Similar attractiveness of maize volatiles induced by Helicoverpa armigera and Pseudaletia separata to the generalist parasitoid Campoletis chlorideae

Campoletis chlorideae Uchida (Hymenoptera: Ichneumonidae), a major larval endoparasitoid of Helicoverpa armigera (Hubner) (Lepidoptera: Noctuidae), also attacks many other noctuid caterpillars. We investigated the attractiveness of H. armigera- and Pseudaletia separata (Lepidoptera: Noctuidae)-infested maize [Zea mays L. (Poaceae)] plants to C. chlorideae, and analyzed the volatiles emitted from infested plants and undamaged plants. Considering the reported specific induction of plant volatiles by elicitors in the caterpillar regurgitant, we also tested the response of the parasitoid to mechanically damaged plants treated with caterpillar regurgitant or water and measured the volatiles released by these plants. In wind-tunnel bioassays, C. chlorideae was strongly attracted to herbivore-induced maize volatiles. Mechanically damaged plants, whether they were treated with caterpillar regurgitant or water, were more attractive to the parasitoid than undamaged plants. The parasitoid did not distinguish between maize seedlings infested by the two noctuid insects, nor did they show a difference in attraction to mechanically damaged plants treated with caterpillar regurgitant or water. Coupled gas chromatography-mass spectrometer (GC-MS) analysis revealed that 15 compounds were commonly emitted by herbivore-infested and mechanically damaged maize plants, whereas only two compounds were released in minor amounts from undamaged plants. Infestation by H. armigera specifically induced four terpenoids, beta-pinene, beta-myrcene, D-limonene, and (E)-nerolidol, which were not induced by infestation of P. separata and mechanical damage, plus caterpillar regurgitant or water. Two compounds, geranyl acetate and beta-sesquiphellandrene, were also induced by the infestation of H. armigera, but not by the infestation of P. separata. All treated maize plants released volatiles in significantly larger total amounts than did undamaged plants. Maize plants infested by H. armigera emitted greater amounts of volatiles than plants infested by P. separata. The treatment with caterpillar regurgitant resulted in larger amounts of volatile emission than the treatment with water did in mechanically damaged plants. The amounts of emissions of individual compounds were also different between differently treated plants.