Rootworm pressure rises in northern Illinois

DEKALB, Ill. — Even as harvest wraps up throughout northern Illinois, farmers already have their thoughts turned to spring planting.

That includes the management of pests, from insects to weeds, that can harm corn crops and cut yields. One of those pests that have made a comeback is the corn rootworm.

Northern and western corn rootworms have plagued corn growers throughout the Corn Belt. But the introduction of the Bt corn hybrids helped reduce rootworm populations.

But in recent years, a variety of factors has contributed to the corn rootworm making a comeback in the Corn Belt.

“It certainly is increasing in northern Illinois and in DeKalb County,” said Nick Seiter, a field crops entomologist with University of Illinois Extension.

As the name describes, the corn rootworm feeds on corn roots, and in northern Illinois, there are a lot of corn roots to be found during the growing season.

“The biggest contributor to that is the relative lack of crop rotation compared to central and southern Illinois. When you look at those areas of the state, they are traditionally more heavily rotated than the northern part of the state,” said Jim Donnelly, DEKALB Asgrow technical agronomist for northern Illinois.

“Productivity is good in northern Illinois and that aids corn. Livestock usage is another big part of that. People just like to grow corn up here and that is a big portion of it. That naturally keeps the rootworm populations higher.”

Environmental factors play a role in rootworm populations. Dry springs tend to favor the survival of large numbers of rootworm larvae in the soil, where cold, wet springs can drown those larvae, reducing the numbers of adult beetles.

In recent years, a major factor in the resurgence of corn rootworms is resistance to existing traits.

“We launched a couple of new traits, SmartStax PRO and VT4PRO, so brand new modes of action for rootworm, which we haven’t had in a while,” Donnelly said.

“We haven’t really had a brand-new mode of action for Bt rootworm control in quite some time, and over that span of time, efficacy has waned a little bit in terms of controlling rootworm with the Bt traits.”

Fields that are not in a corn-soybean rotation tend to present the biggest challenges for growers in controlling corn rootworm.

“In Illinois, we’ve gone back to the scenario where the vast majority of our rootworm problems are in continuous corn,” Seiter said.

“We haven’t seen a lot of impact from rotation resistance in recent years. Most of our problem areas and problem fields are areas where we have a lot of continuous corn.”

In 2022 and 2023, major seed companies, including Bayer and Corteva, announced the introduction of varieties with the RNAi and Cry3Bb1 and Bt technologies to combat corn rootworm.

But even with new seed technologies, Seiter cautioned that those technologies need to be managed to keep them effective for the long term.

“One thing to keep in mind with that RNAi trait, it’s going to be more effective than SmartStax in scenarios where you have a resistant population, like a population that’s resistant to Bt. But keep in mind that those trait packages have the same Bt proteins in them that SmartStax does, they just have an additional RNAi trait,” he said.

“It’s a tool to be used, certainly, but a tool to be used judiciously. It’s something that, if we’re not careful, we could really run through that in a hurry if we throw it into situations where you have a really highly resistant rootworm population.”

Seiter said that while lodging at the end of a season can indicate issues below ground, by the time lodging occurs, the rootworms have become established.

“By the time you see lodging at the end of the season, you are actually several years into a problem at that point. It’s just that it’s big enough that you can see it above ground,” he said.

“I always encourage farmers to dig up a few roots from every control technology that they are using and see what is happening in terms of pruning, in terms of larval injury to those roots. If you want to catch this problem early, you really need to be looking below ground at the roots.”

Donnelly said crop rotation remains the biggest management tool that growers can use to control rootworm populations.

“The things that we promote are, No. 1 is crop rotation and, really, No. 2, 3, 4 and 5 are crop rotation. That is the easiest thing you can do to reduce your issues for next year and reduce your numbers long term,” he said.

Donnelly agreed with Seiter’s caution on using and managing the new seed technologies carefully.

“Nothing is going to cause resistance issues faster than repeated use of the same trait with the same crop, year after year. It’s getting to the point where some of the older traits aren’t working either so rotating to some of the older traits isn’t really going to help you at all,” Donnelly said.

“It’s more about utilizing the new traits, but rotating to soybeans or wheat. It really is about using a comprehensive approach and not just relying on one mode of action.”