Olfactory responses of the parasitic wasp, Trybliographa rapae (Hymenoptera: Figitidae)

Abstract: Delia flies (Diptera: Anthomyiidae) are economically important pests in several horticultural crops. The control is commonly relying on chemical insecticides, though there are possibilities with biological control from the natural enemies in the field. An important natural enemy is the parasitic wasp, Trybliographa rapae Westwood (Hymenoptera: Figitidae) that lays eggs in the Delia fly larvae. The parasitoid larva and the host larva have a parallel development until the host dies within its puparium and an adult parasitoid emerges. The aim of this thesis was to understand the attraction to host- and food-associated plant volatiles of T. rapae, in order to enhance the effectiveness of the parasitic wasp as a biological control agent. The hypothesis was that female T. rapae would be attracted to the volatiles from cabbage plants infested by cabbage root fly larvae (Delia radicum L.) and to the food-associated volatiles from buckwheat flowers (Fagopyrum esculentum Moench). Two-choice experiments were made in an olfactometer, testing root-infested cabbage against non-infested cabbage or against blank odor sources. There was a significant attraction to the infested cabbage plants. Both the below- and aboveground parts of the root-infested cabbage plants were tested against the below- and aboveground parts of non-infested plants. The females significantly chose the belowground parts of infested plants and there was a tendency for attraction to the aboveground parts of root-infested plants. The volatiles collected from infested and non-infested cabbage plants showed both quantitative and qualitative differences. Two compounds from infested plants repeatedly elicited responses from the antennae of female T. rapae in a gas chromatograph coupled with an electroantennograph (GC-EAD). In a semi-field experiment, there was a tendency that the parasitism rate increased due to nectar from buckwheat flowers. A strong attraction to buckwheat of female wasps was shown in two-choice bioassays, whereas the males showed no interest for either buckwheat or infested cabbage. The differences suggested that males would be guided by sex pheromones rather than female-associated plant volatiles when locating mating sites.