Corn rootworm management update
Several recent developments make it important that people carefully consider their management practices for corn rootworms in Nebraska.
Aaron Gassmann, Ph.D., and colleagues at Iowa State University this summer published research which documented that some western corn rootworm populations in northeastern Iowa have reduced susceptibility to the toxin produced by some Bt corn hybrids. He collected rootworm beetles from fields where growers reported greater than expected injury to Bt corn, and tested larvae in the lab using a whole plant bioassay. He compared the results to populations from other parts of the state not having unexpected injury to Bt corn.
The problem fields identified were all hybrids which express the Cry3Bb1 Bt toxin, which is found in YieldGard RW, YieldGard VT RW, YieldGard VT Triple, and Genuity VT Triple Pro. The problem fields were all in corn after corn systems, where hybrids expressing Cry3Bb1 had been used repeatedly over several years. Gassmann et al. reported no reduction in susceptibility to the Cry34/35Ab1 toxin which is present in Herculex RW, Herculex XTRA, and SmartStax hybrids.
Mike Gray, Ph.D., University of Illinois, recently reported fields in northwestern Illinois, just across the Mississippi River from the problem areas in Iowa, with heavy rootworm injury to roots, lodging, and high populations of western corn rootworm adults. He has not yet completed studies to verify the susceptibility of Illinois western corn rootworms to Bt toxins. He stated that many of the cropping practices in fields with heavy rootworm injury were similar to those described by Gassmann et al. in Iowa; corn after corn for several years, and repeated use of Bt corn hybrids with Cry3Bb1 as the rootworm active toxin.
In a news release in August 2011, Monsanto acknowledged that unexpected damage to Bt corn hybrids expressing Cry3Bb1 was being reported in Nebraska and Iowa. The impacted area was estimated to be up to 100,000 acres of corn, which represents less than 1 percent of the total acres planted to hybrids expressing the Cry3Bb1 protein.
We have received several reports of higher than expected injury by rootworms to Bt corn in several locations in Nebraska; all fields reported to us were planted to YieldGard VT Triple hybrids. We have not confirmed the presence of resistance in Nebraska, but studies will be conducted to evaluate potential shifts in rootworm susceptibility to Cry3Bb1.
One thing in common among all of these reports is that problem fields have been in continuous corn production, usually with hybrids producing the same Bt toxin used repeatedly over multiple years. Continuous corn production often leads to higher populations of corn rootworms. Heavy rootworm pressure may result in reduced efficacy of any insect control product, whether it is a Bt corn hybrid or an insecticide.