Dr. Aaron Gassmann 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.

Corn rootworm larvae  (Photos by Jim Kalisch) Corn rootworm beetle

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.

Dr. Mike Gray, 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% 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.

Management Recommendations

The following options are suggested if you have had higher than expected injury from corn rootworms in your Bt corn field this year:

Option #1.  Rotate to a crop other than corn. This is still the best way to reduce corn rootworm populations in Nebraska. Regularly rotating some corn acres can help reduce rootworm densities on a farm. In Nebraska we do not have the “rotation resistant variant” western corn rootworm that has been found in the eastern Corn Belt. It has increased the number of crops in which it will lay eggs to include soybean and other crops, thus reducing the benefit of crop rotation.

Option #2.  If you must plant corn after corn:

• Change to a hybrid containing a different Bt corn toxin active against rootworms, or one containing more than one Bt corn toxin active against corn rootworms. Here is a list of available Bt corns and the toxins they express.
• Follow all refuge requirements for any Bt hybrid.

3.  It is important to use a diversity of control measures to manage rootworm populations, rather than relying on only one Bt corn. Crop rotation and use of different Bt corn hybrids that express different or multiple Bt proteins are important strategies for rootworm management. In addition, conventional insecticides may be used to provide some level of protection as part of a rootworm management program, including the following:

• Neonicotinoid seed treatment (Cruiser, Poncho) at the high rootworm rate
• Liquid or granular insecticide applied at planting
• Postemergence applications targeted for larval or adult control

Resources

For more information see:

• Gassmann, A. J., Petzold, J. L., Keweshan, R. S., and Dunbar, M. W. 2011. Field-evolved resistance to Bt maize by western corn rootworm. PLoS ONE 6(7): e22629. doi:10.1371/journal.pone.0022629
• Mike Gray, 2011, Severe Root Damage to Bt Corn Observed in Northwestern Illinois
• Monsanto Says 100,000 Acres of Corn May Have Resistant Bugs

Bob Wright, Extension Entomology Specialist
Lance Meinke, Professor of Entomology

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