As many as 25 percent of the American farmers growing genetically engineered corn are no longer complying with federal rules intended to maintain the resistance of the crops to damage from insects, according to a report Thursday from an advocacy group.

The increase in farmers skirting the rules, from fewer than 10 percent a few years ago, raises the risk that insects will develop resistance to the toxins in the corn that are meant to kill them, the report says. And it raises questions about whether the Environmental Protection Agency and the agricultural biotechnology industry are adequately enforcing the rules.


The data “should be a wake-up call to E.P.A. that the regulatory system is not working,” Gregory Jaffe, the report’s author, wrote in a letter Thursday to Lisa P. Jackson, the administrator of the federal agency. Mr. Jaffe is the biotechnology project director at the Center for Science in the Public Interest, a Washington advocacy group that does not oppose genetically engineered crops but favors stricter regulation.