Farm Progress

Don’t ignore resistance threat from rootworms to Bt corn

• All problem fields were continuous corn for at least 4 years and used the same Bt toxin each year.• And many, but not all, of the fields lacked appropriate refuge.

January 18, 2012

2 Min Read

You may have heard reports that for the past few years farmers in Midwestern states have been encountering populations of western corn rootworm larvae that are resistant to varieties of Bt corn carrying the Cry3Bb1 toxin, the active ingredient against rootworms in YieldGard varieties.

Thus far, this problem seems to be concentrated in five states: Illinois, Iowa, Minnesota, Nebraska, and South Dakota.

 (For a look at the situation in Georgia, see

Nevertheless, the problem is still relevant for Pennsylvania, because we have fields in our region that share features with the problem areas in the Midwest. All problem fields were continuous corn for at least 4 years and used the same Bt toxin each year. And many, but not all, of the fields lacked appropriate refuge.

To learn from the experiences of our Midwestern counterparts and take steps to avoid similar problems here, I encourage Pennsylvania growers to be proactive and consider the following.

First, all the problems in Midwestern fields occurred in continuous corn acreage, so an easy way to avoid problems would be to rotate fields to soybeans or another crop. Pennsylvania does not have populations of rotational resistant rootworms-that is, rootworm beetles that lay their eggs in soybean fields rather than corn fields so the larvae emerge in first-year corn-and we do not expect them to arrive any time soon.

Second, if you do not want to rotate out of corn and want to continue using rootworm-active Bt hybrids, consider switching to Bt hybrids that express a different corn rootworm-active protein than the one you have been using, including possibly stacked hybrids with more than more protein active against rootworms.

This will provide a different mode of action for the field and delay the possibility of resistance evolution. To check the traits and proteins in different types of Bt corn, check this table produced by entomologists from the University of Wisconsin and Michigan State University: Bt Trait Table Mar 2 2011.pdf.

Third, if the previous options are not possible, consider applying a soil insecticide at planting, the way corn rootworms were controlled prior to the introduction of rootworm Bt events.

Unfortunately, soil insecticides are an imperfect solution, because their efficacy decreases with time and Bt varieties targeting rootworms seem to delay development further into summer.

Finally, ensure that your fields fully comply with refuge requirements. Planting of refuges is required to help prevent the evolution of resistance and not complying simply threatens the viability of the technology.

Any of these approaches will help break the path toward resistance development. The bottom line is that relying on one tactic for too long is a prescription for evolution resistance-change up your approach to stay one step ahead of this important pest.

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