Do Caterpillars Become More Attractive to Parasitoids after Feeding on Host Plants? Effect of Diet on the Attractiveness of Heliothis virescens to Microplitis croceipes

Infested plants emit volatile organic compounds (VOCs) often used by parasitoids to locate herbivore hosts. In the plant-host complex, VOCs emitted by plants likely provide the main cues for host location. However, it is unclear whether hosts become more attractive to parasitoids after feeding on plants. The relatively specialized endoparasitoid Microplitis croceipes (Hymenoptera: Braconidae) and its larval host, Heliothis virescens (Lepidoptera: Noctuidae) were used as model insects to test the hypothesis that parasitoids will show greater attraction to host larvae that recently fed on plant leaves than to larvae that did not feed on leaves. A group of 50 second-third instar larvae (initially reared on a pinto bean-based diet) were fed cotton leaves for 24 h while a similar group was continuously fed pinto bean-based diet. A coupled volatile collection-four choice olfactometer apparatus was used to test parasitoid attraction to host odors. Host body odor was analyzed by GC-MS for possible chemical explanation of parasitoid behavior. In four-choice olfactometer bioassays, female M. croceipes showed a clear preference for foliage-fed larvae over diet-fed larvae. Parasitoids could not significantly discriminate between the two host groups using visual cues alone. The significance of the results to the fitness of both parasitoid and plant is discussed.