Evidence of resistance to Cry34/35Ab1 corn by western corn rootworm: root injury in the field and larval survival in plant-based bioassays

Authors: GASSMANN, AARON - Iowa State University; SHRESTHA, RAM - Iowa State University; JAKKA, SIVA - Iowa State University; DUNBAR, MIKE - Iowa State University; CLIFTON, ERIC - Iowa State University; PAOLINO, AUBREY - Iowa State University; INGBER, DAVID - Iowa State University; French, Bryan; MASLOSKI, KENNETH - Iowa State University; DOUNDA, JOHN - Iowa State University; ST. CLAIR, COY - Iowa State University

Submitted to: Journal of Economic Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/3/2016
Publication Date: 6/21/2016
Citation: Gassmann, A.J., Shrestha, R.B., Jakka, S.R., Dunbar, M.W., Clifton, E.H., Paolino, A.R., Ingber, D.A., French, B.W., Masloski, K.E., Dounda, J.W., St. Clair, C.R. 2016. Evidence of resistance to Cry34/35Ab1 corn by western corn rootworm: root injury in the field and larval survival in plant-based bioassays. Journal of Economic Entomology. 109:1872–1880. doi: 10.1093/jee/tow110.

Interpretive Summary: Western corn rootworm (WCR) is a serious economic pest of corn in the United States. One recent management option for WCR has included planting of Bt corn producing Cry3Bb1, mCry3A, and Cry34/35Ab1 protein toxins. However, in 2009 WCR populations in Iowa and elsewhere were found to be resistance to Cry3Bb1 and mCry3A corn. So far, WCR populations have remained susceptible to corn producing the Cry34/35Ab1 Bt toxin. Using single-plant bioassays to test field populations of WCR for resistance to Cry34/35Ab1 corn, Cry3Bb1 corn, and mCry3A corn, we found incomplete resistance to Cry34/35Ab1 corn among WCR field populations collected from fields where severe injury to corn producing Cry34/35Ab1 had been observed. Resistance to Cry3Bb1 corn and mCry3A corn also was found among the majority of populations tested. These first cases of resistance to Cry34/35Ab1 corn, and the presence of resistance to multiple Bt toxins by WCR, emphasize the potential vulnerability of Bt corn to the evolution of resistance by WCR. In addition to insect resistance management, these results highlight the need for more diversified management practices to thwart resistance and sustain the viability of Bt corn for managing WCR.

Technical Abstract: Western corn rootworm, Diabrotica virgifera virgifera LeConte (Coleoptera: Chrysomelidae), is a serious pest of corn in the United States and recent management of western corn rootworm has included planting of Bt corn. Beginning in 2009, western corn rootworm populations with resistance to Cry3Bb1 corn and mCry3A corn were found in Iowa and elsewhere. To date, western corn rootworm populations have remained susceptible to corn producing Bt toxin Cry34/35Ab1. In this study, we used single-plant bioassays to test field populations of western corn rootworm for resistance to Cry34/35Ab1 corn, Cry3Bb1 corn and mCry3A corn. Bioassays included nine rootworm populations collected from fields where severe injury to Bt corn had been observed and six control populations that had never been exposed to Bt corn. We found incomplete resistance to Cry34/35Ab1 corn among field populations collected from fields where severe injury to corn producing Cry34/35Ab1, either singly or as a pyramid, had been observed. Additionally, resistance to Cry3Bb1 corn and mCry3A corn was found among the majority of populations tested. These first cases of resistance to Cry34/35Ab1 corn, and the presence of resistance to multiple Bt toxins by western corn rootworm, highlight the potential vulnerability of Bt corn to the evolution of resistance by western corn rootworm. The use of more diversified management practices, in addition to insect resistance management, likely will be essential to sustaining the viability of Bt corn for management of western corn rootworm.