Examination of cross-tolerance to Rag1 and thiamethoxam treatments in the soybean aphid, Aphis glycines

Host-plant resistance involving Rag (Resistance to Aphis glycines) genes is a potential tool to manage the soybean aphid. However, multiple soybean aphid biotypes have been identified that are virulent to varieties that possess Rag genes.  The combination of neonicotinoid seed treatments and Rag genes has been proposed as an alternative to manage multiple virulent biotypes of soybean aphid. In order for combined management practices to be effective, the potential for cross-tolerance between neonicotinoid insecticides and Rag genes must first be evaluated. Our objectives were to evaluate the susceptibility of soybean aphid to the combined effects of thiamethoxam and Rag1 and to determine whether aphids exhibit cross-tolerance. Using a whole-plant bioassay method, we exposed age synchronized nymphs and adults of biotype 1(Rag1-avirulent) to two plant types (susceptible control or Rag1) and two doses of thiamethoxam (LC25 and LC50) both alone and in combination. We found that Rag1 and thiamethoxam both alone reduced aphid survival, but when combined no additional mortality occurred. In general, the large effect of the Rag1 gene on soybean aphid survivorship prevented any further effects of  thiamethoxam from being observed when they are evaluated together. In a second experiment, we evaluated possible cross-tolerance by exposing Rag1-virulent and Rag1-avirulent biotypes to Rag1 plants for 24 hours and then transferring them to different thiamethoxam concentrations.