Interactions between resistant plants and foliar insecticides for soybean aphid management

Soybean aphid (Aphis glycines Matsumura), a major pest of soybean in the Midwest, is primarily managed with foliar insecticides, but aphid-resistant soybean are becoming available for growers. However, aphid populations on resistant plants can still build to economically damaging levels, which require treatment with insecticides to protect yields. To examine potential interactions between aphid-resistant soybean foliar insecticides used in aphid management, a field experiment was performed over two years. Aphid populations were monitored in a factorial experimental design with plots of near isogenic soybean lines that were either aphid-susceptible or aphid-resistant (i.e., Rag1). Plots for each soybean line were either untreated or treated with an organophosphate, a pyrethroid, or a mixture of pyrethrum and azadirachtin. A greenhouse bioassay was also conducted in a similar design to the field experiment to specifically monitor mortality and reproduction. In field plots, the organophosphate and pyrethroid treatments significantly reduced cumulative aphid-days (CAD).  Synergistic effects between the aphid-resistant line and the organophosphate or pyrethroid were observed.  Such synergistic effects were not observed with the pyrethroid under greenhouse conditions. The pyrethrum and azadirachtin mixture did not affect CAD in the field or greenhouse bioassays. Our results indicate that synergistic effects between aphid-resistant plants and chemical control are possible for soybean aphid management.  However, to utilize such interactions to improve aphid management, further work is needed to characterize the conditions under which such favorable interactions occur.