Aphelinus albipodus Hayat & Fatima, A. asychis Walker (Hymenoptera: Aphelinidae), Diaeretiella rapae (M’Intosh), Aphidius matricariae Haliday, Aphidius colemani (Viereck), Aphidius picipes (Nees), Aphidius rhopalosiphi DeStefani-Perez, Ephedrus plagiator (Ness), and Praon gallicum Stary (Hymenoptera: Braconidae) were released in southeastern Wyoming for biological control of Diuraphis noxia (Mordvilko), Russian wheat aphid (Homoptera: Aphididae). A total of 407,028 parasitoids in the form of mummified aphids was released from 1989 to 1996. Three species, A. albipodus, A. asychis, and D. rapae, were first detected in winter wheat fields 3 yr after first release and spread throughout the wheat production region within 5 yr of release. Other primary aphid parasitoids were recovered sporadically and in very small quantities, and hyperparasitoids consisted of up to 20% of the specimens recovered. There were significant differences in abundance of A. albipodus, A. asychis, and D. rapae. In 1992, D. rapae was more abundant than the two aphelinids. By 1995, A. albipodus was becoming more prevalent than D. rapae, and A. albipodus was the most abundant species in 1997 and 1998. During these 2 yr, A. albipodus and D. rapae were detected in small grain fields and adjacent grasslands in similar abundance in both habitats. Of the parasitoids released in the D. noxia biological control program, A. albipodus and D. rapae commonly occurred in wheat and barley of southeastern Wyoming, with occurrence of the exotic A. albipodus directly attributable to the release effort. A. albipodus is likely becoming the predominant species in small grain production in the region. And A. albipodus and D. rapae range expansion and occurrence in grassland sites adjacent to wheat and barley may aid in their ability to control D. noxia.
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