Does induced susceptibility occur between virulent and avirulent soybean aphids on resistant soybean?

The soybean germplasm contains several genes that are resistant to the soybean aphid, Aphis glycines Matsumura, (Rag-genes). These genes can limit soybean aphid population growth and reduce the need for insecticide use, however field tests of Rag-containing soybean are never free of aphids. Despite the genetic bottleneck that A. glycines passed through on the way to North America and the sparse use of Rag genes, virulent biotypes are found in North America. To what extant virulent and avirulent A. glycines interact on a shared host is unknown. One possible outcome of an interaction between these biotypes would be induced susceptibility, which would result in increased suitability of soybean for subsequent A. glycines. Our objective was to determine if a population of virulent A. glycines (biotype-2) could induce susceptibility of soybean containing the Rag1 gene leading to increased survival of an avirulent (biotype-1) population. We compared the impact of initial avirulent or virulent A. glycines populations on a subsequent population of either avirulent or virulent A. glycines. The initial population varied by two densities (0 or 50 aphids) and were caged on the first trifoliate. After 24-hours, the subsequent populations of A. glycines was placed on the second trifoliate. The population density of the subsequent populations was measured after 11 days. Results indicate that populations of virulent A. glycines increase the growth of avirulent A. glycines populations on Rag1 soybean. These results suggest that co-occurring avirulent and virulent populations of A. glycines on resistant soybean will be indistinguishable.