Screening for corn rootworm resistance to transgenic Bt corn in North Dakota

Western (WCR), Diabrotica virgifera virgifera LeConte, and northern corn rootworms (NCR), D. barberi Smith & Lawrence, are major economic pests of corn in much of the U.S. Corn Belt.  Western corn rootworm resistance to transgenic corn expressing Bt (Bacillus thuringiensis) endotoxins has been confirmed in some states; however, the status of sensitivity to this technology has not been previously investigated in North Dakota. Our objective was to determine if WCR or NCR have developed resistance to commonly used Bt toxins in corn grown in ND. We screened corn rootworm larvae for potential resistance to Cry3Bb1, Cry34/35Ab1, and pyramided (Cry3Bb1 + Cry34/35Ab1) Bt corn toxins during the summer of 2015 on larval progeny of WCR and NCR beetles collected from problem fields during 2014. There were no significant differences in sensitivity to any of the Bt toxins between a susceptible laboratory colony and the field population of NCR; however, trends suggest that the pest may be in the process of developing resistance to Cry3Bb1 in North Dakota. Also, high survival on corn expressing the Cry34/35AB1 toxin was observed in both the laboratory colony and the field population, whereas, the lowest survival observed was in the pyramided Bt corn. Bioassays on WCR indicated significantly greater survival of the field population than laboratory colony progeny on Cry3Bb1, Cry34/35Ab, and pyramided Bt hybrids. These findings suggest that North Dakota corn rootworm populations may be becoming increasingly resistant to Bt endotoxins and, therefore, underscore the importance of grower adherence to insect resistance management practices.