The Environmental Protection Agency said rootworms in Illinois and Iowa have become resistant to an insecticide produced by Monsanto Co. corn, and four more states likely have resistant worms.
The resistance finding is based on a test developed at the Iowa State University rather than the regulatory definition, which is “flawed” and should be changed, the EPA said Friday in a report examining Monsanto’s product. Probable resistance in South Dakota, Wisconsin, Minnesota and Nebraska fields need to be confirmed, the agency said.
Rootworms affect corn’s ability to draw water and nutrients from the soil and were responsible for about $1 billion a year in damages and pesticide bills until seeds with built-in insecticide were developed a decade ago. The resistance finding comes more than a year after Aaron Gassmann, an Iowa State entomologist, first reported it using a new method.
“Gassmann’s approach of testing for resistance is a better scientific approach than the regulatory criteria outlined,” EPA entomologists said in the annual review of Monsanto’s rootworm- killing corn. At least 23 counties have damaged fields that need to be tested for resistance, the EPA said.
Corn is Creve Coeur-based Monsanto’s biggest business line with $5.81 billion in sales, or 43 percent of total revenue, in its 2012 fiscal year. The EPA’s focus is Monsanto’s YieldGard corn, which is engineered to produce the Cry3Bb1 protein from Bacillus thuringiensis, or Bt, a natural insecticide.
Kelly J. Clauss, a Monsanto spokeswoman, said the company was working with growers to reduce the number of fields with unexpected damage, a sign of suspected resistance.
“Corn rootworm resistance is suspected, but not confirmed, according to the regulatory definition of resistance,” Clauss said by phone. “We are very much in line with the EPA in terms of what needs to be done.”
A separate EPA report Friday recommended Monsanto work with the agency to revise the definition of confirmed resistance by March 31. Monsanto has agreed to transition growers from Yieldgard to SmartStax corn, which has a second mode of action for controlling rootworms, the EPA said.
That could cause problems for Dow Chemical Co. and DuPont Co., which developed SmartStax. Rootworms are more likely to develop resistance to SmartStax in fields where Monsanto’s technology isn’t working, the agency said.
Monsanto also is working with growers with performance issues to help them combat suspected resistance through crop rotation, switching to SmartStax and other means, EPA said.
Problems reported to Monsanto dropped to 45,000 acres last year from 75,000 in 2011. The drop is probably due to Monsanto’s success in these efforts as well as drought conditions in the Midwest, the EPA said.
The agency said it planned to convene a scientific advisory panel this year to evaluate the definition of confirmed resistance.