1. SURVEY FOR RUSSIAN WHEAT APHID BIOTYPES IN COLORADO Terri Randolph, Scott Merrill and Frank Peairs Department of Bioagricultural Sciences and Pest Management Colorado State University, Fort Collins, Colorado 80523 RUSSIAN WHEAT APHID, Diuraphis noxia (Kurdjumov), is a serious pest of wheat and barley in the western United States. Management practices for winter wheat include the use of resistant cultivars. However, biotypes have reduced the value of resistant varieties and have raised concerns for future deployment of resistant wheat cultivars. Biotypic variation had been identified from worldwide cultures (Puterka, et al. 1992) but until 2003, only one North American biotype (RWA1) was known. During the spring of 2003, a possible new biotype was observed in SE Colorado, which severely damaged (leaf rolling, white streaking and plant stunting) a resistant wheat cultivar (Figure 1) (Haley et al. 2004). Experiments confirmed the presence of a new biotype virulent to Dn4 (Figure 2) and many other resistance sources. There was the first report of the occurrence of an additional North American biotype, which has been designated as RWA2. The objectives of this study were (1) to survey Colorado wheat producing counties to determine the distribution of RWA2 relative to RWA 1, and (2) to identify additional potential biotypes. INTRODUCTION METHODS AND MATERIALS Russian wheat aphid samples were collected from 80 and 122 Colorado wheat fields in 2004 and 2005, respectively. In 2004, TAM 107 (susceptible to RWA1 and RWA2) and Prairie Red (RWA1 resistant version of TAM 107, carrying the Dn4 gene) were used as differentials. Three seeds of each were planted in four inch pots. Late in the survey, 94M370 (RWA1 and RWA2 resistant, carrying the Dn7 gene) was added as a third differential. In 2005, Yuma (susceptible to both), Yumar (RWA1 resistant version of Yuma, carrying the Dn4 gene), 94M370, and 2414-11 (STARS 02RWA2414-11, resistant to both, undetermined resistance source) were used as differentials for the survey. After emergence, seedlings were thinned so that one seedling of each differential remained. Russian wheat aphid infested wheat tillers were randomly collected at each location. Ten first and second instar aphids from a sample were placed on each plant in a pot. To avoid cross contamination, each pot was RESULTS Approximately one half (48%) of the 2004 samples were determined as RWA1, and all but one of the remaining samples (52%) were RWA2 (Figure 3). A third biotype was found in a sample collected in Montezuma County (Mtza). This biotype damaged Prairie Red (chlorosis rating of 5) but less severely than RWA2 (chlorosis rating of 7). The Montezuma biotype damaged 94M360 more (chlorosis rating of 4) than RWA1 and RWA2 (chlorosis rating of 3). Approximately one fifth (19%) of the 2005 samples were determined to be RWA1, and the remaining samples (81%) were RWA2 (Figure 4). The biotype found in Montezuma County in 2004 was not recovered. No additional biotypes were encountered in the 2005 survey. Since 2003, RWA2 has become the predominant Russian wheat aphid biotype in Colorado wheat producing counties. A third biotype was found in a part of the state where Dn4 cultivars have not been grown. It is anticipated that additional biotypes will be identified. Currently released Russian wheat aphid resistant wheat cultivars are no longer effective for managing the Russian wheat aphid. Wheats containing multiple resistance mechanisms should be the focus for wheat breeding programs. CONCLUSIONS Figure 3. Caged pots containing differential wheat plants infested with Russian wheat aphids from each site. Figure 1. Map of Colorado depicting the original site where Russian wheat aphid biotype 2 was first found. Note: Counties in green depict the wheat producing areas, with the darker green indicating higher wheat production. Figure 2. Infested TAM 107 and Halt wheat show similar damage indicating the presence of Russian wheat aphid biotype 2. Haley, S. D., F. B. Peairs, C. B. Walker, J. B. Rudolph, and T. L. Randolph. 2004. Occurrence of a new Russian wheat aphid biotype in Colorado. Crop Sci. 44: 1589-1592. Puterka, G. J., J. D. Burd and R. L. Burton. 1992. Biotypic variation in a worldwide collection of Russian wheat aphid (Homptera: Aphididae). J. Econ. Entomol. 85: 1497-1506. REFERENCES Figure 3. Russian wheat aphid bioytpe survey results for 2004. Note: Counties in green depict the wheat producing areas, with the darker green indicating higher wheat production. Figure 4. Russian wheat aphid bioytpe survey results for 2005. Note: Counties in green depict the wheat producing areas, with the darker green indicating higher wheat production. caged with organza cloth supported by a wire frame (Figure 3). Russian wheat aphid chlorosis (1-9 scale) and leaf rolling (1-3 scale) damage symptoms were evaluated when the TAM 107 chlorosis and leaf rolling ratings exceeded 7 and 2, respectively (approximately 2-3 weeks). RWA1 was defined by damage only to TAM 107, and RWA2 was defined by damage to both TAM107 and the Dn4-bearing wheat. Potential additional biotypes were determined by damage to 94M370 and 2414-11