 8 . 5 4
3 3 . 11 7 . 15
3 6 . 9 1 3 1 . 0 3 3 4 . 3 6
, . 2 0 8 1. 074 . 9 0 3
- • 6 5 3 6 .3625 .4229
sample 4 S o u r c e ( N a m e ) df S u m s o f S q u a r e s M e a n S q u a r e . F Rat i o F - P r o b
T o t a l A E x p o s u r e B E a s t - W e s t AB S a m p l i n g E r r o r
23 1 2 2 18
5 3 9 2 8 . 3 2 105.35'
1 1 1 8 6 . 3 8 1 9 3 8 2 . 4 2 2 3 2 5 4 . 1 7 •
2 3 4 4 . 7 1 1 0 5 . 3 5
5 5 9 3 19 9 6 9 1 . 2 1 1 2 9 1 . 9 8
. 0 8 2 4 . 3 2 9 •-7 . 5 0 2
.7785
. '?292
.0043
Cryptolestes spp. sample 3 S o u r c e ( N a m e )
• Cryptolestes spp. sample 3 S o u r c e ( N a m e ) df S u m s of S q u a r e s Mean S q u a r e F R at i o • F - P r o b
T o t a l A E x p o s u r e B E a s t - W e s t AB S a m p l i ng E r r o r
23 1 2 2 18.
7 5 2 . 8 5 18. 39 3 3 . 8 1 6 8 . 8 0
6 4 8 . 6 5
3 2 . 7 3 10. 39 16. 90 30 . 00 3 6 . 84
. 2 8 8
. 4 6 9
. 8 3 3
. 5978
.6330
. 4510
sample 4 S o u r c e ( N a m e )
sample 4 S o u r c e ( N a m e ) df S u m s of S q u a r e s M e a n S q u a r e F R a t i o F - P r o b
T o t a l A E x p o s u r e B E a s t - W e s t AB S a m p l i n g E r r o r
23 1 2 2
13
2 6 4 7 6 5 . 4 0 •28 3 3 . 5 6
7 0 3 7 1 . 7 3 2 7 4 4 7 . 4 1
164912'. 69.
1 1 5 1 1 .-54 2 0 3 3 . 5 6
3 5 1 8 5 . 3 7 1372-3.71 9 1 6 1 ."82
3 . 3 4 0 1. 4 9 8
.6432
."0408
. 2502
.Adults, (STDT) . Sitophilus zeamais
S o u r c e ( N a m e ) df
301.
S u m s pf S q u a r e s M e a n S q u a r e F R a t i o F - P r o b
Tot al 287 8 0 5 9 4 6 4 . 60 2 8 0 8 1 . 76 B l o c k s 2 6 7 3 5 2 0 . 59 3 3 6 7 6 0 . 30 R a. m . / m / p . m 2 9 6 5 6 . 6 1 4 8 2 8 . 30 . 393 .6757 B E a s t - W e s t • 3 2 2 8 9 7 6 0 . 15 7 6 3 2 5 3 . 38 62 . 103 - . 0 0 0 0 C Pos i t i ons 7 1 6 8 4 0 2 4 . 44 2 4 0 5 7 4 . 92 19 . 575 .0000 RB 6 1 8 9 6 7 . 27 3 1 6 1 . 2 1 .257 .9559 AC 14 1 4 2 8 9 0 . 73 1 0 2 0 6 . 48 .830 . 6354 BC 21 6 8 0 5 4 6 . 00 3 2 4 0 6 . 95 2 .637 . 0003 R B C 42 2 2 4 9 8 0 . 9 2 5 3 5 6 . 69 .436 .9990 Block E r r o r 190 2 3 3 5 1 1 7 . 88 1 2 2 9 0 . 89
otroga cerealella S o u r c e ( N a m e ) df S u m s of S q u a r e s M e a n S q u a r e F Rat i o F - P r o b
T o t a l 2 8 7 4 8 2 3 5 . 26 1 6 8 . 07 B l o c k s 2 5 8 8 9 . 56 2 9 4 4 . 78 R a . m . / m / p . m 2 3 7 6 5 . 68 1 8 8 2 . 84 2 1 . 778 .0000 B E a s t - W e s t 3 3 6 6 1 . 7 2 1 2 2 0 . 57 14 . 113 . 0 0 0 0 C Pos i t i o n s 7 9 7 0 1 . 9 1 1 3 8 5 . 99 16 . 025 - . 0 0 0 0 RB 6 6 4 3 . 58 1 0 7 . 26 1 .240 .2875 AC 14 2 0 8 2 . 88 1 4 8 . 78 1 .720 . 0544 BC 21 2 8 4 0 . 14 1 3 5 . 24 1 .564 . 0 6 1 8 R B C 42 3 2 1 6 . 93 7 6 . 59 .886 .6717 Block E r r o r 190 1 6 4 3 2 . 8 5 8 6 . 4 9
pophilus -dimidiatus S o u r c e ( N a m e ) .df S u m s of S q u a r e s M e a n S q u a r e F Rat I o F - P r o b
T o t a l 287 3 6 7 0 0 8 . 6 1 1 2 7 8 . 78 B 1 o c k s 2 6 2 6 1 8 . 4 7 3 1 3 0 5 . 24 R a m / m / p m 2 1 0 9 7 . 76 5 4 8 . 88 . 9 4 1 . 3 9 2 0 B E a s t - W e s t 3 3 3 1 3 3 . 6 5 1 1 0 4 4 . 55 18 .936 ..0000 C Posi t i o n s 7 1 2 2 1 7 5 . 4 9 1 7 4 5 3 . 64 29 . 924 - . 0 0 0 0 RB 6 2 8 4 8 . 34 4 7 3 . 3 9 .812 .5621 AC 14 2 9 2 1 . 24 2 0 8 . 66 . 358 . 9844 BC 2 1 1 6 0 5 3 . 28 7 6 4 . 44 1 . 3 1 1 . 1721 R B C 42 1 5 3 5 6 . 0 1 3 6 5 . 62 . 627' .9628 B l o c k E r r o r 190 1 1 0 8 2 0 . 36 5 8 3 . 27
Gnatocerus maxillosus S o u r c e ( N a m e ) d.f S u m s of S q u a r e s - M e a n S q u a r e F R at i o F - P r o b
T o t a l 2 8 7 1 8 4 8 5 6 . 8 2 644 . 10 B l o c k s 2 3 5 5 6 . 2 1 - 1778 . 1 1 R a . m . / m / p . m 2 .4704.22 2 3 5 2 . 11 4 . 738 .0098 B E a s t - W e s t 3 1 1 7 8 0 . 6 6 3 9 2 6 .89 7 . 910 . 0 0 0 1 C P o s i t i o n s 7 3 1 3 2 6 . 9 1 4 4 7 5 .•27 . 9 . 014 . 0000 RB 6 1 7 3 1 . 4 4 288 .57 5 3 1 . 7 4 5 0 A C 14 3 9 2 9 . 1 3 2 8 0 .65 565 .8895 BC ' 2 1 1 6 1 0 0 . 4 4 7 6 6 .69 1. 544 .0672 R B C 4 2 1 7 3 9 7 . 6 8 41-4 . 2 3 , 834 .7528 B l o c k E r r o r 190 9 4 3 3 0 . 1.3 496 .47 *
Palorus subdepressus "Source (Name/ df S u m s of S q u a r e s M e a n S q u a r e F R a t i o F - P r o b
T o t a l 2 8 7 1 3 9 2 4 9 2 . 32 4 8 5 1 . ,89 B 1 o c k s 2 2 6 1 4 7 4 . 85 1 3 0 7 3 7 . ,42 R a m / m / p m " 2 3 1 4 3 3 . 08 1 5 7 1 6 . ,54 8 . 577 .0003 B E a s t - W e s t 3 3 8 2 4 1 . 7 1 1 2 7 4 7 . ,24 6 . 957 .0002
C P o s i t i o n s '7 5 7 1 7 9 0 . 30 8 1 6 8 4 . ,33 4 4 . 573 - . 0 0 0 0
RB 6 5 0 6 8 . 8 9 8 4 4 . ,82 4 6 1 .8365 RC 14 5 7 5 6 6 . 8 1 4 1 1 1 , ,92 2 . 244 .0077 BC 21 2 9 1 6 7 . 0 9 . 1388, .91 . 758 .7675 RBC 42 4 9 5 9 3 . 76 1180, , SO . 644 .9536 Block E r r o r 190 3 4 8 1 5 5 . 83 1832, .40
yptolest£La_spp. S o u r c e ( N a m e ) df S u m s of S q u a r e s M e a n S q u a r £ F R at i o F - P r o b
T o t a l 2 3 7 2 0 3 8 4 4 . 86 7 1 0 . 26 B l o c k s 2 2 8 5 1 6 . 46 1 4 2 5 8 . 23 R a . m . / m / p . m 2 6 5 8 0 . 9 1 "3290. 45 6 . 940 . 0012 B East-West- 3 3 7 2 3 . 9 9 1 2 4 1 . 33 2 . 618 .0523 C P o s i t i o n s 7 4 0 2 1 5 . , 16 * 5 7 4 5 . 02 12. 116 . 0 0 0 0 RB 6 6 4 1 . , 18 1 0 6 . 86 . 9682 RC • 14 3 7 0 2 . ,82 2 6 4 . 49 '553 . 394 9 BC 2 1 1 7 4 3 5 . ,51 8 3 0 . 26 1. 75.1 .0265 R B C 4 2 1 2 9 3 7 . ,89 . ' 3 0 8 . 05 650 .9506 B l o c k E r r o r 190 9 0 0 9 0 . , 93 4 7 4 . 16
302 .
S o u r c e ( N a m e ) df S u m s of S q u a r e s M e a n S q u a r e F Rat i o F - P r o b
Tot al 287 3 2 2 7 . 3 2 11. 25 B l o c k s 2 3 9 . 8 8 19. 90 R a m / m / p m 2 1.84 . 92 . 103 . 9026 B E a s t - W e s t 3 1 4 3 . 5 7 4 7 . 86 5 . 338 . 0 0 1 5 C P o s i t i o n s 7 4 1 8 . 8 5 5 9 . 84 6 . 674 . 0 0 0 0 AB 6 7 9 . 8 5 1 3 . 3 1 1 . 434 . 1855 RC 14 1 15.37 8 . 24 .919 . 5 3 9 2 BC 21 2 6 5 . 6 2 1 2 . 65 1 .411 . 1169 R B C 42 4 5 8 . 9 6 18. 93 1 .219 . 1875 B l o c k E r r o r 198 1 7 8 3 . 4 5 8 . 97
Litargus balteatus S o u r c e ( N a m e ) df Sums of S q u a r e s M e a n S q u a r e F R a t i o F - P r o b
T o t a l 2 8 7 1 8 9 3 8 . 41 3 8 . 11 B l o c k s 2 4 4 . 0 1 2 2 . 0 1 R a m / m / p m 2 1 5 1 . 75 7 5 . 88 2. ,679 .0712 B E a s t - W e s t ' 3 1 3 7 2 . 32 4 5 7 . 44 16. , 153 . 0 0 0 0 C P o s i t i o n s 7 1 6 3 8 . 31 2 3 2 . 98 8. ,224 . 0 0 0 0 RB 6 8 8 . 54 1 4 . 76 ,521 . 7 9 1 9 AC 14 3 9 5 . 31 2 8 . 24 ,997 . 4 5 7 9 BC ' 21 7 5 3 . 70 3 5 . 89 1. ,267 .2016 R B C 42 1 1 2 1 . 79 2 6 . 7 1 , 9 4 3 .5750 B l o c k E r r o r 198 5 3 8 8 . 68 2 8 . 32
Choetospila elegans S o u r c e ( N a m e ) .—rtf Sums of S q u a r e s ' M e 4 n " S q u a r e F R a t i o F - P r o b
Tot al 287 1 6 5 0 4 . ,83 5 7 . ,51 B l o c k s 2 2 7 2 . ,74 136. ,37 fl a m / m / p m 2 7 4 2 . ,81 3 7 1 . ,40 7. , 769 . 0 0 0 6 B E a s t - W e s t 3 1428. ,38 4 7 6 . , 13 9. , 9 6 0 . 0 0 0 0 C p o s i t i o n s 7 2 2 6 4 . ,86 3 2 3 . ,55 6. ,768 . 0 0 0 0 RB 6 124. ,38 2 8 . ,72 , 4 3 3 . 8 5 6 0 RC 14 4 8 4 . ,43 2 8 . , 89 ,604 .8596 BC 21 5 9 5 . ,37 2 8 . ,35 ,593 . 9 2 0 1 R B C 42 1589. ,32 3 7 . ,84 ,792 .8136 B l o c k E r r o r 190 9 8 8 2 . ,63 4 7 . , 80
Cerocephala dinoderi S o u r c e ( N a m e ) df Sums o f S q u a r e s M e a n S q u a r e F R a t i o F - P r o b
Tot al 2 6 7 6 5 6 2 . ,72 2 2 . , 87 B l o c k s 2 215. ,04 107. ,52 fl a m / m / p m 2 3 6 0 . ,05 180. ,02 9, , 984 . 8 0 0 1 B E a s t - W e s t 3 ' • 9 7 8 . ,82 3 2 6 . • c. r • 18. ,895 . 0 0 0 0 C p o s i t i o n s 7 2 4 8 . ,73 3 5 . ,53 1. ,971 .0610 RB 6 53. ,42 8. ,90 ,494 . .8125 RC 1-4 314. , 15 22. .44 1. .244 .2463 BC • 21 ; 318. ,83 • 15. . 18 , 842 .6655 A B C 42 6 4 7 . ,82 • 15. .42 ,855 .7203 B l o c k E r r o r 190 3425. ,86- 18. ,83 -
Cardiastethus pygmaeus . _ S o u r c e ( N a m e ) df Sums of S q u a r e s M e a n S q u a r e F R a t i o F - P r o b
T o t a l 2 8 7 2 0 7 6 6 . ,74 7 2 . , 36 B 1 o c k s 2 8 8 1 . ,40 4 0 0 . , 78 A a m / m / p m 2 2 5 9 . ,84 129. ,92 .4, ,521 . 0 1 2 1 B E a s t - W e s t 3 5 1 3 6 . ,07 1712. ,02 C p o s i t i o n s 7 4 0 2 9 . ,56 5 7 5 . ,65 2 0 . , 0 3 1 . 0 0 0 0 AB 6 4 2 6 . ,43 7 1 . , 07 2. ,473 . 0 2 5 1 AC 14 3 2 1 . ,66 2 2 . ,98 , 799 . 6 6 9 1 BC 2 1 3 1 3 5 . , 40 149. ,30 5. , 195 . 0 0 0 0 A B C 42 1196. , 04 2 8 . , 48 , 9 9 1 . 4946 B l o c k E r r o r 190 5 4 6 0 , .34 2 8 . .74
Lyctocoris.. cQchici. S o u r c e ( N a m e ) df Sums of S q u a r e s M e a n S q u a r e F R a t i o F - P r o b
28.7 6 7 6 4 . 15 . 2 3 . 5 7
2 3 9 2 . 17 1 9 6 . 0 9
2 2 9 4 . 73 1 4 7 . 3 7 6 . 662 .0016
3 - 3 5 . 45 1 1 . 8 2 . 534 .6594
_. 7 ' • 1022. ,24 1 4 6 . 0 3 6 . 602 ,. 0 0 0 0
6 55. ,73 9 . 2 9 . 420 . 8 6 5 2
" 14 2 0 5 . , 16 1 4 . 6 5 .662 . 8083
21 142. ,46 6 . 7 8 .307 .9987
. 42 413, ,16 • £ . 8 4 .445 .9987.
190 4203, . 04 '22.12
Tot a] B l o c k s . fl a m / m / p m B E a s t - W e s t C p o s i t i o n s FIB RC . BC p B C ' ' B1 ock E r r o r "
3 0 3 .
Scolopoides divareti S o u r c e ( N a m e ) df S u m s of S q u a r e s M e a n S q u a r e F R a t i o F - P r o b
T o t a l 2 8 7 1 8 6 9 8 . 4 5 65 12 Block % 2 3 1 2 1 . 3 8 1568 65 fl a m / m / p m 2 7 3 7 . 8 3 368 51 7 . 884 . O 0 1 2 B E a s t - W e s t 3 4 7 5 . 9 5 158 65 3 . 8 1 5 . 8 3 1 2 C poslt ions 7 2 1 2 7 . 6 6 383 .95 5 . 777 . 0 0 0 0 RB 6 3 1 . 4 4 5 24 . 100 . 9 9 6 4 RC 14 5 1 6 . 2 7 36 88 . 7 8 1 . 7 7 1 9 BC 2 1 5 2 8 . 1 2 25 15 . 4 7 8 . 9 7 5 3 ABC 42 1 1 5 6 . 2 2 27 .53 . 5 2 3 . 9 9 2 7 Block E r r o r 198 9 9 9 6 . 4 6 5 2 .61
grain moisture content S o u r c e ( N a m e ) df S u m s of S q u a r e s M e a n S q u a r e F Rat i o F - P r o b
T o t a l 2 8 7 2 9 4 . ,75 1. .03 B l o c k s 2 13. ,53 6. ,77 fl a m / m / p m 2 2 3 . ,04 11. .52 13 . 906 . 0 0 O 0 B E a s t - W e s t 3 7. ,63 2. ,54 3 .869 . 0 2 9 1 C p o s i t i o n s 7 5 3 . ,49 7, .64 9 .226 . 0 0 0 0 AB 6 4. ,69 .78 . 944 .4651 • AC 14 11. ,33 ,81 . 977 .4787 BC 2 1 17. , 67 , 84 1 .816 . 4 4 6 2 ABC -• 4 2 6. ,02 • - - . 14 . 173 1 . 0 0 0 0 Block E r r o r 190 157. ,37 ,83
grain temperature S o u r c e ( N a m e ) df S u m s of S q u a r e s M e a n S q u a r e F R a t i o F - P r o b
Total 2 8 7 8 0 7 . 8 0 2 . 8 1 B l o c k s 2 9 . 5 9 4 . 79 A a m / m / p m 2 6 8 1 . 7 9 3 4 0 . 8 9 B E a s t - W e s t 3 4 . 4 2 1.47 5 . 206 . 0 0 1 8 C p o s i t i o n s 7 13. 19 1.88 6 . 6 5 9 . 0000, AB 6 2 4 . 7 2 4 . 12 AC 14 1 8 . 64 .76 2 . 6 8 6 .0013 BC 2 1 1.74 .08 . 294 .9991 ABC 4 2 7 . 9 5 . 19 '.669 . 9 3 8 1 Block E r r o r 198 5 3 . 76 .28
> i
emergences (STDT) Sitophilus zeamais
S o u r c e ( N a m e ) df S u m s of S q u a r e s M e a n S q u a r e F R a t i o F - P r o b
T o t a l 95 1 7 9 7 . 24 18. 92 B l o c k s 2 1 9 2 . 52 9 6 . 26 A E a s t - W e s t 3 3 7 8 . 86 1 2 6 . 29 10.410 . 0 0 0 0 B P o s i t i o n 7 2 5 4 . 66 3 6 . 38 2 . 9 9 9 . 0 0 8 9 AB 21 2 1 9 . 05 1 0 . 43 . 860 . 6 3 9 0 Block E r r o r 62 7 5 2 . 15 1 2 . 13
-
Sitotroga cerealella S o u r c e ( N a m e ) df S u m s of S q u a r e s Me an S q u a r e F R a t i o F - P r o b
T o t a l 95 . 3 5 4 . 50 3 . 73 B l o c k s 2 1 5 . 75 7 . 88 A E a s t - W e s t 3 2 0 . 50 6 . 83 1 . 675 .1815 .
B P o s i t i on 7 20. 50 2 . 93 .718 .6571"'
AB 21 4 4 . 83 2 . 13 . 523 . 9 4 9 7
B l o c k E r r o r 62 2 S 2 . 92 4 . ,08
Cryptolestes spp S o u r c e ( N a m e ) d f S u m s -of S q u a r e s M e a n S q u a r e F R a t i o -F-Pr„ob
T o t a l 95 1347. ,99 14, . 19 B 1 o c k s 2 196. ,02 98, .01 A E a s t - W e s t 3 9, .45 3, . 15 .301 . 8 2 4 8 B P o s i t i on 7 328. .57 46, . 94 4 . 482 . 0 0 0 4 AB 21 164, .64 7, . 84 . 749 . 7 6 6 3 B l o c k E r r o r 62
-649, .31 10, .47
- -
Gnatocerus maxillosus S o u r c e ( N a m e ) df S u m s of S q u a r e s M e a n S q u a r e F R a t i o F - P r o b
T o t a l 95 1976 .63 2 0 .81 B l o c k s 2 1 .31 .66 A E a s t - W e s t 3 106 .54 3 5 .51 1.827 . 1515 B P o s i t i o n 7 2 4 0 .29 34 .33 1.766 . 1 104 AB 21 4 2 3 . 12 20
4 A . 15 1.036 . 4 3 6 9
Palorus _ subdepressus % !.... S o u r c e ( N a m e ) df ' S u m s of S q u a r e s M e a n S q u a r e F R a t i o F - P r o b
T o t a l 9 5 . B 1 o c k s ' • 2 A E a s t - W e s t 3 B P o s i t i o n 7 . AB 2 1 B l o c k E r r o r 62
1 0 5 4 . 4 1 1 6 5 . 8 1 1 5 . 6 1
2 9 1 . 8 2 . 7 6 . 9 7
5 0 4 . 1 9
1 1 . 1 0 8 2 . 9 1 5 . 2 0
4 1 . 6 ? 3 . 6 7 8 . 13'
• .640 5 . 127 .451
5 9 2 1 , 0 0 0 1 ,9775
Choetospila elegans S o u r c e ( N a m e ) df S u m s of S q u a r e s M e a n S q u a r e F R a t i o F - P r o b
T o t a l 95 8 8 2 . 24 9 . 2 9
B 1 o c k s 2 6 5 . 40 3 2 . ,70
A E a s t - W e s t 3 6 7 . 78 2 2 . ,59 3. ,109 . 0327
B P o s i t i o n 7 1 5 7 . 99 22*. , 57 3 • , 105 . 0 0 7 1
AB 21 1 4 0 . 47 6. , 69 , 920 • .5676
B l o c k E r r o r 62 4 5 0 . 60 7. , 27
•rocephala dinoderi S o u r c e ( N a m e ) df S u m s of S q u a r e « M e a n S q u a r e F R a t i o F.-Prob
T o t a l 95 3 9 1 . 49 4 . 12
B l o c k s 2 4 3 . 90 2 1 . 9 5
A E a s t - W e s t 3 2 4 . 36 ' ; 12 2\ 686 . 054 1
B P o s i t i o n 7 . 35; 91 5 . 13 1. 6 9 7 . 1263
AB 2 1 9 9 . 89 • 4 . 76 1. 573 . 0 8 6 3
B1 ock. E r r o r 62 1 8 7 . 44 . • 3. 02 \
Acknowledgements
This research project was sponsored by the Tropical Products Institute (Overseas Development Administration) under the London University 'Public Research Institutes' scheme. Field work was carried out at the International Institute of Tropical Agriculture, Ibadan, with the co-operation of the F.A.O. /Danida African Rural Storage Centre.
I would like to express my gratitude to Dr C.P. Haines, who set the project in.motion, and to Dr W.H. Boshoff, project leader at the African Rural Storage Centre, who was generous in making available research facilities and materials and, at a personal level, in providing advice and encouragement. Special thanks are also due to Mr Peter Egbele and Mr Victor Udoh for their good humoured assis-tance with the field work. Among the many friends and colleagues who have given me practical and moral support during the course of this study, I am particularly indebted to Dr T. Kaufmann, Dr Kamil Vanek, Dr Pat Matteson and Ms Deborah Elton. My thanks ate also due to Mrs Maureen Robiiison and Mrs Margaret Clements for their . help in preparing ani typing this thesis.
Finally, I would like to express my warmest and most.sincere thanks to Prof Michael Hassell and Mr Philip Dobie for all that they have contributed in practical help, advice and encouragement.
306 .
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