Electroantennogram and behavioral responses of the parasitoid Microplitis croceipes to single compounds and blends of volatiles identified in the headspace of plant-fed Heliothis virescens larvae

Infested plants emit volatile organic compounds (VOCs) as an indirect defense against herbivore damage. Similarly, herbivores emit VOCs after feeding on plants. These VOCs can be herbivore-specific or plant-related, and can guide parasitoids to their herbivore hosts. The parasitoid, Microplitis croceipes (Hymenoptera: Braconidae) and its herbivore host, Heliothis virescens (Lepidoptera: Noctuidae), a caterpillar pest of cotton plant, were used as model species to test the responses of parasitoids to odors emitted by their herbivore hosts. It was hypothesized that certain compounds play key roles in determining the attractiveness of VOC blends emitted by cotton plant-fed H. virescens larvae to M. croceipes. Previous oviposition choice tests and four-choice olfactometer bioassays showed that plant-fed H. virescens larvae were more attractive than artificial diet-fed larvae (control) to female M. croceipes. Eleven (10 commercially available) of the sixteen compounds identified in the headspace of H. virescens using coupled GC-MS analyses were exclusive to plant-fed hosts and thus, were not emitted by artificial diet-fed hosts. In EAG bioassays, parasitoids showed varying degree of antennal response to synthetic versions of test compounds. In order to investigate the role of key compounds, a series of volatile blends were formulated in hexane using ratios of occurrence in the natural blend. A full blend of 15 commercially available compounds was modified by removing each of the 10 compounds exclusive to plant-fed hosts. In olfactometer bioassays testing parasitoid responses to modified blends, octene-3-ol and decanal were among other compounds identified as key attractive compounds in the natural blend of VOCs emitted by plant-fed H. virescens larvae. The results are discussed in relation to fitness of both plant and parasitoid.