No benefit of a mid-season foliar insecticide application to aphid-resistant soybeans when overall insect pressure is low

Producers currently rely on insecticides to manage pest soybean aphids (Aphis glycines Matsumura), although there are growing concerns about insecticide-resistant aphid populations. Using resistant soybean varieties with naturally occurring, pyramided Rag genes can adequately control soybean aphids and preclude the need to use an insecticide. However, forgoing a mid-season insecticide application could lead to yield loss from other insect pests and thus discourage farmer adoption of aphid-resistant varieties. We conducted a multiyear, multilocation field trial using a pyramided rag1b/rag3b soybean, where all plots received a foliar fungicide and half also received a foliar insecticide. We assessed densities of non-aphid insect pests and natural enemies, along with defoliation, pod feeding, and yield. We confirmed the expected efficacy of the resistant varieties against soybean aphids using speed scouting and screened soybean aphids for virulence against various Rag genes. There was a diverse mix of non-aphid pests and natural enemies, but overall pest pressure was relatively low. The insecticide lowered non-aphid pest densities, but also reduced densities of natural enemies. Yield did not differ between fungicide-only and fungicide + insecticide plots. We found low frequencies of virulence to three types of soybean aphid-resistant plants, and despite low levels of aphids on rag1b/rag3b soybeans, we could not establish a virulent aphid colony. Overall, rag1b/rag3b aphid-resistant soybeans kept aphids at extremely low levels and eliminated the need for a mid-season insecticide application. As per standard IPM guidelines, prophylactic insecticide use on rag1b/rag3b soybeans is not warranted and insecticides should be used only if pests surpass economic thresholds.