Release and recovery of imported parasitoids of the Russian wheat aphid in eastern Colorado

The exotic hymenopterous parasitoids Aphelinus asychis Walker, Aphelinus albipodus Hayat and Fatima, Aphelinus varipes (Forester), Diaeretiella rapae M'Intosh, Aphidius colemani Viereck, Aphidius matricariae Haliday, and Ephedrus plagiator (Nees) were released at several sites in eastern Colorado from 1991 through 1993 in an attempt to establish the species in wheat agroecosystems for biological control of the Russian wheat aphid. Recovery of A. asychis and A. albipodus near Akron and A. asychis near Pritchett in 1994, one year after releases were made at the locations, indicates that exotic species had established populations. Recovery of D. rapae in relatively high numbers in 1994 near Akron where previous surveys had shown rare incidence of parasitism of Russian wheat aphids by this species suggests that an exotic strain of D. rapae may also have established. However, the increase in parasitism of Russian wheat aphids by D. rapae may reflect annual variation in parasitism rates by native D. rapae or adaptation by native D. rapae to the recently introduced host.