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Assess your rootworm control

Reports from Iowa and later Illinois that rootworms were apparently breaking through genetic resistance were headline stories. What does it mean for Indiana?

We posed that question to three members of the Indiana Certified Crop Advisers panel.

Key Points

To prevent resistance, make sure you plant refuge as directed.

Go after rootworms by using multiple Bt events.

Rotate corn when possible to soybeans or a nonhost crop.


I live in north-central Indiana and have read reports about rootworms breaking resistance in Iowa and Illinois. I maintain a strict rotation and rely on resistant hybrids for rootworm control. Will this affect me? Should I make changes?

Betsy Bower, Ceres Solutions, Terre Haute: There has only been confirmation of rootworm resistance to one specific rootworm-resistant protein, the Cry3Bb1 protein, by Aaron Gassman of Iowa State University. Mike Gray documented several cases of severe rootworm damage to Bt corn in northern Illinois, but has not confirmed resistance.

Corn rootworm toxins are not high-dose toxins. Many larvae survive and reach adulthood. Planting refuge is critical to reduce selection pressure of the resistant protein on the rootworm population in a given field. The function of the refuge is to provide an opportunity for rootworms that survive exposure to the protein to mate with susceptible rootworms feeding on refuge corn. Offspring will be susceptible.

Ryan McAllister, Beck’s Hybrids, Parker City: We have learned in ag to never say never. Weeds became resistant to ALS chemistry much quicker than anticipated. We didn’t believe we would see glyphosate resistance to this level. The key to resistance management, be it weeds or insects, is multiple modes of action, following label guidelines, and rotating chemistry or trait platforms.

Resistance to rootworm events in Illinois and Iowa stemmed from a failure to follow refuge requirements, instead planting the same rootworm event year after year in corn after corn. Plant the appropriate refuge. Don’t use your neighbor as refuge. Rotating trait platforms from year to year is a good insect resistance-management plan.

Jeff Nagel, Ceres Solutions, Lafayette: I would not make any changes since you live in an area with less rootworm pressure and you rotate. Most reports from Iowa and Illinois are from continuous corn fields that have relied on one Bt event for several years. This is not a total surprise. History tells us that by using one method to control pests, we put a lot of selection pressure on the control method. In fact, we’re selecting for insects, diseases and weeds that survive the treatment.

Bower: We should still pay attention in Indiana because corn rootworm is a key pest, at least in the northern two-thirds of the state. Continuous corn producers in areas of high rootworm pressure should monitor fields for standability, yield and other signs that rootworm control was less than satisfactory.

Gray recommends rotating to soybeans or a nonhost crop; using a corn rootworm soil insecticide at planting; choosing a Bt hybrid other than the one that failed in Iowa, and using a pyramid Bt hybrid that expresses multiple Cry proteins targeted against corn rootworms.

Nagel: Always plant your required refuge, use a stacked hybrid with more than one Bt event for rootworm control, use a full rate of seed-applied insecticide and consider a soil-applied granular insecticide, as well.

This article published in the December, 2011 edition of INDIANA PRAIRIE FARMER.

All rights reserved. Copyright Farm Progress Cos. 2011.