Ann. appl. Bid . (1978), 88,241-249 Printed in Great Britain

24 7

A technique for measuring the susceptibility of wheat bulb fly larvae to insecticides in the laboratory

BY CLARA SMITH AND D. C. GRIFFITHS Rothamsted Experimental Station, Harpenden, Herts, AL5 2JQ

(Accepted 26 August 1977)

SUMMARY

A simple assay is described for measuring the toxicity of insecticides to larvae of wheat bulb fly (Delia coarctata) in the laboratory. The method should be suitable for other small soil insect larvae that cannot be obtained in large numbers and are dim- cult to treat and confine. A limitation of the technique is that it works only with insecticides that are soluble enough to be toxic in aqueous solution.

I N T R O D U C T I O N

The toxicity of insecticides against soil insects is often tested in soil, where effectiveness is influenced by soil type and soil conditions (Harris, 1964). This paper describes a standard laboratory test which indicates intrinsic toxicity, and is useful (a) to evaluate new insecticides, (b) to monitor susceptibility to insecticides now used so that any future development of resistance can be detected, and (c) to enable factors influencing effectiveness in the field to be investigated. The methods described were developed for wheat bulb fly (Delia coarctata (Fall.)). These small active larvae are difficult to treat with insecticide and to confine in and recover from containers; such problems are general with small soil insects.

M E T H O D S

Adult flies were collected in the field and fed on honey, condensed milk and blood (Bardner & Kenten, 1957) in cages in the laboratory, where they produced fertile eggs. Theeggs were allowed

' Cavity slide

Air bubble Fig. I . Wheat bulb fly larvae trapped in insecticide solution on cavity slide.

24 8 C L A R A S M I T H A N D D . C . G R I F F I T H S

to complete diapause and kept at temperatures below 0 OC (Way, 1959) until required, when they were allowed to thaw. Hatching occurred within 1-5 days, and the first instar larvae were used for the laboratory testing of insecticides.

Groups of five larvae were each immersed in a 0.05 ml drop of aqueous insecticide solution in the centre of a cavity slide. A glass coverslip was placed over each drop in such a way as to enclose an air bubble in the centre. The larvae were thus trapped in the ring of insecticide solution surrounding the central air bubble (Fig. 1). The slides were placed in sealed plastic boxes lined with damp filter paper to maintain a saturated atmosphere and kept for 24 h in a dimly lit, constant temperature room at 15 OC. The coverslips were then removed and the larvae transferred with a fine paint brush to a line drawn across moist black filter paper. The larvae were kept at room temperature and observed every 15 min; those that had not moved from the line within 2 h were recorded as dead.

The insecticides tested were those known to be effective as seed treatments against wheat bulb fly larvae in the field. Samples of pure or technical material (minimum purity 90%) were dissolved in distilled water and a range of dilutions made using water that had been vigorously shaken in air to encourage oxygenation. There were two replicates of each dilution and the tests were repeated three times with each chemical.

R E S U L T S A N D D I S C U S S I O N

Table 1 records the toxicity of carbophenothion, chlorfenvinphos and gamma-HCH, the three materials used commercially as seed treatments against wheat bulb fly. The average EC,,s based on all the tests showed that carbophenothion was more toxic than chlorfenvinphos, which was

Table 1. Number of dead wheat bulb fly larvae (out of 10) in replicated tests Concentration bg/ml)

r A -, EC,, in individual Combined EC,, Insecticide Replicate 10 5 1 0-5 0.1 0.05 0 tests and slope

a Gamma-HCH 6 2 l o - - 1 9.3 b 5 2 1 2 - - 1 11.0 C 6 2 2 2 - - 1 14.8

9.7 & 1.3 Stope = 4-08

Carbophenothion a _ _ 1 0 7 3 1 0 0.21 0.29 & 0.05 Slope = 2.01

C

b _ _ 9 5 1 1 0 0.35 _ _ 9 6 4 2 2 0.34

Chlorfenvinphos a 1 0 1 0 6 5 0 2 1 0.71 0.51 & 0.09

C Slope = 1.95 b - 10 7 6 0 - 0 0.52

- 9 9 1 4 - 2 0.26

more toxic than y-HCH, but all had EC,,s of < 10 pg/ml. Individual tests with each compound gave reasonably similar results and indicate that the susceptibility of small samples of wheat bulb fly larvae could probably be determined by a suitable discriminating dose of about 1 pg/ml carbophenothion or chlorfenvinphos, or 10 ,ug/ml gamma-HCH.

In contrast, saturated aqueous solutions of dieldrin or permethrin gave <50% mortality in these tests, although they are known to be effective when used as seed treatments against wheat bulb fly in the field. These insecticides have very low solubility in water (both <1 pg/ml) whereas, according to Martin & Worthing (1974), the solubilities of gamma-HCH, carbophenothion and chlorfenvinphos are 10, < 40 and 145 pg/g respectively. Concentrations of dieldrin or permethrin might be increased by use of emulsifying or dispersing agents but this has not been investigated. The method described is most suitable for insecticides that are toxic in simple aqueous solution. It was not possible to extend the test with the present species beyond 24 h because control mortality became unacceptably large.

A technique for measuring the toxicity of insecticides 24 9

We are grateful to G. C. Scott for supplying the first instar larvae, and to the insecticide manufacturers for samples of technical materials.

R E F E R E N C E S BARDNER, R. & KENTEN, J. (1957). Notes on the laboratory rearing and biology of the wheat bulb fly

HARRIS, c . R. (1964). Influence of soil type and soil moisture on the toxicity of insecticides in soils to

MARTIN, H. & WORTHING, c . R. (1974). Pesticide Manual, 4th edition. British Crop Protection Council. WAY, M. J . (1959). The effect of temperature, particularly during diapause, on the development of the egg

of Leptohylemyia coarctata Fallen (Diptera: Muscidae). Transactions of the Royal Entomological Society ofLondon 11 1, 35 1-364.

Leptohylemyia coarctata (Fall.). Bulletin of Entomological Research 48, 821-83 1.

insects. Nature, London 202,724.