Minnesota populations of Diabrotica virgifera virgifera LeConte, the western corn rootworm, surviving Cry3Bb1-expressing corn in the field and western corn rootworm populations assumed to be susceptible to all Bt proteins were evaluated for susceptibility to Cry3Bb1, mCry3A, eCry3.1Ab, and Cry34/35Ab1 in diet assays and three different plant-based assays. Rootworm populations originating from Cry3Bb1 fields and that consistently experienced greater than expected damage had increased survival and larval growth compared to control populations assayed on Cry3Bb1 as well as mCry3a and eCry3.1Ab. Cross resistance was documented between Cry3Bb1 and both mCry3A and eCry3.1Ab as single toxins. Despite very high resistance ratios in some comparisons, cross resistance was not complete and also varied with the population being evaluated, the trait measured, and the susceptible rootworm population used for comparison. Regardless of resistance and cross resistance, all proteins, even Cry3Bb1, retained some efficacy in terms of either reducing rootworm larval growth, protecting plants from damage, or both, for all rootworm populations evaluated. For one Cry3Bb1-selected population, a resistance ratio of 9.1-fold was found to Cry34/35Ab1 when evaluating EC50 values relative to a susceptible control population; however, resistance to Cry34/35Ab1 was not evident in all assays in this population. The United States Environmental Protection Agency recently suggested eliminating diet assays as part of the Bt resistance monitoring process. However, given the variability of responses of western corn rootworm populations to different proteins in different assays, both plant and diet assays are needed as options for detecting and fully characterizing resistance.
Bibliographical noteFunding Information:
We would like to thank Monsanto Company, DuPont Pioneer, and Syngenta for the seeds used in this study and the farm families of Minnesota for supporting this research through their corn checkoff. Funding, in part, was also provided by Biotechnology Risk Assessment grant 2009-33120-20256, USDA-NRI Award No. 2009-35302-05256, and Syngenta Biotechnology, Inc. A grant from Monsanto Company's Corn Rootworm Knowledge Research Program helped fund portions of the analysis and writing. DowAgroSciences provided diet and protein for the Cry34/35Ab1 diet assays, Monsanto provided diet and protein for the Cry3Bb1 diet assays, and Syngenta Biotechnology provided diet and protein for the mCry3A and eCry3.1Ab assays. We thank three anonymous reviewers for suggestions that have improved the current draft substantially.
- Bacillus thuringiensis
- Cross resistance
- Diabrotica virgifera virgifera
- Insect resistance management