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Title: Multiple assays indicate varying levels of cross resistance of Cry3Bb1-selected field populations of the western corn rootworm to mCry3A, eCry3.1Ab, and Cry34/35Ab1

Author
item ZUKOFF, SARAH - Kansas State University
item OSTLIE, KENNETH - University Of Minnesota
item POTTER, BRUCE - University Of Minnesota
item Meihls, Lisa
item ZUKOFF, ANTHONY - Kansas State University
item FRENCH, LEE - French Agricultural Research, Inc
item ELLERSIECK, MARK - University Of Missouri
item French, Bryan
item Hibbard, Bruce

Submitted to: Journal of Economic Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/15/2016
Publication Date: 6/6/2016
Publication URL: https://handle.nal.usda.gov/10113/62636
Citation: Zukoff, S.N., Ostlie, K.R., Potter, B., Meihls, L.N., Zukoff, A.L., French, L., Ellersieck, M.R., French, B.W., Hibbard, B.E. 2016. Multiple assays indicate varying levels of cross resistance of Cry3Bb1-selected field populations of the western corn rootworm to mCry3A, eCry3.1Ab, and Cry34/35Ab1. Journal of Economic Entomology. 109(3):1387-1398.

Interpretive Summary: Transgenic corn that expresses its own insecticide (a protein knows as Bt toxin) has been developed targeting the western corn rootworm, the most serious insect pest of corn. This pest has developed resistance to most management tactics targeting it including two types of Bt toxins. Rootworm populations that survived one type of Bt corn in the field was tested on all types of Bt corn along with populations known to be susceptible to all these Bt toxins. This was done both in diet assays and three different plant-based assays. Rootworm populations that had survived the Bt toxin in the field showed increased survival and/or larval growth not only on the toxin it was selected on, but also on all other Bt toxins in some, but not all, plant and diet assays. Although some level of cross resistance was clearly documented between several toxins, it was not complete and also varied with the population being evaluated, the trait assessed, and the reference susceptible rootworm population utilized for comparison. Regardless of resistance and cross resistance, all proteins retained some efficacy in terms of either reducing rootworm larval growth, protecting plants from damage, or both from all rootworm populations evaluated. This information will be valuable to growers, seed companies, and the Environmental Protection Agency as they make decisions regarding appropriate crops to plant in areas with rootworm resistance to Bt.

Technical Abstract: Minnesota populations of the western corn rootworm (WCR) surviving Cry3Bb1-expressing corn in the field and WCR 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. Cry3Bb1-selected WCR populations showed increased survival and/or larval growth on not only Cry3Bb1, but also eCry3.1Ab, mCry3A, and Cry34/35Ab1 in some, but not all, plant and diet assays. Cross resistance was clearly documented between Cry3Bb1 and both mCry3A and eCry3.1Ab. In addition, for one Cry3Bb1-selected population, a resistance ratio of 9.1 was also found to Cry34/35Ab1 when evaluating EC50 values. This same population showed increased survival and weight gain on Cry34/35Ab1 in seedling assays, although we would not necessarily characterize this increased tolerance for Cry34/35Ab1 as cross-resistance. 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 WCR populations to different proteins in different assays, both plant and diet assays are needed as options for detecting and fully characterizing resistance.