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‘Too much of a good thing’: Overuse is making Bt corn less effective against rootworm

March 24, 2025
A damaged corn root is shown in a corn field with a tractor in the background.

The root of a corn plant injured by corn rootworm, the crop's most damaging pest in the Midwest. Photo by Erin Hodgson/Iowa State University.

AMES, Iowa – Widespread planting of corn hybrids designed to combat corn rootworm, the crop’s most damaging pest in the Corn Belt, is reducing both the technology’s effectiveness and some farmers’ profits.

The findings come from a new analysis of 12 years of field trials and seed usage data across 10 Midwestern corn-growing states, including Iowa. The study, recently published in Science, shows rootworms are increasingly resistant to the built-in protection of corn that is genetically engineered to produce insecticidal proteins derived from a bacterium called Bacillus thuringiensis (Bt). 

The title of the Science article, “Too much of a good thing: lessons from compromised rootworm Bt maize in the US Corn Belt,” captures the study’s main thrust, said Aaron Gassmann, professor of plant pathology, entomology and microbiology at Iowa State University.

“‘Too much of a good thing’ is really the key message,” said Gassmann, one of three Iowa State faculty among the study’s 20 co-authors. “Overplanting Bt corn causes farmers to lose some of its benefits for suppressing rootworm populations and preserving yields.”

The study found that farmers in some parts the Midwest were planting rootworm Bt corn at a higher-than-ideal rate, based on an optimum that weighs current-year benefits along with the future impact of pest suppression and resistance. The overplanting gap was especially pronounced in states where it is rare for farmers to continuously plant the same fields with corn, the study found.

“When we crunched the numbers, we found that some of those farmers – from their own immediate private profit perspective, not considering the broader effects – were planting more rootworm Bt corn than they should,” said David Hennessy, Cargill Professor in Economic Systems at Iowa State and one of the paper’s co-authors. 

Powerful tool weakening

Adult corn rootworm beetles lay their eggs in the summer to overwinter in soil before hatching in late spring, when the larvae feed on developing corn roots. Bt corn designed to combat rootworm imbues the plant with proteins that are toxic to the insect but harmless for most other organisms, including humans and livestock.

By comparing root injury in Bt and non-Bt corn, researchers can estimate the efficacy of Bt protection. In the analysis of field trial data, effectiveness fell from 2005 to 2016 for all three Bt proteins studied. The pooled efficacy was 92% in 2005, two years after rootworm Bt corn was first introduced. By 2016, it had fallen to 80%.

Slowing rootworm’s Bt resistance is important because it’s a highly damaging pest that costs U.S. farmers up to $2 billion per year in management costs and reduced yield. 

“Probably 80% of what I talk about is corn rootworm management. If I’m going to offer a range of topics to choose from, they always pick rootworm,” said study co-author Erin Hodgson, a professor of plant pathology, entomology and microbiology and ISU Extension and Outreach extension entomologist.

Bt hybrids are an appealing rootworm solution for farmers because they’re a simple, effective and safe alternative to applying pesticides, Hodgson said. The vast majority of Iowa’s corn crop is Bt corn. 

“When rootworm Bt corn became available, it was a big step up. Farmers really liked it. Regardless of what sort of pest pressure they have, they continued to use it because it worked so well. They don’t necessarily want to back off,” Hodgson said. 

Rotation reduces Bt need

Overall populations of rootworm have fallen thanks to Bt corn, but fields where corn is grown continuously give the pest an ideal habitat to develop resistance and damage crops. 

In the western states included in the study – Iowa, Illinois, Wisconsin, Minnesota, Nebraska and the Dakotas – continuous corn is relatively common, and yield loss from rootworm in the final three years of the analyzed trials averaged 47.5 bushels per acre. Crop rotation’s dominance in the study’s eastern states – Indiana, Michigan and Ohio – helped reduce rootworm pressure, leading to yield loss of only 8.5 bushels per acre. 

The difference in yield protection benefits made the ideal Bt rate far lower in eastern states, though actual Bt usage didn’t differ nearly as much throughout the regions. In eastern states, Bt corn rates were about 50% from 2014 to 2016. If farmers throughout Indiana, Michigan and Ohio had instead planted Bt corn at an optimum 18% rate, they collectively would have saved $99 million per year by avoiding the $25.7 per acre premium for Bt seed. Optimum Bt rates in the western states ranged from 45% to more than 75%.

“We were still managing the pest using Bt hybrids as if rootworm was a prime driver of yield loss in states like Indiana. It wasn’t, and it hadn’t been for some time,” said Christian Krupke, a field crop entomology professor at Purdue University and the study’s lead co-author, along with Ziwei Ye, assistant professor of agricultural economics and rural development at Renmin University of China.

The wide difference in ideal rates for western and eastern corn shows how critical crop rotation is for controlling rootworm. Rootworm pressure builds in fields over time, so any interruption in corn planting helps keep rootworm populations in check, Gassmann said. 

While there are often economic incentives to plant continuous corn, even giving fields a break from corn every few years helps manage rootworm pressure, Hodgson said. 

“My soapbox talking point to farmers is to check your fields. Go below ground and evaluate root injury. Seeing where you are at will help you make better choices about the coming year,” she said. 

Providing more choice

Offering farmers more seed choices also would help reduce Bt overuse, researchers said. Bt traits targeting insect pests such as rootworm and corn borer usually are bundled with other valued genetic attributes, such as yield performance and drought resistance. Farmers have little opportunity to customize by only selecting traits they prefer.

In a separate study, published last week in the Proceedings of the National Academy of Sciences (PNAS), Gassmann described another seed-design factor contributing to rootworm’s growing resistance to Bt corn. Modifying crops to produce two or more different Bt toxins is a common method for delaying resistance, a “pyramid” approach that slows down natural selection.

But seed companies often offer Bt pyramids in which one of the traits is already seeing significant resistance, which undermines the intended redundancy of incluing multiple toxins. The PNAS study showed rootworm pests resistant to one trait in a pyramid tend to develop resistance to the other trait, as well.

Combinations of Bt traits need to both be effective for the approach to work, Gassmann said. Increasing the typical 5% level of non-Bt “refuges” in Bt corn fields, and considering Bt technology as part of a rootworm strategy instead of the entirety of it, would also help. 

“Novel pyramids and more diversified management would make those Bt traits more durable,” he said. 

Bt traits are expensive and time-consuming for seed companies to develop, and improving strategies to delay resistance can help increase the longevity of those traits, Gassmann said.   

Study’s value

Reports from individual field and laboratory studies have documented the presence of rootworm’s resistance to Bt corn for about 15 years, including prior studies by Gassmann. But the Science study covers multiple states over many years, analyzed by a group of entomologists, plant pathologists, crop scientists and economists from 13 institutions.

“It’s rare to have data this good. Getting the plots year after year is a challenge,” Hennessy said. 

The study originated from observations of a U.S. Department of Agriculture-organized working group for corn entomologists, and it shows the value of public datasets generated by long-term applied research, Krupke said. 

“These are among the most useful types of data for developing policy recommendations,” he said.

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