Assessing the benefits of pyramids and seed treatments for soybean aphid (Aphis glycines) host plant resistance

The soybean aphid is currently the leading insect threat to soybean production in the Midwestern United States. Host plant resistant varieties have recently been released for soybean aphid control. These varieties all contain the Rag1 gene conferring antibiosis resistance to the soybean aphid. In other systems aphid pests have rapidly developed virulence to single gene resistance traits. We conducted a small plot field experiment with six locations across five states, Kansas, Minnesota, Ohio, South Dakota, and Iowa (2 locations). We evaluated near isolines of soybean for the ability of single gene resistant lines containing the Rag1 and Rag2 genes and a pyramided line containing both genes to limit plant exposure to aphids (CAD) and protect yield as compared with a susceptible line. At the two Iowa locations, the efficacy of an insecticide seed treatment alone and in concert with host plant resistance to limit CAD and protect yield was measured. For all locations, we used a split-plot design with soybean line as the whole plot effect and aphid exposure, “aphid-free” or “untreated”, as the sub-plot effect. Aphid-free sub-plots were kept at densities of >50 aphids plant-1 with foliar applications of insecticide as needed. A third sub-plot treatment, “seed treated”, was added in the Iowa locations. Seed treated sub-plots received a seed treatment of thiamethoxam before planting. All resistant lines significantly reduced CAD compared to the susceptible line, with CAD lowest for the pyramided line. Thiamethoxam significantly lowered CAD on the susceptible line, but not for any of the resistant lines.