Parasitization and Timing of Diapause in Virginia Microplitis croceipes (Hymenoptera: Braconidae): Implications for Biocontrol of Helicoverpa zea (Lepidoptera: Noctuidae) in Soybean

Abstract

Field surveys and field-cage studies were conducted to evaluate the parasitoid Microplitis croceipes (Cresson) as a biological control candidate for release against corn earworm, Helicoverpa zea (Boddie), in Virginia soybeans, and to gain knowledge of its diapause behavior. Corn earworm larvae were collected in 1990 and 1991 from corn and soybean fields to monitor parasitization of native H. zea and to provide host material for cage experiments. Of 609 H. zea collected from corn, only one was parasitized (by Campoletis sp.) indicating that field corn is not an early-season reservoir for H. zea parasitoids. Incidence of natural M. croceipes parasitism of H. zea in soybean also was very low (only 10 of 717 larvae collected were parasitized). In total, 1,960 laboratory-reared and 1,060 field-collected H. zea larvae were released with laboratory-reared M. croceipes adult females into field cages weekly from August to October in 1990 and 1991. An average of 65% (1,975) of H. zea larvae (over all cage releases) were recovered. Of these, 990 (50%) were parasitized by M. croceipes. M. croceipes effectively parasitized wild H. zea as efficiently as laboratory-reared hosts. In total, 816 (82%) M. croceipes successfully spun cocoons that were recovered and maintained outdoors. Adult wasps emerged from 83% (676) of the cocoons, most within 2—3 wk. The remaining 123 (15%) cocoons entered hibernal diapause. Percentages of recovered cocoons that entered diapause increased with later cage-release dates (<20% before early September to 50% by early October).