Are two genes better than one for soybean aphid management?

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 seven locations across six states, Minnesota, South Dakota, Iowa (2 locations), Kansas, Wisconsin, and Pennsylvania. We evaluated near-isolines of soybean for the ability of single gene resistant lines containing the Rag1 and Rag2 genes and a pyramid line containing both genes to limit plant exposure to aphids (CAD) and protect yield as compared with a susceptible line.  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.  All three soybean aphid-resistant lines significantly decreased CAD at all locations but the Wisconsin site.  The pyramid line accumulated significantly fewer CAD than both single gene lines at four of the seven locations.  Yield data was pooled from the three locations with the highest aphid pressure, Minnesota, South Dakota, and northern Iowa.  Yield was significantly reduced in the untreated sub-plot for the susceptible line and numerically reduced for both single gene lines.  We were unable to measure a yield decrease, significant or numerical, for the pyramid line.