University entomologists report that field-based western corn rootworm (CRW) resistance to the Bt toxin Cry3Bb1 is now found in four states, with confirmation recently announced in Nebraska. Resistance has been confirmed by researchers previously in Illinois, Iowa and Minnesota. Scientists in Colorado, Kansas, Missouri, New York, South Dakota and Wisconsin also are reporting significant damage in fields planted to corn hybrids containing the single Bt toxin. However, researchers in those states have not made a final determination that resistance is there. While Cry3Bb1 has been suspect in most of the CRW resistance cases, the practice of continuous corn and the repeated use of a single toxin are putting increased stress on all of the Bt toxins currently available. In 2013, Aaron Gassmann, an entomologist at Iowa State University, confirmed some cases of western corn rootworm resistance to the mCry3A toxin in that state. Ken Ostlie, University of Minnesota Extension entomologist, is finding similar problems in that state. "We continue to receive reports of Cry3Bb1 performance problem fields, although with VT Triple and VT Triple Pro being phased out, the issue will be masked by Cry34/35Ab1 in SmartStax," he notes. "We've also verified cross resistance with mCry3A at several locations. We also have had some troubling reports the last two years of performance problems with SmartStax and even Herculex Xtra, but don't have confirmed resistance to Cry34/35Ab1 yet." Most scientists are concerned the resistance issue will continue to worsen if farmers don’t adopt a more comprehensive integrated pest management (IPM) program. Consider these recommendations to reduce your risk in 2014:
1. Rotate your crops. If you must plant continuous corn, make it a point to rotate fields on a schedule every three to four years.
2. If you must grow continuous corn, rotate modes of action, just as you would with herbicides. Avoid using the same Bt year after year by planting a hybrid with a different Bt trait or multiple Bt traits for rootworm. Alternatively, plant a conventional hybrid with a soil insecticide. The use of a soil insecticide on top of a Bt hybrid is not recommended by most corn entomologists.
"We have shown in our field trials that insecticide on top of a single trait only improves relative root protection if the single trait is starting to fail in the field (i.e. due to resistance evolution)," explains Lance Meinke, University of Nebraska-Lincoln entomologist. "In this case, root protection improves but selection for resistance to the trait continues, so this is not a good long-term solution to the problem.
"Root protection from single traits that work well does not increase when insecticide is added on top of the trait," he adds. "Also, we have not seen an increase in root protection when an insecticide is added on top of a pyramid containing Cry34/35Ab1. So in most cases, we do not recommend using a soil insecticide on top of a pyramid that contains Cry34/35Ab1."
3. Scout cornfields for damage this summer and report any problems identified to your state Extension entomologist.
Western Corn Rootworm Bt Resistance Now Includes mCry3A Toxin
The problem has been confirmed in some continuous cornfields in Iowa and likely extends to other Midwest states, an Iowa State University entomologist says.
Be on the Lookout for Corn Rootworm Beetles
In appearance, adult western corn rootworms are about 1⁄4"-long with yellow bodies and three black vertical stripes across their backs.
Western corn rootworm breaks through Bt control.