Abstract. Adult Aleochara bipustulata L. and Aleochara bilineata Gyllenhal (Coleoptera: Staphylinidae) are predatory on immature stages of cabbage root fly Delia radicum (L.) (Diptera: Anthomyiidae). Larvae of the two Aleochara are parasitoids of D. radicum pupae. Female Aleochara lay eggs near D. radicum puparia; the newly-hatched Aleochara larvae enter puparia and consume the contents. Delia radicum-infested roots of brassicas give off dimethyl disulphide (DMDS). In the field, DMDS attracts adult Aleochara to pitfall traps but does not enhance the biological control of D. radicum. In the present study, we investigate the behavioural responses of the Aleochara to DMDS in still air, as well as in moving air in a Y-tube olfactometer, and also investigate the influence of DMDS on host selection. In larvae of both Aleochara species, DMDS induces a restricted-area search in still air, resulting in elevated frequencies of attack of D. radicum puparia close to a source of DMDS. In the olfactometer, newly-emerged virgin adults of both sexes of both Aleochara species choose alternatives to DMDS, older recently-mated females are attracted to DMDS, and older males and older mate-deprived females show no preference. Mating status of males determines the switch of their response to DMDS from avoidance to indifference. We conclude that DMDS is an important cue for host-finding, although other cues are involved in mate-finding. We discuss the implications for use of DMDS to enhance D. radicum mortality and for parasitism of nontarget species if A. bipustulata is introduced to Canada for biological control of D. radicum.

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