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 EPA 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. 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. The crops in question, called BT corn, have bacterial genes spliced into their DNA that cause the plants to make toxins that kill certain insects when they feed on the crop. In 2008, about 49 million acres of BT corn was grown, accounting for 57 percent of domestic corn acreage. So far there appears to be little sign that insects are growing resistant to the toxins in the corn. If they were to, however, it would not only render the crops ineffective but would hurt organic farmers who use sprays of bacterial BT toxins as natural pesticides. To stave off such resistance, EPA requires farmers in the Corn Belt to plant 20 percent of their fields with non-BT corn to serve as a refuge for insects. The idea is that if an insect becomes impervious to the BT toxin, it is likely to mate with a nonresistant insect from the refuge, and the offspring would not be resistant. Four big biotechnology companies -- Monsanto, Pioneer Hi-Bred, Syngenta and Dow AgroSciences -- jointly do an annual survey of corn growers to assess compliance. Jaffe obtained these reports from the EPA under the Freedom of Information Act. From 2003 to 2006, about 90 percent of farmers growing corn resistant to the corn borer established refuges of the required size. But the rate fell to 80 percent in 2007 and 78 percent in 2008. 