Comparing the behavioral responses of a specialist (Microplitis croceipes) and a generalist (Cotesia marginiventris) parasitoid to different types of host-related volatiles

Many parasitoids use host-related volatile compounds for host location. One current theory regarding the evolution of parasitoid host location strategies is that the degree of specificity of the signals needed by a parasitoid to successfully locate its host correlates with its level of host specificity. This hypothesis was tested using as models two parasitoids (Hymenoptera:Braconidae) of cotton caterpillars with different degrees of host specificity: Microplitis croceipes, a specialist parasitoid of Heliothis spp. and Cotesia marginiventris, a generalist parasitoid of several caterpillar genera including Heliothis spp. and Spodoptera spp. Y-tube olfactometer bioassays were conducted to compare responses of naïve male and females of both parasitoid species to select synthetic host-related compounds representing green leaf volatiles (GLVs) and herbivore-induced plant volatiles (HIPVs). Based on the results of previous studies by our group that showed key differences in the electrophysiological responses of both parasitoid species to host-related volatiles, we hypothesized that i) M. croceipes (specialist) would show relatively greater behavioral responses to the HIPVs, whereas C. marginiventris, (generalist) would show greater behavioral responses to GLVs , and ii) female parasitoids of both species would show greater responses than conspecific males to the host-related volatiles. As predicted, C. marginiventris showed relatively greater behavioral responses to the GLVs, whereas M. croceipes was more responsive to the HIPVs. Females of both species also showed greater responses than conspecific males to most of the tested volatiles. The results are discussed in relation to the role of HIPVs in mediating host specificity in specialist parasitoids.