Wheat, Triticum aestivum L., with Russian wheat aphid, Diuraphis noxia (Kurdjumov) (Hemiptera: Aphididae) resistance based on the Dn4 gene has been important in managing Russian wheat aphid since 1994. Recently, five biotypes (RWA1–RWA5) of this aphid have been described based on their ability to differentially damage RWA resistance genes in wheat. RWA2, RWA4, and RWA5 are of great concern because they can kill wheat with Dn4 resistance. In 2005, 365 Russian wheat aphid clone colonies were made from collections taken from 98 fields of wheat or barley, Hordeum vulgare L., in Oklahoma, Texas, New Mexico, Colorado, Kansas, Nebraska, and Wyoming to determine their biotypic status. The biotype of each clone was determined through its ability to differentially damage two resistant and two susceptible wheat entries in two phases of screening. The first phase determined the damage responses of Russian wheat aphid wheat entries with resistance genes Dn4, Dn7, and susceptible ‘Custer’ to infestations by each clone to identify RWA1 to RWA4. The second phase used the responses of Custer and ‘Yuma’ wheat to identify RWA1 and RWA5. Only two biotypes, RWA1 and RWA2, were identified in this study. The biotype composition across all collection sites was 27.2% RWA1 and 72.8% RWA2. RWA biotype frequency by state indicated that RWA2 was the predominant biotype and composed 73–95% of the biotype complex in Texas, Oklahoma, Colorado, and Wyoming. Our study indicated that RWA2 is widely distributed and that it has rapidly dominated the biotype complex in wheat and barley within its primary range from Texas to Wyoming. Wheat with the Dn4 resistance gene will have little value in managing RWA in the United States, based on the predominance of RWA2.
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Vol. 100 • No. 5