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Monsanto Co. corn that’s genetically engineered to kill insects may be
losing its effectiveness against rootworms in four states, the U.S.
Environmental Protection Agency said.

Rootworms in Iowa, Illinois, Minnesota and Nebraska are suspected of developing tolerance to the plants’ insecticide, based on documented cases of severe crop damage and reports from entomologists, the EPA said in a memo dated Nov. 22 and posted Nov. 30 on a government website. Monsanto’s program for monitoring suspected cases of resistance is “inadequate,” the EPA said.

“Resistance is suspected in at least some portions of four states in which ‘unexpected damage’ reports originated,” the EPA said in the memo, which reviewed damage reports.

The insects, which begin life as root-chewing grubs before developing into adult beetles, are among the most destructive corn pests, costing U.S. farmers about $1 billion a year in damages and chemical pesticides, according to the U.S. Department of Agriculture.

Monsanto fell 2.5 percent to $72.40 at 9:54 a.m. in New York, the third-biggest decline among companies in the Standard & Poor’s 500 Index.