Corn farmers fight rootworm resistance

New research confirms what many Midwest farmers have already suspected: The corn rootworm can develop resistance to varieties genetically modified to thwart the pest. Here, rootworm damage in an Iowa field ruined a corn crop. (Photo courtesy Aaron Gassmann)
Rootworms are raised from larvae into adult beetles in a growth chamber at Iowa State University, where researchers confirmed the pests can develop resistance to the corn hybrids engineered to kill them. (Photo by Amy Mayer, Harvest Public Media)
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April 14, 2014 - 6:30am

After a long battle with corn rootworm, Midwest farmers thought they’d found relief in genetically modified seeds engineered to produce toxins deadly to the pest. But recent research confirms what farmers have been noticing for several years: the western corn rootworm has been evolving to outwit the technology.


Researcher Aaron Gassmann has been studying the western corn rootworm’s ability to develop resistance to Bt corn for the past several years. (Photo by Amy Mayer, Harvest Public Media)

Bt corn, as it’s called, is genetically modified to produce a bacterium that can kill insects. But recently, farmers all over the Midwest have reported rootworm damage to corn they had expected to fight off rootworm. When Aaron Gassmann, a bug researcher at Iowa State University, started answering calls to look at some cornfields, he quickly had a hunch. Now, he has the research that proves his fear.

“These insects can adapt rapidly to this Bt technology,” Gassmann said. “So it highlights the potential vulnerability of the technology and the need to not set up scenarios where this type of adaptation occurs.”

Gassmann recently published a study showing that after about three-and-a-half years the poison produced by Bt corn no longer works. That’s because when corn varieties with the same toxic traits are planted in consecutive years, the worms with some natural resistance survive. Then, natural selection makes those bugs the winners, and farmers the losers. Their corn suffers devastating root damage, complicating harvest and lowering yields.

“Farmers could use Bt corn, but they should also be using other things like crop rotation,” Gassmann said, “so these patterns don’t arise then, where the rootworm population gets selected year after year on this same Bt trait.” 

Alternating soybeans or some other crop on the field limits the root worm’s food source and creates a fresh start for the following year’s new corn crop. Although the certainty that the worms evolve to survive the toxins in Bt corn is new, the damage from the rootworm and the advice on crop rotation are not. That’s why some farmers are moving away from depending on the latest in genetically modified technology and back towards older pest control methods.

Iowa State entomologist Erin Hodgson, at a research farm in 2013, suggests farmers should plan for rootworm resistance. (Photo by Amy Mayer, Harvest Public Media)

Confuse the pests

Researchers like Iowa State’s Erin Hodgson have for years been highlighting the need to watch and plan for the rootworm population developing resistance.

“I hear a lot of reasons for growing continuous corn,” Hodgson said at a 2013 field day at an ISU research farm in Kanawah, Iowa. “My recommendation to you guys would be to throw a confusion at the larvae. Mix up your traits so that you’re not planting the same trait year after year after year.”

Crop rotation once was the norm, with fields growing corn, alfalfa, oats, soybeans, and perhaps other crops as well, in order to maximize soil health and diminish pest problems. Gradually, as current agricultural trends took hold, many farms dropped to just corn and soybeans and, in some cases, corn alone.

Sticking with corn

Mostly, it comes down to dollars and cents. Farmers wanted to capitalize on the high price of corn. Despite the risk of rootworm, farmer Doug Adams of Humboldt, Iowa, says farmers with landlords to pay can feel beholden to market forces.

“You have to make decisions based on the markets for that year,” Adams said. “Typically, your cash rent is year-to-year as well and that’s kind of what forces you into a shorter-term outlook.”

Adams says he recognizes the importance of crop rotation and uses it on most of his fields. But not all of his acres.

“I’ve got a few acres where I do corn on corn,” he said. On those, he tries to use a variety of the available Bt traits to limit the rootworm’s ability to adapt.

The USDA is already predicting that farmers will plant fewer acres of corn this year, partly thanks to lower prices. And analysts are expecting a record-high for soybean acres. That could help quell the rootworm’s ability to develop resistance. But scientists and farmers both know that pests are likely to continue adapting and seed companies will keep trying to stay a step ahead.

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