Exploitation of host mating behaviour by a parasitoid fly

Egg-laying decisions are crucial in insect parasitoids because the host constitutes the only resource available for immature development. We estimated the extent to which host sex, size and mating behaviour are involved and interact in shaping host selection in the association between the Japanese beetle, Popillia japonica, and a parasitoid fly, Istocheta aldrichi. Under field conditions, we found that females and larger individuals of the Japanese beetle, both males and females, are associated with the highest levels of parasitism, regardless of seasonal variations in host size and sex ratio. Furthermore, we propose that foraging I. aldrichi females exploit the sexual selection behaviour of the host to their own advantage. The increased vulnerability to parasitism of Japanese beetle females during mating when immobilized by males, the extended duration of mating and the preference of males for larger females contribute to the strong bias towards large females being the most parasitized. This leads to the hypothesis that I. aldrichi females have evolved to 'merely' rely on Japanese beetle males to select the most suitable hosts for reproduction. Finally, we discuss the possible impact on both seasonal and long-term demographics of the Japanese beetle populations and biological control.