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2007 Russian wheat aphid biotype survey results releasedBy Scott Merrill, Terri Randolph, Thia Walker and Frank Peairs Colorado State University The Russian wheat aphid is a serious pest of wheat and barley in the western United States. One of the most common management practices for reducing yield damage to winter wheat by RWA feeding is the use of resistant cultivars. Many RWA biotypes occur worldwide but until 2003, only one North American biotype (RWA Biotype 1) was known. Unfortunately, during the spring of 2003, a new biotype was observed in SE Colorado, which severely damaged a resistant wheat cultivar. Symptoms of RWA damage on the resistant cultivar included leaf rolling, white streaking and plant stunting. Experiments confirmed the presence of a new biotype virulent to the Dn4 gene, which is the gene that provides resistance to the original RWA biotype. The RWA biotype discovered in 2003 and virulent to our commercially available winter wheat cultivars has been designated as RWA Biotype 2. RWA surveys have been performed over multiple years to determine the extent to which this biotype has spread throughout Colorado. Russian wheat aphid samples were collected from 51 Colorado wheat fields in 2007. Three cultivars, referred to as differentials, were used to screen the samples: Yuma (susceptible to RWA Biotypes 1 and 2), Yumar (RWA Biotype 1 resistant version of Yuma, carrying the Dn4 gene), 2414-11 (STARS 02RWA2414-11, resistant to RWA Biotypes 1 and 2, undetermined resistance source. This line is not yet commercially available.). Symptoms caused by RWA feeding on these three differentials were used to determine the biotype. Russian wheat aphids were collected at sampling locations across the state based on the amount of winter wheat harvested in each county. That is, counties that harvested more wheat were more thoroughly sampled. Aphids from the sampled locations were placed on each of the differential plants. Russian wheat aphid damage symptoms were evaluated when the Yuma cultivar (the cultivar susceptible to all known North American biotypes) exhibited strong symptoms (e.g., leaf rolling, chlorosis or death). RWA Biotype 1 was defined by damage only to Yuma, and RWA Biotype 2 was defined by damage to both Yuma and Yumar. Potential new biotypes were determined based on damage to 2414-11. Extensive snow cover across much of Colorado, during the 2006-2007 winter, appears to have resulted in a late and reduced population of RWA. Collection of samples frequently took intensive field scouting, and heavily infested fields were rarely encountered. As a result, only 50 samples were obtained in comparison to the more than 100 samples collected in each of the past surveys. Of the sampled locations in 2007, 4 percent tested as RWA Biotype 1 (2 of the sampled sites), and 96 percent of the locations tested as RWA Biotype 2. Figure 1 depicts the 2007 survey results and can be contrasted to the 2006, 2005 and 2004 Biotype surveys (Page 3: Figures 2, 3 and 4). Survey results indicate that RWA Biotype 2 continues to displace RWA Biotype 1. RWA Biotype 1 results were reduced from 18 percent of the 2005 survey sample results, to 7 percent of the 2006 sample results, and finally to 4 percent of the 2007 results. However, it is important to remember that collected samples may have populations of mixed biotypes and if Biotype 2 were present in the sample, then the sample will appear to be RWA Biotype 2. Therefore, a RWA Biotype 2 result from differentials is equivalent to stating that RWA Biotype 2 was collected at the location, but that we cannot rule out that RWA Biotype 1 also exists at that location. Date: 11/20/07
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