Why corn rootworms haven’t conquered the planetWhy corn rootworms haven’t conquered the planet

Corn Chatter: Doomsday predictions about resistance to Bt traits haven’t come true, but they weren’t totally wrong either.

Tom J. Bechman, Midwest Crops Editor

January 17, 2025

3 Min Read
close-up of roots on a corn plant
BETTER PROTECTION: Roots of this SmartStax Pro corn plant from Bayer carried three modes of action against corn rootworm, including the RNAi trait. Tom J. Bechman

It was the late 1980s and I was at a twilight field day. That is where I heard my first reports of western corn rootworms that didn’t read the rule book. They were ravaging first-year cornfields in east-central Ford County, Ill. If this wasn’t a fluke, controlling rootworms through crop rotation could be in jeopardy.

But that’s not what I remember most. I was shocked when the speaker didn’t want to talk about it with me because I was a reporter. His logic? He didn’t want to scare people. Report no evil, and there is no evil?

You know how that turned out. Ignoring a problem leads to bigger problems. By the early ’90s, farmers over a large area in Illinois and northern Indiana were treating first-year corn for rootworms.

Fast-forward to the late ’90s. I was in the front row at a seminar, listening to someone explain how environmentalists predicted that new Bt traits for insect control would be useless within two years, four years at most, because insects would develop resistance rapidly.

That was a shocking prediction. European corn borer was hard to control effectively with sprays. Prevention with Bt traits was the best control. Plus, rootworm insecticides are potent chemicals that require careful handling. This was before the days of closed systems for insecticides on planters.

Mixed bag

So, more than 25 years later, how did things turn out? Were doomsday predictions overblown?

Bt traits for corn borer and corn rootworm control are still effective in many locations. So, no, total effectiveness for this technology wasn’t lost in four years, or even 25 years. However, where farmers didn’t practice crop rotation, yes, insect resistance developed, nullifying effectiveness of multiple Bt traits in those areas.

“Populations of both western and northern corn rootworm have resistance to all available Bt traits in some areas,” says Nick Seiter, field crops entomologist at the University of Illinois. “This resistance is most pronounced where corn on corn is common.” That includes several counties in northern Illinois and parts of Iowa and Minnesota.

But rootworm pressure in many areas is low. In Illinois, pressure has increased slightly since 2019, and bioassays on both major rootworm species show increasing resistance to all traits. But rootworm levels are still below 2009 numbers.

What happened to the variant that wreaked havoc in first-year corn? “Injury to first-year corn from the western corn rootworm variant is rare today,” Seiter says.

Credit many farmers and companies with responding to early alarmist cries and following rotation and refuge requirements. That is at least partly why rootworm numbers in many areas are lower now than 25 years ago. To be sure, rootworms are still an issue where rotation isn’t followed. But new technologies are providing another tool for farmers.

SmartStax Pro hybrids from Bayer, Vorceed Enlist hybrids from Corteva Agriscience and VT4Pro hybrids from Bayer now have the DvSnf7 RNAi (RNA-interference) trait to protect against corn rootworm. Seiter notes that while the potential for insects to develop resistance to RNAi traits is real, it hasn’t been found in the field yet.

Want to continue delaying doomsday predictions? Then take Seiter’s advice, and practice sound management practices. Rotate crops if you can, and if not, rotate to a soil insecticide or RNAi trait package. Above all, stay vigilant!

About the Author

Tom J. Bechman

Midwest Crops Editor, Farm Progress

Tom J. Bechman became the Midwest Crops editor at Farm Progress in 2024 after serving as editor of Indiana Prairie Farmer for 23 years. He joined Farm Progress in 1981 as a field editor, first writing stories to help farmers adjust to a difficult harvest after a tough weather year. His goal today is the same — writing stories that help farmers adjust to a changing environment in a profitable manner.

Bechman knows about Indiana agriculture because he grew up on a small dairy farm and worked with young farmers as a vocational agriculture teacher and FFA advisor before joining Farm Progress. He works closely with Purdue University specialists, Indiana Farm Bureau and commodity groups to cover cutting-edge issues affecting farmers. He specializes in writing crop stories with a focus on obtaining the highest and most economical yields possible.

Tom and his wife, Carla, have four children: Allison, Ashley, Daniel and Kayla, plus eight grandchildren. They raise produce for the food pantry and house 4-H animals for the grandkids on their small acreage near Franklin, Ind.

Subscribe to receive top agriculture news
Be informed daily with these free e-newsletters

You May Also Like