Field surveys and field-cage studies were conducted to evaluate the parasitoidMicroplitis 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 nativeH. zeaand to provide host material for cage experiments. Of 609H. zeacollected from corn, only one was parasitized (byCampoletissp.) indicating that field corn is not an early-season reservoir forH. zeaparasitoids. Incidence of naturalM. croceipesparasitism ofH. zeain soybean also was very low (only 10 of 717 larvae collected were parasitized). In total, 1,960 laboratory-reared and 1,060 field-collectedH. zealarvae were released with laboratory-rearedM. croceipesadult females into field cages weekly from August to October in 1990 and 1991. An average of 65% (1,975) ofH. zealarvae (over all cage releases) were recovered. Of these, 990 (50%) were parasitized byM. croceipes. M. croceipeseffectively parasitized wildH. zeaas efficiently as laboratory-reared hosts. In total, 816 (82%)M. croceipessuccessfully 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).