LANSING – The overuse of rootworm-resistant corn has led to the crop becoming less effective at controlling the destructive pest – a problem that could cost an estimated $1.6 billion in lifetime economic losses for U.S. farmers, including those in Michigan.s

That’s according to a new study of the effectiveness of genetically modified seed as a weapon to combat rootworm, beetles that feed on corn.

Their larvae eat and burrow into roots, while adults feed on corn leaves, silk and pollen.

That damage can significantly reduce yield.

Michigan ranks 12th nationally in corn production, growing 346 million bushels in 2023, U.S. Department of Agriculture data shows.. Lenawee and Sanilac counties grow the most corn in the state.

Seed companies have developed genetically modified corn to defend against pests like rootworm.

A bacteria is inserted into corn DNA, which causes the plant to produce a protein that’s toxic to pests.

Larvae die when they eat the modified corn before they can cause damage.

Today, most corn grown in the U.S. is genetically modified, which is a safer and more convenient way of controlling pests than using insecticides, said Purdue University entomology professor Christain Krupke, one of the study’s researchers.

However, the study – conducted over a 12-year period – found rootworms are gaining resistance to the toxin.

Like antibiotic resistance in humans, pest resistance happens because the toxin doesn’t kill every grub, Krupke said. The few that live can breed, passing on the traits that caused their survival. This speeds up evolution.

“Every time we use these [modified] crops, you get a risk of resistance happening,” Krupke said, “so you want to only use it ideally when it’s needed.”

Krupke said rootworm isn’t as big of a problem in the eastern part of the Corn Belt, which includes Michigan, Ohio, Indiana and Illinois, as it is elsewhere.

Since rootworm-resistant corn was introduced in 2003, the number of rootworms declined more quickly there than in the western part of the region: Iowa, Kansas, Nebraska, South Dakota and Minnesota.

“Farmers are using the approach much more than they would actually need to if it was based on the actual numbers out there,” Krupke said. “There aren’t enough of the pests there to justify spending the time, effort and money to manage it to that degree.”

Christina DiFonzo is an entomology professor at Michigan State University and works with farmers to conduct research and provide solutions for insects in field crops. She helped write the study published in the journal Science.

DiFonzo first saw rootworms gain resistance in Michigan in 2012 at a St. Johns cattle farm. The farmer grew corn for animal feed.

“It was a field where there were ears,” DiFonzo said. “There were so many beetles that volcanoed out of the ground from the larvae. The adults will eat the corn silks. Each silk is attached to a kernel, and that’s the pollination.”

“They had eaten the silk, so there was none. There was just an ear without kernels on it,” she said.

The farmer normally grew about 220 bushels of corn, she said. That season he produced 13.

In another St. Johns field, all the corn fell over because the roots were gone from rootworm larvae feeding, she said.

But most often, rootworms are not a problem in Michigan fields because farmers rotate crops, DiFonzo said.

Western Corn Belt states tend to specialize in corn, while Michigan grows other crops, like soybeans, wheat, sugar beets and vegetables.

Farmers decide which crop will be most profitable each season and plant that rather than grow corn every year.

“Females go into the ground in little soil cracks and lay their eggs, so a continuous cornfield would have a lot of eggs,” DiFonzo said. “If you rotate to soybeans, the eggs hatch and have nothing to feed on.”

Because of that rotation and the early success of genetically modified corn, growers in Michigan generally don’t need rootworm-resistant seed, she said

Preferably, a farmer would plant rootworm-resistant seed only if it was needed.

“But that’s not how it’s sold, typically,” DiFonzo said. “It’s sold as a package.”

Seed distributors usually sell genetically modified hybrid seeds.

For example, a seed may have herbicide resistance so a farmer can spray herbicides to kill weeds but not the crop. The same seed might also have rootworm and borer resistance.

A farmer might need only one of those traits, but it’s on the market as a bundle.

Genetically modified seeds are more expensive than natural-grown ones, and each additional feature is an added cost.

DiFonzo said that means farmers are buying traits they don’t need.

It also means rootworm populations are being exposed to – and therefore developing resistance to – the modified corn. That makes the crop less effective when it is actually needed for pest control, she said.

DiFonzo said there are options, and farmers should try to buy seeds without extra traits.

But she said options might not be available in every place, and farming is a busy profession. A seed dealer will recommend a product, and the farmer might not have time to look into the details.

“Convenience is often our main opposing force on this quest because people don’t want more things to worry about,” Krupke, the Purdue researcher, said.

Scott Miller of Elsie is the owner of Miller Family Farms, which has about 2,900 acres across Clinton, Shiawassee, Saginaw and Gratiot counties. He grows mainly corn and soybeans.

He said all corn is traited except for organic, and he grows a lot of hybrid crops.

“Quite often, you’re going to buy seed that has traits in it you don’t need, but it’s in with what you already need,” Miller said, “and it’s actually cheaper to buy it that way.”

He said farmers might pay for extra traits they don’t need, but it’s a trade off. He compared it to cable TV bundles – people pay for channels they don’t watch.

If farmers want a seed package a company doesn’t offer, they can go to a different one, he said.

“Seed companies take that very seriously, and they’re not going to waste anybody’s time and money by trying to sell something that’s not needed in an area,” Miller said.

He said a lesson the industry learned is that farmers can’t use only one pest solution.

“Even if it’s a weed, if you spray the same chemical for long enough, it’s going to eventually be resistant,” Miller said. “You have to keep switching it up.”

DiFonzo said developing new approaches to resistance helps everyone. Farmers and seed companies want to avoid pest resistance so genetically modified crops continue to work.

And Krupke said, “The whole premise of the study is that we will continue to find ways to get plants to produce their own insecticidal toxins. That, I think, is the future.”