Midwestern field trials suggest overuse of rootworm-resistant corn reduces farmers' profits

The project, led by Purdue University entomologist Christian Krupke, documented greater rootworm pest pressure in the western Corn Belt states of Illinois, Iowa, Minnesota, Nebraska, North Dakota, South Dakota and Wisconsin. In these states, farmers commonly plant corn continuously. In the eastern states Corn Belt states of Indiana, Michigan and Ohio, farmers practice crop rotation that reduces the need for control through genetically engineered seed or applied insecticides. However, the use of transgenic corn hybrids targeting rootworm pests has been remarkably similar across the entire region. This study explored the consequences of this disconnect.

Krupke, Dean's Fellow and professor of entomology, and 19 co-authors from 12 other universities in the U.S., China and Canada reported their results in the journal Science. The retrospective study demonstrated that the pest landscape has fundamentally changed in many key corn-growing areas of the U.S. since 2004, the initial year of broad adoption of transgenic corn hybrids targeting corn rootworms. The study shows pest pressures in much of the region declined dramatically following introduction of these hybrids, reducing the need for broad deployment of the rootworm-specific traits.

"This study shows the value of long term, applied research datasets from public sector field research," said Krupke. The short duration and shifting priorities of research funding streams make it increasingly rare to have such scope and coverage in biological data. "These are among the most useful types of data for developing policy recommendations," Krupke said.

In 2003, the first genetically engineered corn hybrid trait lethal to corn rootworms and derived from the bacterium Bacillus thuringiensis (Bt) was introduced. Since then, farmers have extensively used "Bt corn" hybrids. Subsequently, multiple field studies have reported Bt resistance in rootworms since 2009, raising doubts about the long-term viability of the Bt traits.

The study emerged from observations made by a working group of corn entomologists who meet annually to discuss the impact of pests on the commodity. Several years ago, Krupke noticed a difference between corn rootworm reports in the eastern and western Corn Belt states.

"We were all in different regions of the country, yet doing much the same thing," Krupke said. "We were still managing the pest using Bt hybrids as if rootwormwas a prime driver of yield loss in states like Indiana. It wasn't and it hadn't been for some time."

From 2014 to 2016, yield losses attributed to rootworm damage amounted to 47.5 bushels per acre in western states, and 8.5 bushels per acre in eastern states, the researchers estimated.

The study quantified the two costs of using the Bt hybrid. One cost was the substantial technology fee, a premium growers pay when buying the seed. The other cost was the erosion of pest susceptibility to the Bt toxin.

"If you think of a Bt hybrid as a finite resource, like a declining debit card, every time you use it, you erode a little bit of susceptibility" Krupke said. "So it's a little less likely that it will work as well next time."

For their part, growers select elite hybrid seed genetics that will produce the best yields. Those hybrids often bundle a variety of other desirable traits, including expression of multiple Bt toxins, whether they are needed or not.

Christina DiFonzo, the field crops entomologist at Michigan State University, compared bundling seed traits to the packages formerly offered for cable TV. Consumers paid for hundreds of channels when they only wanted 10 or 15. Streaming services and on-demand programming has ended all that. "Farmers would welcome a similar 'pick list' for the seed supply and that would help insect resistance management."

The key difference is that insect-resistant corn carries an ongoing collateral price to pay along with out-of-pocket costs. "Money can be replaced but loss of susceptibility to the technology only goes in one direction and is irreversible," DiFonzo said.

These lessons should be applied to as-yet undefined new transgenic trait technologies for in-plant protection, said the study's lead author, Ziwei Ye, assistant professor at the School of Agricultural Economics and Rural Development at Renmin University of China. She advised against going back to insecticides, such as organophosphates and pyrethroids, for rootworm control.

"The Bt technology is and was a net benefit that helps safeguard beneficial organisms and soil ecosystems and enhance drought tolerance," Ye said. "These are increasingly appreciated as key resources in feeding the growing world population as our climate changes."

Bt and antibiotics are both examples of the "biological commons." Individuals may perceive that it makes sense to use them as insurance, even when risk of harm from the pest seems limited. If many growers do this over a long period, resistance will set in and the technology's decline will affect them all.

"Overusing Bt may make sense for each individual, but all will eventually lose the technology sooner than might have been the case," said study co-author David Hennessy, the Cargill Professor in Economic Systems at Iowa State University. "Essentially, each individual does not take account of the harm done to others due to the buildup of genetic resistance to the toxin."

Farmers thus have tended to use too much of Bt seed targeting rootworms for their bottom-line profit, especially in the eastern Corn Belt.

"In many cases, they overuse the input not just for the common good but also for their own private benefit," said ISU's Hennessy. Simply alerting farmers to focus on comparing the financial costs and benefits to themselves when using the rootworm Bt trait might help protect the trait's future effectiveness.

The use of transgenic technologies such as Bt maize as a key pest-management tool is likely to continue, Krupke noted. The technology is both user-friendly for growers and reduces insecticide applications.

"To keep this and future, similar technologies around and functional for the long-term is in everyone's best interest," he said.

This work was supported by the State Agricultural Experiment Station (SAES) from the Hatch Multistate Research Fund provided by the National Institute for Food and Agriculture (NIFA Project NC246: Ecology and Management of Arthropods in Corn) and the National Natural Science Foundation of China.