Update on corn rootworm resistance

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Corn rootworms continue to make news. Most readers are familiar with the reports of resistance from the western part of the Corn Belt. These originated in Iowa and were summarized first in a publication by Dr. Aaron Gassmann’s lab at Iowa State and can be accessed at this link (click here).

Briefly stated, field resistance has been confirmed in Iowa and other states west of us, including parts of Illinois. There has been some confusion and parsing of words in media and industry reports about whether this is truly resistance, or just due to high pressure in fields. However, among corn rootworm scientists at the USDA and public universities there is consensus that rootworms in these fields are indeed resistant to the Cry3Bb1 toxin, found in many commercial corn hybrids – including the popular multi-trait stacks (e.g., SmartStax). This was the first toxin marketed for rootworm control, back in 2003, when it was sold under the YieldGard brand name.

This week a report on NPR highlighted current performance issues in Nebraska, that report (including audio) can be accessed here (click here).

This may cause some consternation and worry among Indiana producers, but it really shouldn’t. Most producers should keep doing what they have been doing (including planting a refuge), and be vigilant as we are now passing through the peak feeding period of rootworm larvae. Indiana is in a good spot to weather this storm, at least for a while, for several reasons.

First off, the reports indicate that resistance has arisen independently in these different areas – as opposed to spreading out from a focal point. In problem fields, there has been a history of several years of continuous corn expressing the Cry3Bb1 rootworm trait. Few Indiana fields meet that description, largely because the vast majority of producers here rotate with soybeans. Rotation to a non-host crop like soybeans remains the single most powerful and effective method of delaying rootworm resistance to Bt. Zero rootworm larvae will survive in a “clean” soybean field. This underscores the importance of removing glyphosate-tolerant volunteer corn, easily done with a grass herbicide. Don’t be confused by the term “soybean variant” – this just means females will lay eggs in soybeans (and other non-corn) this fall. The larvae still need the same food source – corn – when they hatch the following spring. If they don’t find that corn within about 24 hours, they die.

In addition, corn growers will want to consider rotating technologies for rootworm control if at all possible. This includes switching Bt toxins. An even better option is occasionally switching to granular and/or liquid insecticides, especially when rootworm populations are a low-risk. Layering the granular products “on top” of Bt traits is not recommended – this will provide control, but will also keep the pressure on insects to evolve resistance to the Bt toxins, which is exactly what we need to avoid. A final point in our favor is that our rootworm populations have been very low for several years running. That means the chances of resistance development are lower here than in states with very high pressure. That could change in the future, but for now we are in a good spot and need to make sure things stay that way.

For a full summary of corn entomologists’ recommendations to keep Bt in the pest management arsenal, please see this link (click here). 

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