Laboratory evaluation of sulphur-bearing chemicals as attractants for larvae of the onion fly, Delia antiqua (Meigen) (Diptera: Anthomyiidae)

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Abstract
A laboratory method was developed to evaluate the effectiveness of volatile sulphur-bearing chemicals as attraotants for newly hatched larvae of Delia antiqua (Mg.). The 27 chemicals evaluated included mono-, di-and trisulphides, mercaptans, thiosulphinates, thiosulphonates, thiopropanal S-oxide and ‘onion oil’ (a complex mixture of volatiles). Larvae were placed in the centre of a petri dish and chemicals were presented either in glass tubes or on white absorbent discs 6 mm in diameter. One tube or disc contained 10 000, 1000, 100, 10, 1 or 0·1 nl of the test chemical and the other a similar amount of diethyl ether. The solvent did not attract the larvae and was used to dispense 0·1–100 nl amounts of the test chemicals. Most of the chemicals tested were unattractive at 0·1 nl and repellent at 10 000 nl. At the intermediate amounts, all attracted at least 40% of the larvae and 13 attracted more than 80%. Onion oil at 1 nl and methyl trisulphide at 10 nl elicited the greatest response, attracting 95% of the larvae. A possible method is suggested for using these chemicals in the field to attract newly hatched larvae away from onion seedlings.

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