Identification of Plant-Related Volatiles Emitted by Heliothis virescens Larvae that Attract the Parasitoid, Microplitis croceipes

Infested plants emit volatile organic compounds (VOCs) as an indirect defense against herbivore damage. Similarly, herbivores emit VOCs after feeding on plants. It was hypothesized that certain plant-related compounds play key roles in determining attractiveness of VOC blends emitted by herbivores to larval parasitoids. To test this hypothesis, electroantennogram (EAG) and behavioral responses of the parasitoid, Microplitis croceipes (Hymenoptera: Braconidae) to odor cues from its herbivore host, Heliothis virescens (Lepidoptera: Noctuidae) was investigated. Previous studies reported that cotton plant-fed H. virescens larvae were more attractive than artificial diet-fed larvae to female M. croceipes. In the present study, comparative GC-MS analyses indicated that 10 of 15 compounds  in the headspace of H. virescens larvae were exclusive to plant-fed hosts. Parasitoids showed varying EAG responses to synthetic versions of test compounds. In order to identify key attractive compounds, a full blend of 15 compounds was modified by removing each of the 10 compounds exclusive to plant-fed hosts. In Y-tube olfactometer bioassays testing parasitoid responses to modified blends, octene-3-ol, decanal, α-pinene, β-caryophyllene, α-humulene, α-farnescene and β-pinene were identified as key attractants in the natural blend of VOCs emitted by cotton-fed hosts. The results are discussed in relation to fitness of plant and parasitoid.