Induced susceptibility: A density dependent response that explains Aphis glycines populations on resistant soybean in field research plots

Despite the genetic bottleneck that Aphis glycines 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. Plant defenses may be up regulated based on the density of herbivores attacking the plant. However, large populations of insects can overcome plant basal defenses thereby inducing susceptibility. Induced plant susceptibility allows for avirulent populations of insects to feed on resistant plants. Our objective was to determine if the basal defenses of soybean could be overcome by varying densities of an avirulent A. glycines population (i.e. biotype 1) on soybean. We compared the impact of an initial population of A. glycines on a subsequent population on both susceptible and resistant soybean. The initial population varied by three densities (0, 25, or 50 aphids) and were caged on the first trifoliate of a V2 soybean plant. After 24-hours, a secondary population of aphids was placed on the second trifoliate, and the growth rate of this secondary population was measured over 10 days. Larger initial populations of A. glycines increased the population growth of the secondary infestation. This effect occurred on both aphid susceptible and aphid-resistant soybean. These results suggest that non-genetic factors contribute to the observations of soybean aphid biotypes in field research plots in North America.