Olfactory host-finding behaviour of Oulema melanopus (Coleoptera: Chrysomelidae) and its parasitoid, Tetrastichus julis (Hymenoptera: Eulophidae)

Behavioural responses to the host-associated olfactory cues have not been completely understood for the cereal leaf beetle, Oulema melanopus, and its principal parasitoid, Tetrastichus julis. We, therefore, investigated the role of olfactory cues in the host-finding behaviour of these species using olfactory bioassays. Behavioural responses of O. melanopus to odours emanating from intact host plants (wheat, oat, barley) vs. a clean-air control were tested using multichoice and two-choice bioassays. For T. julis, responses of naïve and experienced adult female wasps to odours associated with the faecal coat of O. melanopus larvae were measured under multichoice and two-choice conditions. Our results indicate that olfactory cues are involved in the host-finding behaviour of both O. melanopus and T. julis. Olfactory responses of O. melanopus were influenced by the sex of the beetle and the physiological stage of adults (reproductively active vs. in reproductive diapause). Females respond to olfactory cues in greater proportions than males, and reproductively active, overwintered adults show greater responsiveness than teneral adults in reproductive diapause. Behavioural responses to cues emanating from different crop species were different in multichoice bioassays but not in two-choice bioassays. Further, we report for the first time that the olfactory cues associated with the faecal coat of O. melanopus evoke host-finding behaviour of its parasitoid, T. julis. Naïve female wasps are more likely to use these cues to locate the potential host than experienced females. The results of this investigation provide insights into host finding by both the species and the nature of behavioural response brought about by olfactory stimuli, and the results can help to design strategies to improve parasitoid activity by enhancing the crop environment to generate cues for host finding and to manage O. melanopus populations.