rice whorl maggot action & defoliator action thresholds

Evaluation of action thresholds for chronic rice insect pests in thePhilippines: II. Whorl maggot and defoliators

J. A. LITSINGER1, J. P. BANDONG2, B. L. CANAPI3, C. G. DELA CRUZ2, P. C. PANTUA2,

A. L. ALVIOLA2, & E. H. BATAY-AN III4

1Dixon, CA, USA, 2International Rice Research Institute, Metro Manila, Philippines, 3Monsanto Philippines, Makati,

Metro Manila, Philippines, and 4Philippine Department of Agriculture, Philippines

AbstractAction thresholds as insecticide decision tools were developed and evaluated against chronic vegetative stage insect pests ofirrigated transplanted rice in the Philippines: whorl maggot Hydrellia philippina Ferino (Diptera: Ephydridae) and twolepidopterous defoliators, Naranga aenescens Moore and Rivula atimeta (Swinhoe) (Lepidoptera: Noctuidae). Through aniterative process, new threshold characters and insecticide technologies were evaluated at four sites (categorized as high andlow pest density sites) over a 13-year period with the objectives of minimizing insecticide usage and maximizing farmeradoption. Characters were evaluated as to their ability to predict significant pest densities and yield loss. Significantdifferences occurred between high and low pest density sites among the best characters and monitoring methods for bothpests. Earlier-planted fields were used as a forecasting tool, showing best results in the high pest density sites. Overall, mostthreshold characters performed well but the insecticide response was poor. Insecticide control was better with some thresholdcharacters than others due to better timing. Seedling root soak with isofenphos was more effective than granules or foliarsprays against whorl maggot but required a decision before planting. Foliar sprays were most effective if applied twice (2 and7 days after transplanting). The best performing characters for whorl maggot were 1 – 2 eggs/hill for the low pest densitysites and 15 – 30% damaged leaves in earlier planted fields in high density sites. The best performing characters fordefoliators were 10% damaged leaves monitored in earlier planted fields for high density sites and 10% damaged leaves in lowdensity sites. Carbaryl was the most effective insecticide spray against defoliators that combined performance and relativesafety.

Keywords: Pest control, irrigated rice, insecticides, decision-making, yield loss, planting date, damage pattern

1. Introduction

In the Philippines rice whorl maggot, Hydrellia

philippina Ferino (Diptera: Ephydridae), and the two

lepidopterous defoliators, Naranga aenescens Moore

and Rivula atimeta (Swinhoe)(Noctuidae), are among

the first insect pests to colonize transplanted rice

crops. Damage is concentrated in the vegetative stage

as their densities markedly decline after maximum

tillering for reasons that differ for each pest group.

Whorl maggot larvae feed internally on unfurled

leaves, thus their abundance is correlated with the

issuance of new tillers (Ferino 1968). As tillering

declines after canopy closure, marking the end of the

vegetative stage, new feeding sites become scarce.

On the other hand, natural enemies, which exert a

minimal influence on the whorl maggot, rapidly rise

toward the end of the vegetative stage to curtail the

surface feeding defoliators (van den Berg et al. 1988).

Chlorotic damage symptoms are similar between

both pest groups and are thus often confused by

farmers, but through training, can be distinguished,

as defoliators scrape off plant tissue, whereas whorl

maggot injury causes necrosis of leaf margins (Reissig

et al. 1986).

Decision thresholds have been documented from

surveys of rice farmers (Bandong et al. 2002), where

the unit of measure is often whole fields or the

presence of moths flushed while walking along dykes.

The first action threshold characters developed by

researchers for both whorl maggot and defoliators

were based on percentage damaged leaves with values

ranging from 10 to 30% (Way et al. 1991).

The aim of this study was to improve upon farmers’

methods by developing thresholds based on plant

monitoring to derive the most effective quantifiable,

and statistically reliable, characters for insecticide

decision-making. The type of thresholds was action

thresholds (ATs) rather than economic thresholds as

damage functions (Pedigo et al. 1986; Way et al.

1991) have not been developed.

Selection of a character is an important component

of an AT and is based either on pest damage or a pest

life stage. AT characters that were tested were drawn

from the most promising from farmers as well as ones

we developed. It was noticed that pest abundance

Correspondence: J. A. Litsinger, 1365 Jacobs Place, Dixon CA 95620, USA. Tel: þ1 707 678 9068. Fax: þ1 707 678 9069. E-mail: [email protected]

International Journal of Pest Management, July–September 2006; 52(3): 167 – 180

ISSN 0967-0874 print/ISSN 1366-5863 online � 2006 Taylor & Francis

DOI: 10.1080/09670870600653337

increased in a community in some sites (Department

of Entomology 1984) thus a strategy to improve

timeliness was to monitor earlier-planted, neighbor-

ing fields rather than the target field.

Improvements were sought in the insecticide

response once a threshold was reached. Foliar sprays

and broadcast granules were most popular, but car-

bofuran granules incorporated into the paddy soil

during the last harrowing before transplanting af-

forded protection for the first month after trans-

planting (Seiber et al. 1978). Over 480% control was

readily achieved at a dosage of 1 kg a.i./ha (Arceo and

Heinrichs 1980), but lower dosages would be needed

for farmer adoption.

This is the second of a series of four papers on the

development and evaluation of ATs for chronic insect

pests of rice. The first in the series (Litsinger et al.

2005) provided an overall evaluation of ATs as tools

which contain an economic analysis as well as

substitution of insecticide with nitrogen. Subsequent

papers on leaffolders and stemborers will follow.

2. Materials and methods

The study sites, research teams, and experimental

design were discussed in Litsinger et al. (2005).

2.1. Action thresholds

Thresholds involve a number of variables, any one

of which can affect efficacy. The first variable is a char-

acter such as an insect stage (egg or larva) or its damage

symptom (damaged leaves). Second is the sampling

unit and number of samples to measure the character

(usually 20 hills). Third is the density of the char-

acter per sampling unit (e.g., one whorl maggot egg or

one defoliator larva per hill or percentage damaged

leaves). Normally a single AT character with two

threshold levels for each pest group was tested each

season per site, the lower of which was termed ‘low

level’ (e.g., one egg or larva per hill or 15% damaged

leaves) and the other the ‘high level’ (e.g., two eggs

or larvae per hill or 30% damaged leaves).

New characters were continually being developed

in an iterative process to improve performance. The

levels of each of these thresholds were adjusted

season to season depending on performance (e.g.,

0.5, 1, 2, or 4 whorl maggot eggs/hill). Generally

character densities at sites with higher pest pressure

and more rapid colonization rates were lowered in

both the low and high level treatments and vice versa

for sites with lower pest pressure and less rapid

colonization rates. Lower levels were needed to

respond to heavy infestations as the damage curves

were steeper and earlier warning was required.

Comparing two or more levels each season enabled

more reliable adjustments to be made.

As development of ATs was iterative there was no

balanced design to test the full complement of

characters and response variables in a given trial.

New ideas were tested in the following season’s trials,

some coming from the farmers. Most characters were

tested in multiple sites over several years. Data

analysis after each season entailed comparing yield in

the threshold treatments to that in the untreated

control. Yield loss results were evaluated field by

field to determine if yield loss occurred in each

growth stage where thresholds were reached and

benchmark losses were established. If a benchmark

loss was reached but the threshold was not, the level

was lowered the following season and vice versa.

Another set of variables is associated with the

corrective insecticide response including the chemi-

cal, dosage, and method, timing, and frequency of

application. Insecticide technology likewise evolved

through an iterative process. The first goal was to

achieve the minimum effective dosage whenever

possible for cost savings. Foliar sprays were applied

as described in (Litsinger et al. 2005). Graphs were

drawn to illustrate the weekly pest abundance and

degree of control compared to the untreated check in

each field (e.g., Department of Entomology 1984,

1988). Thus, if efficacy was low, adjustments were

made and research was carried out to improve

performance. AT treatments were assessed for a

period of 1 – 4 weeks after treatment to measure

activity, allowing ample time for the crop to recover

by generating new leaves. In the tropics new leaves

emerge about every 4 days during the vegetative stage,

each leaf having a life of ca. 3 weeks (Yoshida 1981).

Percentage control of each threshold character was

calculated as the change (positive and negative) from

the untreated check. A standard of 480% control

was established (Litsinger et al. 1980a).

2.2. Whorl maggot thresholds

Two AT characters were compared: (1) damaged

leaves and (2) number of eggs per hill. Levels of

damaged leaves (DL) tested included 10, 15, 25, and

30% and were consolidated during analysis into

fewer characters to increase replication: e.g., 10%

and 15%DL became 15%DL, likewise 25% and

30%DL became 30%DL.

The egg density character was designed to achieve

earlier warnings than damaged leaves with a goal to

improve insecticide control. The 0.75-mm whitish

eggs laid singly scattered on either side of a leaf

(usually near the midrib) can be detected with a

trained eye. Sampling could be quickly accomplished

on a young crop due to few tillers per hill. Numbers

of eggs per hill tested ranged from 0.5 (termed as

0.5E), 1 (as 1E), and 2 (as 2E). Four eggs per hill

were tested for only 1 year but it became apparent

this level was too high. In the analysis this character

was combined under 2E.

After several years testing, it was deemed necessary

to find ways to achieve even earlier warnings, and an

idea borrowed from farmers (Bandong et al. 2002)

was to monitor earlier-planted, neighboring fields.

168 J. A. Litsinger et al.

Neighboring fields (NF) were defined as the nearest

two fields planted 1 – 2 weeks earlier than the target

field. Monitoring NF was done twice 7 days apart

beginning 2 – 7 days after transplanting (d.a.t.). Two

NF were monitored per target field with results

averaged. As farmers in irrigated areas tend to plant

within 2 months of one another, such fields were

readily available. NF were selected within 100 m of

each other. The NF characters were tested from 10

to 30%DL (consolidated as above into NF15%DL

and NF30%DL). The egg character also was tested

with NF as NF0.5E, NF1E, and NF2E.

Two further variations involving the egg character

were tested. The first was tested in Zaragoza

where eggs were sampled from the field itself and a

decision was triggered when both the level surpassed

0.5 eggs/hill and incidence started to decline, thus

the response was timed at the ‘egg peak’ (termed

0.5E-pk). It was only tested at the 0.5 egg/hill

level. The second variation termed ‘trap crop’ (tc)

involved removing 20 hills from a neighboring

field 2 – 3 weeks after transplanting (WT) and re-

transplanting them in a 1 m2 block in the co-

operator’s field during the interval between land

preparation and transplanting. The isolated hills were

thought to be attractive to ovipositing females. The

seedlings were monitored for eggs at 4-day intervals

beginning 3 d.a.t. Different levels (0.5, 1, and 2

eggs/hill) were evaluated and combined for analysis

as 1E-tc.

Foliar spraying is the most popular application

method with rice farmers (Litsinger et al. 1980b),

thus four of the most effective chemicals (mono-

crotophos, azinphos-ethyl, triazophos, deltamethrin)

(Arceo and Heinrichs 1980) were compared. But as

farmers rarely utilize manufacturers’ recommended

dosages of any material (Litsinger et al. 1980b),

lower dosages (0.4 kg a.i./ha) were tested (about half

the manufacturers’ recommendation). After several

seasons with below standard control, the single spray

response was changed to double spraying at a 10-day

interval. To reduce costs, dosages were reduced in

each double application to 0.2 kg a.i./ha.

Broadcasting diazinon granules into paddy water is

an alternative to spraying (Pathak 1966) as is soil

incorporation of carbofuran (dela Cruz et al. 1981).

A further improvement in whorl maggot control

involved immersing the roots of seedlings carbosul-

fan SP or isofenphos DS/ha for 4 – 24 h prior to

transplanting in a plastic sheet-lined, small-bunded

paddy constructed next to the seedbed. This seedling

root soak method was performed for both wetbed

and dapog seedlings, but like soil incorporation

required early warning.

2.3. Defoliator thresholds

The first characters tested were percentage da-

maged leaves in the field itself and later in NF. Both

low (10%DL and NF10%DL) and high (30%DL

and NF30%DL) (20 – 30%) threshold levels were

tested. As a means to achieve early warning, larval

(L) densities were censused as a new character at

three levels: 0.5 (0.5L), 1 (1L), and 2 (2L) larvae/hill.

A higher level of 4 larvae/hill was initially tested and

later combined with 2L. A clue in detecting the

highly camouflaged 15 – 25-mm long leaf-green

colored larvae of both species is their characteristic

defoliation injury. Another method utilized when the

plants were small was to bend the rice hill and slap

the foliage to dislodge the larvae which floated on the

water surface for easy detection. During the first

2 years, leaf damage from whorl maggot and defoli-

ators was lumped together in a single %DL

character. But as whorl maggot proved the more

difficult to control, it required a more rigorous

insecticide regime and separate threshold characters.

Three insecticides were evaluated as foliar sprays

against defoliators at their minimum effective do-

sages (0.4 kg a.i. monocrotophos/ha, 0.5 kg a.i.

carbaryl/ha, and 12 g a.i. deltamethrin/ha). Mono-

crotophos was further tested by applying two sprays

at half the dosage (0.2 kg a.i./ha each) with the

second spray 10 days later.

2.4. Sampling methods

Monitoring of AT characters was carried out in the

threshold and untreated plots. Whorl maggot eggs

were counted at 4-day intervals beginning 3 d.a.t.

Defoliator larvae were monitored on a weekly basis.

All sampling was on a per-hill basis from 20 hills

taken in a stratified pattern. The number of tillers

and leaves per hill with pest damage were recorded

on those plant parts as appropriate.

Percentage DL was monitored in the untreated

and AT treatments on a weekly basis from 2 to 5 WT

for both whorl maggot and defoliators as a standard

indicator of pest density as well as to measure

percentage control if insecticide were used. AT

characters were judged by the percentage reduction

of damaged leaves as well as yield gain over the

untreated.

2.5. Threshold assessment

In order to assess the outcome of each AT

character, the pest infestation and yield loss were

both scored against benchmark infestation levels and

associated yield loss in each growth stage. Combin-

ing pest damage and yield loss into a single

benchmark was necessary as yield loss could only

be calculated in a given growth stage and not by pest.

The method was developed in order to evaluate

thresholds for each pest individually. The benchmark

levels were based on Smith et al. (1988). The

benchmark for both whorl maggot and defoliators

for each replicate was attained if infestation levels

exceeded 15% DL (for each pest) and yield loss was

greater than 250 kg/ha in the vegetative stage.

Evaluation of action thresholds for rice whorl maggot and defoliators 169

Four outcomes emerged: (1) if the AT was not

surpassed and was not justified based on both

benchmarks of yield and damage, it was scored

‘correct not to treat’, (2) if the AT was surpassed and

was justified by both benchmarks it was scored

‘correct to treat’, (3) if the AT were not surpassed

but was justified it was scored ‘should have treated’,

and (4) if the AT were reached but was not justified it

was scored ‘should not have treated’. The frequen-

cies of these four outcomes add to 100%.

Six criteria were assessed for each pest’s ATs:

(1) most justified decisions based on the damage þyield loss benchmark, (2) most percentage of ‘correct’

decisions, (3) fewest ‘incorrect’ ‘should have treated’

decisions, (4) fewest ‘incorrect’ ‘should not have

treated’ decisions, (5) highest ratio of errors to

‘correct decisions to treat’, and (6) highest correlation

to yield gain over the untreated. The fifth criterion

rewarded characters that triggered moderate levels of

AT decisions and in doing so made proportionally

fewer errors, as distinguished from characters which

had predominantly ‘correct not to treat’ results.

Chemical control efficacy for treated plots was

measured as the percentage reduction in insect den-

sity (damaged leaves or larvae) in the treated versus

the untreated plot divided by the density in the

untreated plot multiplied by 100. Because each pest

was monitored weekly in the threshold plots, there

was an opportunity to measure the effect of applying

insecticide against non-target pests, termed ‘colla-

teral control’. Such data were analyzed in the same

way as for target pest control.

2.6. Crop age and seasonal damage patterns

Whorl maggot and defoliator damage patterns

were constructed to describe the rates of damage as

the crop aged from an expected low point early in

the crop cycle to a peak sometime later. Knowledge

of such patterns could indicate the minimal AT

monitoring requirements in terms of timing and

frequency. The crop-age damage pattern was des-

cribed for each pest group and site separately from

the averages of each of the four weekly sampling

dates (2 – 5 WT) for each field in the untreated plots.

The results were then averaged over each crop.

Using the same data set a second analysis was

made on the effect of planting date. The hypothesis

supporting monitoring earlier planted fields assumes

progressively increasing pest densities from earlier to

later fields over the season. Data were used from the

untreated plots in the field trials, each of which was

purposely selected to achieve a range of planting

dates over the breadth of each season. The number of

elapsed days between the date the earliest field was

transplanted and date for each succeeding field was

calculated over all crops by site. The number of

elapsed days (seasonal age) from the date the first

field was planted to that for each successive field was

regressed against the mean damage (averaging the

mean weekly counts 2 – 5 WT, and then calculated

as the percentage change from the earliest field).

Regression was carried out for each site separately

on a per field basis and a significant positive corre-

lation would indicate a rising population over the

season.

2.7. Statistical analysis

Results were subjected to one-way ANOVA and

regression/correlation analysis where appropriate.

Treatment means were separated using the paired

t-test for two variables or least significant difference

(LSD) test for more than two variables. Means are

shown with standard errors of the mean (SEM) using

a pooled estimate of error variance.

3. Results

3.1. Pest densities

Whorl maggot infestations averaged 15% damaged

leaves across all crops over the four sites 2 – 5 WT

(Table I). Damage levels in Zaragoza and Koronadal

were twice those in the other two sites with no

significant difference between seasons within any

site. Defoliator damage averaged 5%, three times less

than that of whorl maggot. Defoliator incidence only

surpassed that of whorl maggot in Guimba during

wet seasons. As with whorl maggot, greatest defo-

liator densities occurred in Zaragoza and Koronadal.

No seasonal differences in abundance were apparent

within a site.


Examining crop age effects for whorl maggot, only

in Zaragoza did damage show a rising trend (from 14

to 22% damaged leaves) over the entire vegetative

period (Figure 1a). In Koronadal, damage had

peaked by 3 WT, before declining steadily thereafter.

No change in trend, either positive or negative, was

noted in the other two sites. Zaragoza and Koronadal

were the sites with highest damage, and it was noted

that AT levels were often exceeded by 2 WT, the

earliest date that damage is expressed.

Similar flat damage patterns over increasing crop

age for defoliators were observed in all four sites

(Figure 1b). The only increasing trend came in

Koronadal, with a slight increase from 6% 2 WT to a

peak of 8% damaged leaves 3 WT and levelling off

thereafter. In Zaragoza damage had peaked on 2 WT

at 12% and actually fell thereafter. Thus except for

whorl maggot in Zaragoza, damage levels had peaked

by 3 WT for both pest groups.

With seasonal effects, average damage incidence of

both pests increased significantly from the earliest to

the latest planted fields in the two sites with highest

damage incidence (Table II). Thus, monitoring

earlier planted fields was justified in Zaragoza and


Koronadal which showed significant linear build

up in infestation levels over the season for whorl

maggot and defoliators. There was no such pattern in

Guimba and Calauan.


3.3.1. Decision threshold characters. Across the four

sites, whorl maggot incidence surpassed threshold

levels in 40% of fields (column 1), the most for any

one pest in a given growth stage as seen in the upper

half of Table III. Note that the total number of crops

and fields in the site data section is lower than the

totals under the individual threshold data section

because more than one threshold character was

normally tested per field. Greatest frequency of

surpassing thresholds occurred in Koronadal and

Zaragoza (55 – 57% of fields), twice that of the other

two sites. In these sites 74 – 93% of fields surpassed

the damage benchmark of 15% damaged leaves, with

only 31 – 33% of fields in Calauan and Guimba,

underscoring significant site variation in pest density.

Collectively the thresholds seemed conservative

when measured against the damage benchmark

(column 2) (58%), but not when the yield loss bench-

mark (4250 kg/ha) is added (column 3) (28%).

Note that both the damage and yield loss bench-

marks need to be satisfied for a ‘correct’ decision

score. Following these criteria, most of the correct

decisions made were ‘correct not to treat’ decisions

(51%) (column 4), while ‘correct to treat’ decisions

averaged 20% over all sites and crops (column 5)

giving a total of 72% correct decisions. Incorrect

decisions totaling 29% were divided into two error

types: (1) ‘should have treated’ (12%) and (2) ‘should

not have treated’ (17%) based on benchmarks.

Both Zaragoza and Koronadal, the two sites with

the highest damage incidence, had the highest

‘correct to treat’ (27 – 30%) and lowest ‘correct not

to treat’ (30%) decisions and vice versa for the two

low incidence sites. The breakdown of ‘incorrect’

decisions showed fields in both sites errored more in

both error types, due to inappropriate thresholds.

Guimba and Calauan, with the fewest 4AT

decisions, had the lowest rates of incorrect decisions.

Most (76%) of the ‘should not have treated’

decisions had sufficient pest pressure but yield loss

was less than the benchmark level. Likewise of the

erroneous ‘should not have treated’ decisions, most

(56%) met the damage benchmark but not yield loss.

Overall yield gain for threshold treatments averaged

176 kg/ha over the untreated control (column 9),

highly significant for all sites, despite differences in

pest pressure.

Three-quarters of the fields tested thresholds

based on the eggs/hill character with the balance

going to damaged leaves. Comparing individual

characters in the lower half of Table III, 0.5E-pk,

15%DL, NF15%DL, and NF30%DL (see Table III

for explanation of abbreviations) had the highest

4AT rates (64 – 87%). It is important that threshold

characters have accurate predictability for both types

of correct decisions ‘correct to treat’ and ‘correct not

to treat’. Some thresholds were tested when damage

levels were consistently either high or low, thus

yielding biased results. For example, damage levels

over the five crops where NF2E and 1E-tc were

tested occurred under exceedingly low damage

incidence (4AT only occurred in 3 – 17% fields).

But NF15%DL and NF30%DL were tested in crops

with very high damage levels (4AT occurred in 85 –

98% of fields). A further bias was introduced as

characters with higher levels were more utilized in

low density sites and vice versa.

The development of ATs was an iterative process

during the field testing phase. When a new character

Table I. Comparison of whorl maggot and defoliator pest densities by season in four sites, Philippines.a

Damaged leaves (%)c

Site Seasonb

Crops

(no.)

Fields

(no.)

Whorl

maggot Defoliators

Zaragoza WS 12 72 20.4+ 2.2 a 11.0+2.4 a

DS 11 69 19.2+ 2.6 a 3.6+2.6 ab

Koronadal 1st 7 52 25.0+ 2.9 a 9.7+2.8 a

2nd 8 57 19.5+ 2.7 a 6.0+2.6 ab

Guimba WS 7 44 7.4+ 2.9 b 7.5+2.8 ab

DS 6 44 11.5+ 3.1 b 1.5 +3.0 b

Calauan WS 9 44 11.4+ 3.1 b 0.9+3.0 b

DS 8 37 5.8+ 2.7 b 0.8+2.8 b

total 68 419

avg 15.0+ 1.1 5.1+1.0

P 50.0001 0.05

F 6.38 2.17

df 67 67

aIn a column, means+SEM followed by a common letter are not significantly different (P� 0.05) by LSD test. bWS, wet season; DS, dry

season. cNon-insecticide treated plots, mean of weekly sampling 2 – 5 weeks after transplanting per crop.


was developed most sites tested it over the same

range of levels (e.g., 1E and 2E), but based on

evaluation each season those levels were adjusted

lower (e.g., 0.5E and 1E) if yield loss occurred but

the AT was not reached, or higher (e.g., 2E and 3E)

if ATs surpassed thresholds but no yield loss

occurred. Normally the lowest AT levels performed

better in the sites with highest incidence. It can be

Table II. Regression correlations between rice planting date and whorl maggot and defoliator damage in four sites, Philippines.a

Site Pest damage Linear regression

Zaragoza Whorl maggot (damaged leaves) y¼18.1 þ 4.6 x, r¼ 0.376, P�0.0001, df¼ 106

Defoliators (damaged leaves) y¼0.3 þ 2.3 x, r¼ 0.219, P¼0.03, df¼95

Koronadal Whorl maggot (damaged leaves) y¼15.2 þ 0.09 x, r¼ 0.541, P�0.0001, df¼ 62

Defoliators (damaged leaves) y¼73.9 þ 4.3 x, r¼0.424, P¼0.0005, df¼63

Guimba Whorl maggot (damaged leaves) ns, df¼67

Defoliators (damaged leaves) ns, df¼67

Calauan Whorl maggot (damaged leaves) ns, df¼65

Defoliators (damaged leaves) ns, df¼33

aPest damage is the dependent variable (y) measured weekly, planting date is the independent variable (x) based on the number of elapsed

days after the first planted field, level of significance (P� 0.05).

Figure 1. Damage patterns in the rice vegetative stage by (a) whorl maggot and (b) defoliators in four locations, Philippines.


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seen that the lower threshold levels, as expected, had

higher 4AT frequencies. Thus, the ranges from two

characters—0.5E to 2E and NF0.5E to NF2E—each

show decreasing rates of justification based on dam-

age with higher threshold levels.

The criteria of the best characters should be those

that have the greatest justification rates comparing

the values in column 1 with those of columns 2

and 3. If column 2 were higher than column 1, there

should be a high rate of incorrect ‘should have

treated’ decisions (e.g., NF1E, 15%DL). Vice versa

if column 2 were lower than column 1, there should

be a high rate of ‘should not have treated’ error (e.g.,

0.5E-pk). Those characters with the greatest similar-

ity (+5%) between frequencies of column 1 and

column 3 levels were NF0.5E, NF1E, NF2E, and

30%DL (Table III).

Characters that resulted in the most correct

responses (475% of occasions) were 1E and 2E,

NF2E, and 1E-tc. It is notable these characters relied

on egg densities rather than damaged leaves. The 2E

threshold had the highest rate of correct decisions

(89%) and achieved the lowest levels of both error

types. The thresholds NF0.5E, NF1E, NF2E

and 30%DL had the highest rates of the ‘should

have treated’ error. A wider array of characters—

0.5E, NF0.5E, 0.5E-pk, 15%DL, NF15%DL, and

NF30%DL—had the highest rates of the ‘should

not have treated’ error. NF0.5E had high rates of

both error types. This can occur when tested in

both high and low pest density sites. Those charac-

ters with the best ratios (values51) of error types to

‘correct to treat’ decisions (column 8) were 0.5E-pk,

NF15%DL, and NF30%DL which meant that more

correct decisions than errors were made. This is a

measure of a character in which performance was

based on more correct positive decisions than those

that resulted in mostly correct negative decisions.

There were no significant differences among

the characters in yield gain over the untreated

(Table III). But 0.5E, 1E, 2E, 0.5E-pk, 15%DL,

and 30%DL showed significant yield gains over the

untreated when analyzed individually. The characters

with the best scores for the five criteria were 1E and

2E which did well in all categories except having poor

ratios. No other character had passing scores on

more than two of the five criteria.

3.3.2. Insecticide response. Overall the insecticide

response was poor. None of the twelve insecticide

regimes tested against whorl maggot met the

standard of 80% control (80% fewer damaged leaves

compared to the untreated) (Table IV). Highest

efficacy (45% control) occurred with isofenphos as a

seedling root soak (at 3 weeks after treatment [WT]),

whereas carbosulfan as a root soak showed signs of

phytotoxicity and performed poorly. The two gran-

ules, carbofuran and diazinon, were poor performers

when soil incorporated or broadcast, although

carbofuran resulted in high yield gain. None of the

four foliar spray materials applied as a single

application between 7 and 28 d.a.t. performed well,

although triazophos and azinphos-ethyl were the best

(25 – 28% control). Deltamethrin as a single foliar

application performed sub par and resulted in lowest

yields despite lack of phytotoxicity symptoms.

Seedling root soak was superior during the first

3 weeks after treatment before performance declined.

Carbofuran, on the other hand, showed low initial

control, performing comparatively well only at

3 WT. Diazinon provided modest initial control

(1 WT) but efficacy declined steadily thereafter.

Double sprays based on damaged leaf thresholds

improved efficacy while single sprays performed

poorly from the onset, and greatly declined after

3 WT. Thresholds based on egg counts performed

Table IV. Insecticide efficacy in response to whorl maggot action thresholds.a

Insecticide

Application

method/timing

Dosage per

application

(kg a.i./ha)

Damaged leaves

3 WTb,c

(% control) Yield gain (kg/ha)b n

Monocrotophos One spray 7 – 28 d.a.t. 0.4 20.6+ 4.7 d 154+ 84 b 45

Two sprays 7 and 14 d.a.t. 0.4 25.1+ 5.1 c 264+ 94 a 38

Two sprays 2 and 9 d.a.t. 0.4 31.8+ 4.3 b 336+ 103 a 54

Azinphos-ethyl One spray 7 – 28 d.a.t. 0.4 24.5+ 4.5 c 159+ 94 b 49

Two sprays 2 and 9 d.a.t. 0.4 29.7+ 9.1 bc 341+ 187 a 12

Deltamethrin One spray 7 – 28 d.a.t. 0.012 721.6+ 10.0 e 7198+ 149 c 10

Two sprays 2 and 9 d.a.t. 0.012 36.7+ 8.2 b 171+ 160 b 15

Triazophos One spray 7 – 28 d.a.t. 0.4 28.2+ 6.7 bc 177+ 139 b 22

Diazinon Broadcast granules 14 d.a.t. 0.75 13.0+ 6.5 d 133+ 126 b 24

Carbofuran Soil incorporated granules 0.5 22.6+ 8.2 cd 414+ 160 a 15

Isofenphos Seedling root soak 0.5 45.4+ 4.8 a 233+ 94 ab 44

Carbosulfan Seedling root soak 0.5 17.0+ 11.9 d 7127+ 230 c 11

P 50.0001 50.0001

F 4.81 4.17

df 357 336

aWT, weeks after treatment, d.a.t., days after transplanting, mean+SEM. bIn a column, means followed by a common letter are not

significantly different (P� 0.05) by LSD test. cAverage of weekly sampling 1 – 4 WT.


well initially but both single and double declined

similarly thereafter.

Only deltamethrin of the insecticides tested at low

dosages (0.5 kg a.i. carbofuran granules/ha, 0.75 a.i.

diazinon granules/ha, and sprays at 0.4 kg a.i./ha,

deltamethrin 12 g a.i./ha) attained 450% control

1 – 4 WT even though some spray materials were

applied twice. Incidental control of coterminous rice

pests (defoliators, leaffolders, and stemborers) ranged

from 23 to 40% depending on pest group (Table V).

The benefit in terms of both control and yield gain

of multiple and early spray application was illustrated

by monocrotophos where two sprays were superior

to one spray particularly when initiated earlier

(2 and 9 d.a.t. versus 7 and 14 d.a.t.) (Table IV).

Two azinphos-ethyl sprays also resulted in significant

yield gain over a single spray although efficacy was

insignificant. Surprisingly two sprays of deltamethrin

overcame the poor control and low yield that plagued

the single spray.

Early warning is necessary to take advantage of the

best performing insecticide regimes. Sampling eggs

rather than damaged leaves allows earlier warning

when monitoring the field itself. The best results

from egg monitoring came with 4-day intervals from

3 to 19 d.a.t. Decisions made after the 7 d.a.t.

sampling date are too late for the best insecticide

options. But if earlier-planted, neighboring fields are

monitored any character can be used. When the data

were pooled to contrast the field itself with neighbor-

ing fields over all insecticide regimes, greatest control

occurred from monitoring neighboring fields (35 –

42% control) whether egg or damaged leaf characters

were used (Table VI). Least control occurred when

monitoring the field itself with damaged leaves

(15%), while intermediate levels of control occurred

with egg monitoring in the field itself. There was no

significant yield difference, however, between the

four characters.

3.4. Defoliator thresholds

3.4.1. Decision threshold characters. Naranga and

Rivula defoliators thresholds were surpassed in only

12% of fields (Table VII). Among the four sites,

highest frequency of threshold responses occurred

in Zaragoza and Koronadal (18 – 19% of fields) with

an intermediate level in Guimba. Remarkably in

Calauan not one field was treated as a result of any of

the seven characters tested in 13 crops, nor in any of

the 81 fields monitored was the benchmark of

15% damaged leaves exceeded.

Over all sites, most decisions fell within ‘correct not

to treat’ (83% of fields), with Calauan scoring 100%

averaged over all threshold characters. Due to the low

populations, there were no differences between

thresholds among sites within the decision class

‘correct to treat’. The threshold incidence (column

1) matched well with the rate of damage levels

4benchmark (column 2) despite most characters

(65% of fields) being based on larval densities. All

sites averaged 480% correct decisions with Calauan

reaching 100% followed by Guimba with 90%. Most

of the thresholds errored in being aggressive (7% of

the time), mostly in Koronadal and Zaragoza. There

were no significant differences between sites in yield

gain from the threshold treatments but in all sites

there were significant gains over the untreated.

Decisions to treat were very high for NF10%DL

and NF30%DL while very low for the others. All the

characters responded well to pest density, as despite

wide variation among characters in reaching thresh-

olds, remarkably there were no differences among the

seven characters in making correct decisions, which

ranged from 81 to 100%. For example NF10%DL

exceeded the threshold on 100% of occasions while

30%DL and 2L only reached threshold levels in

4% of occasions each. The reason for the 100%

frequency with NF10%DL was that it was tested

during three crops of very high incidence, averaging

73% of fields surpassing the damage benchmark.

There were greater correlations with the bench-

marks between characters based on damaged leaves

than those based on larvae. Only 0.5 L, among the

larval characters, correlated with the damage þ yield

loss benchmark, while all those based on damaged

leaves did. There were no differences among char-

acters in correct decisions (85 – 90%). Also there

were no significant differences among characters

regarding frequency of both error types, but numeri-

cally there was a higher frequency of decisions that

errored from ‘should not have treated’ than ‘should

have treated’ errors. The ratios of both error types

to ‘correct to treat’ decisions were highly favorable

with NF10%DL (0.2) but only marginally with

NF25%DL (0.9).

There were also very large differences in yield loss

between characters that most likely reflected differ-

ences in infestation levels. However, there were no

significant differences in yield gain among the

Table V. Control of collateral vegetative stage pests by insecticides based on thresholds for whorl maggot and defoliators.

Control of nontarget pest (%)a

Target pest Whorl maggot n Defoliators n Leaffolders n Stemborers n

Whorl maggot 36.1+ 4.9 200 40.0+ 3.7 180 22.9+6.4 162

Defoliators 26.4+4.8 48 24.6+ 7.9 58 15.4+8.9 55

aDamaged leaves for whorl maggot, defoliators. Average of four sites, n¼number of fields, mean+SEM.


threshold characters. But when compared to the

untreated and not each other, significant yield gain

occurred with all characters that sampled the field

itself rather than neighboring fields. Overall the

characters with the best scores (four of the five

criteria) were 10%DL, NF10%DL, and NF25%DL.

Preference would go to NF10%DL based on its low

ratio score.

A case was made earlier to separate whorl maggot

and defoliator characters due to differences in

preferred control practices, but since a synergistic

relationship regarding their damage and yield loss has

since been detected (Litsinger 1993), there is cause

to re-examine this relationship. Abundance of both

pests was highly site dependent, although those sites

with high whorl maggot also had high defoliator

damage. Defoliators were too low in Calauan for a

relationship between the two pests to be detected. In

the other three sites, from 11 to 24% of fields had

both whorl maggot and defoliators surpassing thresh-

olds in the same fields (Table VIII). While only whorl

maggot exceeded thresholds in Zaragoza and

Koronadal in 74 – 75% of fields, in Guimba more

(23%) fields had only defoliators exceeding thresh-

olds. If a threshold of 15% damaged leaves from both

pests combined and 250 kg/ha yield loss benchmark

would have been established, 25% of fields in

Guimba and 10% in Zaragoza, would have exceeded

the threshold than with one pest alone.

3.4.2. Insecticide response. Monocrotophos per-

formed better as a double spray in preventing leaf

damage from defoliators (Table IX). Level of

control, measured both as damaged leaves and larval

mortality, was quite low overall, barely exceeding

50% with the best insecticide as a single spray.

Monocrotophos as a double spray achieved 490%

larval mortality 1 – 4 WT.

Over all characters tested, carbaryl provided equal

control to monocrotophos as a single spray against

larvae but was inferior on the basis of damaged

leaves. Deltamethrin provided over 90% larval

control 1 WT, but mortality quickly declined to nil

after 2 WT giving a low 15% residual mortality when

averaged 1 – 4 WT. There was no difference in yield

gain from any of the insecticides despite significant

differences in efficacy. Insecticides targeted against

defoliators resulted in 15 – 26% control on non-

target pests (Table V).

In contrast to whorl maggot there was no signi-

ficant advantage in terms of level of control among

threshold characters—percentage damaged leaves

or larvae per hill—whether monitored in the field

itself or a neighboring field (F¼ 1.23, df¼ 143,

P40.05).

4. Discussion


ATs are most suitable for chronic pests that only

periodically rise above threshold levels in a given field

from initial subeconomic levels (Stern et al. 1959).

Studies on crop age showed that early crop monitor-

ing did not produce the desired results. In the

high pest density sites of Zaragoza and Koronadal,

ATs based on damaged leaves were often exceeded

by the first sampling date 2 WT while in the low pest

density sites of Guimba and Calauan there was no

rising pattern in damage from the first to last

sampling dates (Figure 1). The former case supports

monitoring earlier planted fields or use of the egg

character to be able to take advantage of the best

chemical control options while in the latter there was

no benefit from sampling beyond 2 WT, although

weekly monitoring is still advised throughout the

crop cycle.

There were also site differences with respect to

seasonal effects with the high pest density sites

showing significant pest build up from earlier to later

planted fields, while no build up occurred in the low

density sites (Table II). Thus, different monitoring

strategies and AT characters may be needed for high

and low pest density sites. Sites with higher pest

densities have characteristics of longer periods of rice

availability throughout the year, greater planting

asynchrony, and larger rice areas (Loevinsohn et al.

1988).


ATs need to be tailored to site pest density

characteristics with respect to matching monitoring

tools with chemical control methods, as the former

are not necessarily compatible with the latter. For

example, the best insecticide methods (root soak, soil

incorporation, and double sprayings) by definition

require a decision prior to or soon after transplanting.

The only characters that provide such a warning are

those based on eggs or by monitoring earlier planted

Table VI. Comparison of whorl maggot threshold characters and

monitoring sites on insecticide efficacy and yield.

Character

Sampling

site

Damaged

leavesa

1 – 4 WTb

(% control)

Yield

gain

(kg/ha) n

Damaged

leaves

Field itself 15.2+ 5.5 c 195+ 85 38

Neighbouring

field

34.7+ 3.4 a 245+ 52 102

Egg Field itself 21.2+ 3.2 b 202+ 50 111

Neighbouring

field

41.5+ 3.6 a 188+ 55 91

P 50.0001 ns

F 9.18 1.13

df 341 298

aIn a column, means+SEM followed by a common letter are

not significantly different (P� 0.05) by LSD test, mean+SEM.bAverage of weekly means 1 – 4 weeks after treatment (WT) across

four sites.


Tab

leV

II.

Def

oliat

or

acti

on

thre

sho

ldan

alys

isb

ylo

cati

on

and

char

acte

rfr

om

fou

rsi

tes

ove

ra

13-y

ear

per

iod

,P

hilip

pin

es.

Fre

qu

ency

per

fiel

d(%

)aD

ecis

ion

s(%

)a

Just

ified

Co

rrec

td

ecis

ion

Inco

rrec

td

ecis

ion

Cro

ps

Fie

lds

Pes

tF

rom

Fro

m

dam

ageb

Co

rrec

t

no

tto

Co

rrec

t

Sh

ou

ld

hav

e

Sh

ou

ld

no

th

ave

Rat

io

(6)þ

(7)

Yie

ldgai

n(A

Tvs

un

trea

ted

)d

(no

.)(n

o.)

4th

resh

old

dam

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,bþ

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fields. Monitoring earlier planted fields did not

produce good results in low pest density sites. The

best performing characters 1E and 2E are limited in

terms of utilization with the best insecticide control

as there is only a narrow window during the first two

monitoring periods for eggs (3 and 7 d.a.t.) as

insecticide efficacy is best as two foliar sprays applied

within the first week after transplanting.

Examining the insecticide regimes when the 1E

and 2E characters were used revealed they were

predominantly single sprays 5 – 15 d.a.t. or double

sprays beginning 5 – 7 d.a.t. triggered just after one or

two samplings 3 and 7 d.a.t. Single sprays performed

poorly but were rarely applied, as most outcomes

(83% of occasions) were ‘correct not to treat’

decisions.

The best performing spray materials acted by

contact against adults alighting on the treated crop

and against larvae immediately after hatch before

entering tillers and also were ovicidal (Pantua and

Litsinger 1987). Monocrotophos is noted to have

translaminar systemic activity but is not sufficient to

reach larvae feeding at the base of tillers. Carbofuran

is highly systemic, but because it binds readily with

soil (Siddaramappa et al. 1978), is not effective at

economically acceptable dosages as a paddy water

broadcast. Soil incorporation increases dosages that

reach the root zone and reduces loss to achieve

greater residual activity (Seiber et al. 1978). Diazinon

has only pseudo-systemic activity (Pathak 1966), but

even if applied at high dosages (41.5 kg a.i./ha)

requires deeper ponding than most rice farms can

manage in order to effect capillary action (Bandong

and Litsinger 1979).

The most effective and economical insecticide

treatment was the single-application seedling root

soak, but its use requires a pre-planting decision.

Most farmers opt for the cheaper sprays. Double

spray application is the next most effective method

but doubles the cost, and best results occurred with

early application and higher doses. The high yield

gain from carbofuran was probably due to phytotonic

effects (Venugopal and Litsinger 1984).

The best early warning came from monitoring

earlier planted fields (eggs or damaged leaves) which

resulted in more effective chemical control methods

than were possible from monitoring the field itself

(Table VI). Overall egg characters utilizing earlier

planted fields did not perform as well as the NF-DL

characters. The NF character associated with the

most correct decisions was NF2E, which, however,

was highly conservative with only a rate of 1%

‘correct decisions to treat’ along with a surprisingly

high 17% ‘should have treated’ rate. More ‘correct

decisions to treat’ occurred from lowering the level to

NF1E and NF0.5E, but in doing so both error types

also increased. But NF15%DL and NF30%DL had

the highest frequencies of ‘correct to treat’ decisions

along with favorable 0.6 ratios. Both NF15%DL and

NF30%DL performed equally well, with the only

detraction of not being correlated with yield. The

NF%DL character, tested over a range from 10 to

30% damaged leaves, is suggested for Zaragoza and

Koronadal where seasonal effects were significant.

Perhaps farmers can fine tune the percentage dam-

aged leaves with experience.

The best performing characters 1E and 2E should

be confined to low density sites. Which character and

control method will be optimal will be the result of

other considerations: (1) ability to afford the more

costly insecticide regimes, e.g., two spray applica-

tions or granules versus a single or lower dosage

application, (2) keenness of sight to monitor whorl

maggot eggs (a problem with the majority of

older farmers), and (3) willingness to adopt new

and more elaborate technologies such as trap crop

monitoring and seedling root soak method. A

number of these constraints could be overcome if

farmers made decisions in groups, thus those with

poorer eyesight could have younger farmers in-

spect the crop (Matteson 2000). More difficult-to-

learn practices could be reinforced by the benefits of

group learning. Some farmers may be hesitant to

perform a corrective action before damage is seen

and thus would be hesitant to adopt early warning

monitoring.

Monitoring damaged leaves would be preferred

by many farmers over eggs as it does not require

keen eyesight, and with experience assessing

damaged leaves can be learned by ‘gestalt’ patterns

Table VIII. Frequency of whorl maggot and defoliators surpassing thresholds alone or combined.

Frequency 4AT per field (%)a

Site Whorl maggot alone

Defoliators

alone Both Only when damage combined

Zaragoza 74.9+6.0 a 3.5+3.7 b 11.2+ 4.9 10.4+3.7b

Guimba 34.0+7.5 b 23.3+4.6 a 18.2+ 6.1 24.5+4.6a

Koronadal 74.3+7.2 a 1.3+4.5 b 24.3+ 5.9 0+4.6b

P 50.0001 0.001 ns 0.001

F 10.66 7.32 1.49 7.28

df 86 86 86 86

aAT¼ action threshold of 15% damaged leavesþ 250 kg/ha yield loss benchmarks met. In a column, means+SEM followed by a

common letter are not significantly different (P� 0.05) by LSD test.


(Bandong et al. 2002). Most weekly monitoring in

practice can be quickly assessed and the tedious

number based monitoring could be left to only those

cases where there is doubt.

In Table III it was noted that even if columns 1

and 2 were highly similar, it does not signify that

column 3 will follow suit as those fields with high

damage may not be the same fields showing high

yield loss. This variance is evidence of the problem

of obtaining reliable damage functions with rice

(Litsinger et al. 2005). In addition damage may not

necessarily correlate with yield loss as more than one

pest may occur at damaging levels in each crop. Thus

yield loss may be high but whorl maggot damage low

as loss may be attributed to other pests or interac-

tions with other crop stresses.

4.3. Defoliators

The best performing characters for defolia-

tors were 10%DL, NF10%DL, and NF25%DL,

although the results should be viewed with caution as

the three main characters—(1) larvae per hill, (2)

percentage damaged leaves in the field itself, and (3)

percentage damaged leaves in neighboring fields—

were tested under different infestation levels by

chance. 0.5L, 1L, and 2L were tested under very

low damage levels while 10%DL and 30%DL were

tested under intermediate levels, with NF10%DL

and NF30% DL under very high levels (column 2

Table VII). Therefore, until each character is tested

in both high and low infestation levels, the results are

to be viewed as preliminary. In addition there was

no differentiation between characters in the cate-

gories of total correct decisions or both error types.

In Zaragoza and Guimba, combining damaged leaves

with whorl maggot may have merit. The lack of

correlation with planting date in low pest density sites

and lack of significant yield gain among the NF

characters are negative reasons regarding combining.

The 0.2 ratio (incorrect to treat:correct to treat) in

the NF10%DL treatment is more convincing and is

the suggested choice in the high density sites with

10%DL in the low density sites.

Monocrotophos was the best performing insecti-

cide but the less hazardous carbaryl is suggested as it

performed as well against larvae. A double spray

doubles cost making it less attractive and as there was

no significant yield gain despite significant increase

in degree of control, a single spray appears adequate.

Acknowledgements

We are highly appreciative of the generous coopera-

tion provided by over 400 farmers in the study sites.

Their willingness to become experimenters with the

research teams and devote at times a tenth of their

rice lands to trials is a testament to their desire to

seek improvements in rice production technology.

Many locally hired project staff were responsible for

Tab

leIX

.C

hem

ical

con

tro

lef

fica

cyo

fm

ater

ials

and

spra

yfr

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s.

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(no

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no

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57

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82

8

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conducting the trials and their invaluable contribu-

tions are acknowledged. Those assisting in Zaragoza

were Catalino Andrion and Rodolfo Gabriel, in

Guimba George Romero, in Calauan Mariano

Leron, Eduardo Micosa, and Carlos de Castro, and

in Koronadal Hector Corpuz, Joseph Siazon, Beatriz

Velasco, and Anita Labarinto. Cooperation of the

staff in the Central Luzon and Mindanao regions of

the Philippine Department of Agriculture is highly

appreciated.

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rice whorl maggot action & defoliator action thresholds

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