Do biotype-1 soybean aphids benefit from the presence of biotype-2 on resistant soybean?

Despite the genetic bottleneck that soybean aphids passed through on the way to North America and the sparse use of aphid-resistant soybeans (i.e. Rag genes) in North America, biotypes have been identified that overcome this resistance. In addition, aphid-resistant soybeans often have large populations of soybean aphids in areas where these genes have not been used for management. Plant defenses may be up regulated based on the density of herbivores attacking the plant. However, large insect populations can overcome plant basal defenses thereby inducing susceptibility. Induced plant susceptibility may allow avirulent insects to feed on resistant plants. Our objective was to determine if a population of virulent biotype (biotype-2) soybean aphids could facilitate the growth of an avirulent (biotype-1) population on soybean containing the Rag1 gene. We compared the impact of an initial population of biotype-1 and biotype-2 soybean aphids on a subsequent population of both biotype-1 and biotype-2. The initial population varied by three densities (0, 25, or 50 aphids) and were caged on the first trifoliate of a soybean plant. After 24-hours, a second population of aphids (either biotype-1 or biotype-2) was placed on the second trifoliate, and the growth rate of this second population was measured over 10 days. The population growth rate of the second population increased with increasing density of the initial population. Furthermore, biotype-2 facilitates the growth of biotype-1 on Rag1 soybean significantly. These results suggest that insect resistance management for soybean aphid may need to be designed in a manner unique to soybean.