Entomol. exp. appl. 45: 283-288, 1987 �9 Dr W. Junk Publishers, Dordrecht - Printed in the Netherlands 283

The effect of a chemical deterrent, released from the frass of caterpillars of the garden pebble moth, on cabbage root fly oviposition

T.H. Jones & S. Finch Institute of Horticultural Research, Wellesbourne, Warwick, CV35 9EF U.K.

Accepted: July 7, 1987

Key words." Delia radicum, cabbage root fly, Evergestisforficalis, garden pebble moth, frass, deterrent, ovipo- sition, interspecific interactions

Abstract

The cabbage root fly, Delia radicum (L.), was deterred from laying eggs on cauliflower plants that had been sprayed with a suspension of the frass of caterpillars of the garden pebble moth, Evergestisforficalis (L.). Polar extracts of the frass deterred oviposition irrespective of the cruciferous plant species on which the caterpillars had been feeding. Non-polar extracts of the frass had no effect. Spraying plants with macerates from Brassica leaves stimulated fly oviposition whereas spraying plants with macerates from garlic mustard leaves deterred fly oviposition. Macerates from the leaves of all other plants tested had no effect. In field experiments the deterrent effect persisted 2 -3 days after leaves were sprayed with frass extracts. Plants infested with feeding caterpillars and contaminated with only a few discrete frass pellets were as deterrent to the fly as those sprayed with frass suspensions.

Introduction

Plant odour (Finch, 1978), plant colour (Prokopy et al., 1983) and the numerous interactions between these two stimuli as the plant age (Ellis et al., 1979) influence host selection by the cabbage root fly, De- lia radicum (L.) (Diptera:Anthomyiidae). While intra-specific interactions are known to play an im- portant role during host plant selection in some spe- cies (Rothschild & Schoonhoven, 1977; Renwick & Radke, 1980; McNeil & Quiring, 1983), the effects of inter-specific behavioural relationships have not been determined. This is true even for the pest com- plex on crucifers, which has been studied in con- siderable detail by a number of authors (e.g. Pimen- tel, 1961a & b; Root, 1973).

Earlier studies have shown that the cabbage root fly avoids plants with caterpillars or frass of the

caterpillars of the garden pebble moth, Evergestis forficalis (L.) (Lepidoptera: Pyralidae) (Finch, 1983; Finch & Jones, 1987). This moth is considered to be of only low pest status in the British Isles. This paper describes how chemicals emanating from the frass of caterpillars of E. forficalis (L.), affect host plant selection by D. radicum, a species which normally lays its eggs in the soil alongside the selected plant and in which oviposition by one female does not de- ter oviposition by others (Finch & Coaker, 1969).

Materials" and methods

Insects. D. radicum was reared using the method described by Finch & Coaker (1969). Caterpillars of E. forficalis were reared on a range of cruciferous plants in a constant environment room maintained

284

at 20 _+ 1 ~ and 65 _+ 5~ r.h. and illuminated at a L 18: D 6 h photoperiod.

Plants. Brussels sprout (Brassica oleracea var. gem- mifera Zenk.), cabbage (B. oleracea var. capitata (L.) Alef), cauliflower (B. oleracea var. botrytis L.), Chi- nese radish (Raphanus sativus L. var. longipinnatus Bailey), garlic mustard (Alliaria petiolata (Bieb.) Cavara and Grande), horse-radish (Armoracia rusticana Gaertn., May and Scherb), swede (B. na- pus (L.) var. napobrassica DC (L.) Reichenb) and wallflower (Cheiranthus cheiri L.) plants were used as food for the caterpillars ofE. forficalis. All plants were grown from seed in 7.5 cm diameter pots of John Innes compost and used 8 - 10 weeks after sow- ing.

Cauliflower plants (cv. White Rock) were used in both laboratory and field-cage experiments. Plants were raised from seed sown in 7.5 cm diameter pots of John Innes compost maintained in a glasshouse at temperatures of 20 _+ 2~ during the day and 10 _+ 2 ~ during the night. Plants for laboratory ex- periments were used 8 -10 weeks after sowing. For the field-cage experiments, 10-week old plants were re-potted into 10 cm diameter pots and used one week later.

Preparation of frass suspensions. Frass was collected daily from filter paper placed beneath foliage on which caterpillars of E. forficalis were feeding. A sprayable suspension was made by stirring 1 g of frass, or 1 g of macerated leaf tissue, into 50 ml of distilled water and then adding two drops of deter- gent solution ('Savona', Premiere Products) as a wet- ting agent.

Preparation offrass extracts. Frass (10 g) from cater- pillars feeding on cauliflower, garlic mustard and swede was macerated in 10 ml di-chloro-methane (DCM) and 20 ml of distilled water. The mixture was centrifuged and the water layer decanted from the DCM. By filtering the suspended particles from the water layer and collecting the filtrate, a solution of polar chemicals was obtained. The DCM layer was evaporated to dryness and shaken vigorously in 15 ml distilled water to produce a suspension of the non-polar chemicals.

Laboratory experiments. The compost in which the plants were growing was covered with silver sand to standardize the egg-laying medium before exposure to the flies. Except for experiments with frass ex- tracts, three 'check' plants were sprayed with 4 ml distilled water and three with 4 ml of either macerat- ed leaf suspension or frass suspension. In the other experiments, water and either the aqueous or the DCM extract were used. All experiments were done in a test chamber consisting of three 65 cm • 65 cm and 45 cm high cages arranged one above the other (Ellis & Hardman, 1975). Each cage contained a 60 cm diameter turntable which rotated once every 4 minutes. Test plants were arranged regularly (12 cm apart), though in random order, on the turn- tables.

Twenty 5 - 6 day-old gravid, mated, female D. radicum were released into each cage and allowed to lay eggs for 1 day before the eggs on each plant were counted. Each experiment was repeated three times and the results were analysed using the variance of log e transformed data and the values then back- transformed for the tables.

Field experiments. These were all done in net cages (6 m x 3 m and 2 m high) (Finch, 1971). In each cage, 24 potted cauliflower plants were 'transplant- ed' in four rows of six plants 40 cm from each other and cage sides. Prior to transplanting, the pots were topped-up with finely-sieved soil which was made contiguous with the surface of the surrounding soil in the cages. A 3 m • 1 m strip of the herbaceous plants from alongside a hedgerow was transplanted inside the southern end of each cage to provide shelter for the flies. These strips contained dormant plants of cow parsley (Anthriscus sylvestris (L.)), which later flowered and provided the flies with nec- tar, a source of sugar solutions which are essential for flies to survive under field condition (Finch & Coak'er, 1969; Finch, 1971). Each experiment was repeated simultaneously in three cages.

In Expt. l, 200 male and 200 gravid female, 5 - 6 day-old D. radicum were released into each cage. Af- ter 2 days the soil around each plant was removed and the eggs counted. Using the numbers of eggs laid, plants of similar attractiveness were paired be- fore one plant of each pair was sprayed with 8 ml of

frass suspension and the other with 8 ml of distilled water. More liquid was applied to plants in the field than in the laboratory experiments because they were approximately twice as large. The plants were again exposed to flies for 2 days before the eggs were count- ed.

The first part ofExpt . 2 was similar to Expt. 1 but the 24 plants tested were divided, on the basis of the initial egg-laying period, into groups of three. One

plant in each tree was sprayed with distilled water and the other two with frass suspension. The soil around the plants was sampled for eggs on each of the following 4 consecutive days. The experiment was then repeated but after 2 days, one of the treated plants in each three was re-sprayed with frass suspen- sion. Eggs were again sampled daily.

In Expt. 3, groups of three plants were selected as in Expt. 2..One plant was sprayed with water, anoth-

er with frass suspension and the third was 'inoculat- ed' with ten, third instar caterpillars ofE. forficalis. After 48 h, the first two plants were resprayed as be- fore. The soil surrounding the plants was sampled every two days for D. radicum eggs. Every 5 days, a further 75 flies of each sex were released into the cages used for Expts 2 and 3 to maintain an ade- quate level of oviposition.

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Results

Frass produced by caterpillars of E. forficalis (in fu- ture referred to only as caterpillars) deterred oviposi- tion by D. radicum, regardless of the host-plant on which the caterpillars had been feeding (Table 1). Approximately three times as many eggs were laid on the 'check' plants as on plants sprayed with frass sus- pensions. Spraying suspensions of macerated leaf tissues produced variable effects. Only the macerate from the leaves of garlic mustard deterred oviposi- tion as much as the frass suspensions. Generally, macerates from leaves of Brassica species stimulated cabbage root fly oviposition whereas suspensions of macerated leaves of other genera had no effect.

Separate testing of the polar and non-polar con- stituents of the frass revealed that it was the polar (water-soluble) constituents of the frass from cater- pillars fed on cauliflowers, garlic mustard and swede that deterred oviposition (Fig. 1).

Results from the field cages in Expt. 1 were similar to those from the laboratory experiments. Fewer D. radicum eggs were laid around cauliflower plants sprayed with the frass suspension (~ = 22 eggs/plant) than with distilled water (X = 59 eggs/plant) ( P < 0.05).

The persistence of the deterrent effect of the frass suspension on D. radicum oviposition was studied

Table 1. Mean numbers of D. radicum eggs recovered/plant (nine plants) f rom cauliflower plants sprayed with 4 ml of either distilled water ( 'check') or (a) leaf macerate or (b) suspension of frass produced by caterpillars of E. forficalis.

Plants f rom which leaf

tissue and frass were obtained

Material sprayed on plants

(a) (b)

Water Macerate of Water Suspension

leaf tissue of frass

Brussels sprout 49 157"** 108 38***

Cabbage 43 120"* 103 32"** Cauliflower 50 142"* 123 38"**

Chinese radish 60 46 140 49**

Garlic mustard 150 43** 103 33*** Horse-radish 67 74 81 24"*

Swede 59 164"* 168 38*** Wallflower 63 67 110 28"* *

Difference significant from water, **P = 0.01 ; ***P= 0.001.

8 0 r

m

z

4 0 -

F 2 0 -

I

oi

8 0 -

6 0 -

4 0 -

2 0 -

0 �84

Cauliflower Garlic mustard

Origin of frasa

286

T !

i Swede

POLAR

NON POLAR

Fig. 1. Mean numbers of D. radicum eggs recovered from cauliflower plants sprayed with 4 ml of distilled water (blank) or polar (striped) or non-polar (cross hatched) extracts of the frass produced by caterpillars of E. forficalis feeding on different host- plants. Bar indicates L.S.D. (P<0.05).

over several days (Expt. 2). Oviposition was reduced for at least 2 days but the effect was not evident'after 4 days (Table 2). Spraying host plants at 2-day inter- vals maintained the repellent effect of the frass sus- pension.

The effect of caterpillars on the leaves was com- parable to spraying the plants with the frass suspen- sion (Table 3), The suspension was more repellent than the caterpillars for the first 4 days after spraying but thereafter produced similar effects.

Discussion

Delia radicum females thoroughly examine the sur- face of the foliage of cruciferous plants with both the

Table 2. Mean numbers of eggs laid by D. radicum on cauliflower plants (24 per cage) sprayed with 8 ml of either distilled water ('check') or a suspension of the frass produced by caterpillars of E. forficalis over an 1 l-day period. Eg. sa." Eggs sampled; Gr. = Group; Material sprayed on plants: Gr. A = Water; Gr. B & Gr. C = Suspension of frass.

Day of Plant treatments Mean no. eggs/plant

experiment Gr. A Gr. B Gr. C

1 Levels of D. radicurn oviposition used for selecting plants

2 All plants sprayed:

3 Eg. sa. 4 Eg. sa. 5 Eg. sa. 6 Eg. sa. 6 All plants re-sprayed: 7 Eg. sa. 8 Eg. sa. 8 Gr. A and B plants re-sprayed: 9 Eg. sa.

10 Eg. sa. 10 Gr. A and B plants re-sprayed: 11 Eg. sa.

28 28 26

33 10" 11" 40 13" 12" 40 39 3I* 56 61 57

42 11"* 11"* 37 11" 13"

37 8** 32 38 16" 38

54 20** 55

Difference significant from water (Gr. A) at *P<0.05; **P<0.01.

287

Table 3. Mean numbers of D. radicum eggs laid around plants sprayed every 2 days with 4 ml of either distilled water ( 'check') or

a suspension of frass produced by caterpillars of E. forficalis compared with plants infested with caterpillars. Gr. = Group; Gr. A

= plant sprayed with water; Gr. B = plants sprayed with suspension of frass; Gr. C = ten caterpillars added/plant .

Mean no. eggs laid/plant

Gr. A Gr. B Gr. C

Plants selected for stimulating similar levels of D. radicum oviposition 60 65 66

No. days plants exposed to oviposition following treatment

2 66 17"* 25 **+

4 74 15"* 39 **+

6 78 25** 26**

8 63 20** 27**

Differences significant (P<0 .01) from Gr. A** and between (P<0 .05) Gr. B and Gr. C § .

labellum and their tarsal receptors to determine the plant's suitability as an oviposition site before laying their eggs, generally in the soil next to the stem (De Wilde, 1947; Traynier, 1967). Finch & Jones (1987) showed that, during this process, host-plant selec- tion was influenced by the frass of other insects and that these effects were influenced sometimes by the host plants on which the frass-producing insects were feeding. The results in this paper show that, ir- respective of host-plant, frass produced by the cater- pillars of E. forficalis deterred oviposition by D. radicum.

Some ways in which chemicals released from in- sect frass influence intra-specific behavioural rela- tionships during host-plant selection have been described (Renwick & Radke, 1985) but there is little comparative data on such effects during inter- specific interactions.

D. radicum were deterred from laying eggs by the polar fraction of the frass of E. forficalis. Polar de- terrents from non-host plants were also responsible for rejection of plants by gravid Pieris rapae (Ren- wick & Radke, 1985). Many other plant-derived chemicals may also be involved when once insect species influences another during host-plant selec- tion. For example, Finch & Jones (1987) showed that caterpillars of six species attacking cruciferous plants reduced the non-polar sulphur compounds that characterise the leaves of their host plants and excreted them as nitriles in their frass. In certain in- stances, the feeding caterpillars stimulated D. radi- cure oviposition, largely by releasing chemicals simi-

lar to those released normally from damaged leaves. With other species of caterpillars, the additional chemicals did not affect host plant selection by D. radicum.

The deterrent effect of the frass suspension lasted only 2 to 3 days in the field but, under controlled laboratory conditions, the effect lasted for up to 7 days (T.H. Jones, unpublished). Therefore, some component of the deterrent appears to be relatively stable and non-volatile. The persistence of chemicals which alter the spatial distribution of phytophagous insects varies considerably. The oviposition- deterrent pheromone secreted when Pieris brassicae lays eggs retains its activity for at least 7 weeks (Schoonhoven et al., 1981), the marking pheromone of the apple maggot fly, Rhagoletispomonella, per- sists for about 3 weeks (Averill & Prokopy, 1987) whereas other more volatile deterrents/pheromone markers often persist for less than one day (e.g. Brantjes, 1976; Schurr & Holdaway, 1970).

The presence of t?. forficalis caterpillars feeding on cauliflower plants had a deterrent effect similar to spraying the plants with a suspension of their frass. Differences in the results obtained during the first four days of this comparative experiment may have resulted from either of two factors, or from both. Firstly, the sprayed suspensions covered more leaf area than the frass which tended to remain as discrete, compact pellets. Also, damage caused to the leaves by caterpillars feeding may have released additional chemicals that stimulated D. radicum to iay eggs and thus partially nullified the deterrent

288

effect. Secondly, caterpillars of E. forficalis ingest leaf tissue at a slower rate than many other species (Theunissen et al., 1985). Differences observed dur- ing the initial 4 days of the experiment may be the result of the time required for the caterpillars to pro- duce sufficient frass to elicit a response comparable to that of the frass suspension.

R~sum~

Action sur Delia radicum d'un dissuadant chimique, libdrd par les excrements de Evergestis forficalis.

D. radicum a 6vit6 de pondre sur des pieds de chou-fleur, qui avaient 6t6 pulv6ris6s avec une sus- pension d'excr6ments d'E. forficalis. Les extraits polaires de ces excr6ments emp~chent la ponte quelle que soit l'esp6ce de crucif6re sur laquelle les chenilles se sont aliment6es. Les extraits non polares sont sans effet. Dans la nature, l'action dissuadante persiste 2

3 jours apr~s la pulv6risation des feuilles avec les extraits d'excr6ments. La pulv6risation tousles deux jours a maintenu l'action dissuadante. Des plantes contamin6es par des chenilles en train de s'alimenter et souill6es par quelques crottes sont aussi dissua- dantes pour la mouche que celles pulv6ris6es avec une suspension d'excr6ments.

Acknowledgements

We thank Drs R. A. Cole, P. R. Ellis and A. R. Thompson for their comments. This study was financed by the Agricultural and Food Research Council New Initiative Scheme.

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