Electrophysiological and Behavioral Responses of Microplitis mediator (Hymenoptera: Braconidae) to Caterpillar-Induced Volatiles From Cotton

Microplitis mediator</it> Haliday (Hymenoptera: Braconidae) is an important larval endoparasitoid of various lepidopteran pests, including Helicoverpa armigera</it> (Hübner). In China, H. armigera</it> is a key pest of cotton and is currently the focus of several biological control efforts that use M. mediator</it> as principal natural enemy of this pest. To improve the success of biological control efforts, behavioral studies are needed that shed light on the interaction between M. mediator</it> and H. armigera</it>. In this study, we determined M. mediator</it> response to volatile compounds from undamaged, mechanically injured, or H. armigera-</it>-damaged plants and identified attractive volatiles. In Y-tube olfactometer assays, we found that mechanically damaged plants and/or plants treated with H. armigera</it> oral secretions did not attract wasps. However, volatiles from H. armigera</it>–damaged plants elicited a strong attraction of both M. mediator</it> sexes. Headspace extracts from H. armigera</it>–damaged cotton were analyzed by coupled gas chromatography–electroantennographic detection (GC-EAD), and a total of seven different compounds were found to elicit electroantennogram (EAG) responses, including an unknown compound. Six different EAD-active volatiles were identified from caterpillar-damaged cotton plants, of which 3, 7-dimethyl-1, 3, 6-octatriene and (Z)-3-hexenyl acetate were the principal compounds. Olfactometer assays indicated that individual synthetic compounds of 3, 7-dimethyl-1, 3, 6-octatriene, (Z)-3-hexenyl acetate, and nonanal were attractive to M. mediator</it>. Field cage studies showed that parasitism of H. armigera</it> larvae by M. mediator</it> was higher on cotton plants to which 3,7-dimethyl-1,3, 6-octatriene was applied. Our results show that the combination of terpenoids and green leaf volatiles may not only facilitate host, mate, or food location but may also increase H. armigera</it> parasitism by M. mediator</it>.