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

Authors: PRISCHMANN-VOLDSETH, DEIRDRE - North Dakota State University; Hesler, Louis; Beckendorf, Eric; PENTON-MICHAEL, ERIN - The Ohio State University; MICHEL, ANDY - The Ohio State University

Submitted to: Journal of Economic Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/30/2026
Publication Date: 8/21/2026
Citation: Prischmann-Voldseth, D., Hesler, L.S., Beckendorf, E.A., Penton-Michael, E., Michel, A. 2026. No benefit of a mid-season foliar insecticide application to aphid-resistant soybeans when overall insect pressure is low. Journal of Economic Entomology. https://doi.org/10.1093/jee/toag261.
DOI: https://doi.org/10.1093/jee/toag261

Interpretive Summary: Insecticides help manage pests like the soybean aphid, although there are growing concerns that certain populations are becoming resistant. Using resistant host plants with naturally-occurring genes (i.e., Rag genes) is one alternative option to insecticides as soybean varieties with Rag genes can help control aphids. However, reducing insecticide application might allow other herbivorous insect pests to cause damage and thus discourage farmer adoption. We conducted a multi-year, multi-location field trial using a rag1b/rag3b soybean line, where all plots received a foliar fungicide and half also received a foliar insecticide. We measured densities of non-aphid insect pests and natural enemies, along with defoliation, pod feeding and yield, and screened soybean aphids for virulence against various Rag genes. Soybean aphid densities were extremely low on resistant varieties within experimental plots. We found low frequencies of virulence to three types of soybean aphid resistance. The insecticide lowered pest densities, but also reduced densities of natural enemies. Yield did not differ between fungicide only and fungicide + insecticide plots. Overall, aphid-resistant soybeans kept soybean aphids at extremely low levels and eliminated the need for a mid-season insecticide application. These results suggest that prophylactic insecticide use in rag1b/rag3b soybeans is not warranted and that insecticides should be used only if insect pests surpass economic thresholds.

Technical Abstract: 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 may might lead to yield loss from other insect pests and thus discourage farmer adoption of aphid-resistant varieties. We conducted a multi-year, multi-location 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 also 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 their presence on rag1b/rag3b soybeans, we could not establish a virulent aphid colony. Overall, rag1b/rag3b aphid-resistant soybeans kept soybean aphids at extremely low levels and eliminated the need for a mid-season insecticide application. These results suggest that prophylactic insecticide use in rag1b/rag3b soybeans is not warranted and that insecticides should be used only if insect pests surpass economic thresholds.