Movement and response to semiochemicals by parasitoids of Plathypena scabra (Lepidoptera: Noctuidae)

On legumes, Plathypena scabra (F.), are parasitized by Cotesia marginiventris (Cresson), Diolcogaster facetosa Ashmead, and Aleiodes nolophanae (Ashmead); these primary parasitoids are hyperparasitized by Mesochorus discitergus (Say). D. facetosa and A. nolophanae are host-specialists and C. marginiventris and M. discitergus are host-generalists. In the first experiment, we tested the hypothesis that the specialists are more vagile than the generalists by placing trap plots consisting of either the native tick-trefoil, Desmodium paniculatum L., or soybean, Glycine max (L.) Merrill, between and within legume fields. D. facetosa parasitism of P. scabra placed in trap plots outside legume fields was high, while parasitism by C. marginiventris was low in those plots. This result suggested higher vagility of the host-specialist compared with the host-generalist. In addition, tick-trefoil and soybean plots were equally apparent to both parasitoid species. In the second experiment, we tested the hypothesis that generalist parasitoids respond to a broader range of volatiles than specialist parasitoids. The flight response of the four parasitoid species to volatiles was measured in a flight tunnel in no-choice assays. The 2 test odors used were green leaf volatiles released from soybeans damaged mechanically, and the plant-host complex that included soybeans fed upon by P. scabra larvae and P. scabra larvae plus associated kairomones. All 4 parasitoid species initiated upwind flight to green leaf volatiles, but the host-specialist, A. nolophanae, responded more strongly to the plant-host complex than to green leaf volatiles. Some upwind flights in the absence of volatile release were observed, but they were too few to explain the response of the parasitoids to green leaf volatiles or the plant-host complex.