PLANT RESISTANCE Biotypic Variation Among North American Russian Wheat Aphid (Homoptera: Aphididae) Populations JOHN D. BURD, 1 DAVID R. PORTER, GARY J. PUTERKA, SCOTT D. HALEY, 2 AND FRANK B. PEAIRS 3 Plant Science Research Laboratory, USDAÐARS, 1301 N. Western Road, Stillwater, OK 74075 J. Econ. Entomol. 99(5): 1862Ð1866 (2006) ABSTRACT The Russian wheat aphid, Diuraphis noxia (Mordvilko) (Homoptera: Aphididae), has been a major economic pest of small grains in the western United States since its introduction in 1986. Recently, a new Russian wheat aphid biotype was discovered in southeastern Colorado that damaged previously resistant wheat, Triticum aestivum L. Biotype development jeopardizes the durability of plant resistance, which has been a cornerstone for Russian wheat aphid management. Our objective was to assess the relative amount of biotypic diversity among Russian wheat aphid populations collected from cultivated wheat and barley, Hordeum vulgare L. We conducted Þeld surveys from May through June 2002 and August 2003 from seven counties within Texas, Kansas, Nebraska, and Wyoming. Based upon a foliar chlorosis damage rating, three new Russian wheat aphid biotypes were identiÞed, one of which was virulent to all characterized sources of Russian wheat aphid resistance. The future success of Russian wheat aphid resistance breeding programs will depend upon the continual monitoring of extant biotypic diversity and determination of the ecological and genetic factors underlying the development of Russian wheat aphid biotypes. KEY WORDS biotype, plant resistance, Diuraphis noxia, Triticum aestivum The Russian wheat aphid, Diuraphis noxia (Mord- vilko) (Homoptera: Aphididae), is one of the more economically important and widely distributed pests of wheat, Triticum aestivum L., and barley, Hordeum vulgare L. (Stoetzel 1987). A native of central Asia (Durr 1983), it now occurs, except for Australia, throughout the major small grain production areas of the world. It is believed to have reached the United States in 1986 from the northward movement of pop- ulations originating near El Batan, Mexico (Gilchrist et al. 1983). In the United States, economic infestations occur annually, and where resistant varieties are not avail- able, are primarily controlled by insecticides. Reliance on chemicals to control Russian wheat aphid has been mitigated by the development and deployment of re- sistant cultivars, particularly in wheat production ar- eas prone to Russian wheat aphid infestations (Ber- zonsky et al. 2002). However, the recent discovery of a new Russian wheat aphid biotype in Colorado that is virulent to all currently deployed Russian wheat aphid-resistant cultivars has raised major concerns regarding durability of future Russian wheat aphid resistance sources (Haley et al. 2004). Of particular concern was the Þnding that the newly discovered Russian wheat aphid biotype was virulent to eight of the nine known Russian wheat aphid resistance gene sources. Biotypic variation has been known to occur among Russian wheat aphid populations for some time. Based on differential responses of a limited number of plant resistance sources, Puterka et al. (1992) found eight Russian wheat aphid isolates from a worldwide col- lection to be biotypically unique. Since then, Russian wheat aphid biotypic variation has been recognized among populations in Hungary (Basky 2003) and Chile (Smith et al. 2004). However, biotypic variation had not been detected from surveys conducted in the United States until March 2003 when Russian wheat aphid-resistant winter wheat grown in southeastern Colorado was severely damaged (Shufran et al. 1997, Haley et al. 2004). The discovery of this new biotype jeopardizes wheat production in areas where resistant cultivars have served as the cornerstone for Russian wheat aphid management. The objective of this study was to assess the relative amount of biotypic diversity among Russian wheat aphid populations collected from cultivated wheat and barley throughout the United States Great Plains. Mention of trade names or commercial products in this article is solely for the purpose of providing speciÞc information and does not imply recommendation or endorsement by the U.S. Department of Agriculture. 1 Corresponding author, e-mail: john.burd@ars.usda.gov. 2 Soil and Crop Science Department, Colorado State University, Fort Collins, CO 80523. 3 Bioagricultural Science and Pest Management Department, Col- orado State University, Fort Collins, CO 80523.