Evidence of western corn rootworm (Coleoptera: Chrysomelidae) field-evolved resistance to Cry3Bb1 + Cry34/35Ab1 maize in Nebraska

Western corn rootworm (WCR; Diabrotica virgifera virgifera) field-evolved resistance to transgenic maize expressing the Cry3Bb1 protein derived from Bacillus thuringiensis (Bt) has been confirmed across the United States (U.S.) Corn Belt. Although the use of pyramided hybrids expressing Cry3Bb1 + Cry34/35Ab1 has increased in recent years to mitigate or slow existing WCR Bt resistance, the susceptibility of Nebraska WCR populations to this rootworm-Bt pyramid has not been assessed. The present study aimed to characterize the susceptibility of Nebraska WCR populations to Cry3Bb1 and Cry3Bb1 + Cry34/35Ab1 in areas with a history of Cry3Bb1 and Cry34/35Ab1 cultivation by conducting plant-based bioassays on F₁ progeny of WCR populations collected in 2017 and 2018 from fields in northeast Nebraska.

Most WCR populations were highly resistant to Cry3Bb1 and exhibited similar larval development when feeding on Cry3Bb1 and non-Bt maize, as indicated by head capsule width and fresh weight metrics. In contrast, most WCR populations exhibited a lower level of resistance to Cry3Bb1 + Cry34/35Ab1 maize and sublethal exposure to the rootworm-Bt pyramid resulted in more variable results in larval development compared to non-Bt maize. The present study documents a neighborhood WCR Cry3Bb1 resistance pattern and confirms the first cases of field-evolved resistance to Cry3Bb1 + Cry34/35Ab1 maize in Nebraska. Use of a multi-tactic integrated pest management approach is needed in areas of continuous maize production to slow or mitigate WCR resistance evolution to Bt maize.