More farmers are failing to plant required "refuge" zones near fields where genetically modified corn is grown, possibly increasing chances that pesticide-resistant bugs will harm crops in years to come, according to a report released Thursday by an advocacy group.

The Center for Science in the Public Interest, a Washington-based group critical of government oversight of the biotech industry, said that as much as 15 percent of the country's corn acreage was out of compliance with federal rules. In recent years, compliance rates have dropped from around 90 percent to between 60 and 80 percent, the group's report concluded.

"Clearly farmers aren't doing as good a job as they should be," said Greg Jaffe, the report's author.

Roughly 60 percent of the corn grown in the United States last year was "Bt" corn that has been genetically modified with genes from the Bacillus thuringiensis bacteria to produce toxins that will kill rootworms and corn borers, pests that have the capacity to damage vast amounts of corn. Rules set by the Environmental Protection Agency in 2001 say that farmers who grow Bt corn must also grow a certain percentage of non-GM corn nearby. These "refuge zones," in theory, are supposed to reduce the risk that rootworms or corn borers that survive the toxins breed with each other, thereby developing resistance to the technology.

But the number of farmers planting these required refuge zones has dropped, and compliance rates plunged from above 90 percent in 2003 and 2005 to as low as 63 percent for certain Bt varieties, according to the report, which is based on federal data.

Four companies are registered to market Bt corn — Creve Couer-based Monsanto, Syngenta, Dow AgroSciences and Pioneer

Hi-Bred International. They are responsible for educating farmers about the proper size and location of refuge zones. Each year, a group representing these companies called the Agricultural Biotech Stewardship Technical Committee, submits a required annual report to federal regulators, based on the industry's assessments, showing the level of farmer compliance with refuge zone requirements.

"Obviously this isn't new to us," said Nicholas Storer, chairman of the group. "We've been acting on this information for a year."

Storer said the drop in compliance, which the group does not refute, is the result of a combination of factors.

"This happened at the same time the price of corn started to increase, and there were concerns about food shortages globally," he explained. "Farmers felt some pressure to maximize production."

In other words, farmers may have planted in these refuge zones.

At the same time, Storer and others said, Bt corn with "stacked" traits that provide resistance to both corn borer and rootworm, hit the market. These stacked trait varieties had new and more complicated refuge requirements. In areas where genetically modified cotton is grown, largely in the south, the requirements are even more stringent.

"It's getting a little more confusing than it used to because they're stacked traits and different requirements," said Mike Geske, the recent president of the Missouri Corn Growers Association.

"I raise 1,000 acres of corn, and I might have 20 fields, and each one has a compliance requirement. I could have one field that falls short, and the whole [farm] is out of compliance."

Geske pointed out, however, that fewer farmers in the southern cornbelt are planting Bt corn these days.

"Down here we're not getting as much of a yield bump, and it's interfering with our no-till practices because the Bt stalks are much stronger," Geske said. "The seed is more expensive, and we're getting a premium for non-GMO."

In a letter sent to EPA administrator Lisa Jackson, the Center for Science in the Public Interest called on the agency to pressure registered Bt corn companies to impose fines on growers who fail to comply with the rules. So far, Storer said, the companies have withheld the technology from at least 12 farmers who have repeatedly violated refuge guidelines.

Farmers, industry and the center, all pointed out Thursday that no resistance in the pests has been discovered so far. But, they say, it's only a matter of time.

"We've come into this assuming that pests are clever and they'll find a way," Storer said. "It's in our interest to [comply]. We're certainly not sitting still and assuming resistance won't happen."

Still, the performance recently has been dipping.

"There's enough fault to go around," said Jaffe. "Clearly, the registrants aren't putting enough pressure on the farmers, and the EPA isn't putting enough pressure on the registrants."