A Bayer CropScience pesticide is at the center of a legal battle for research data that could help explain what's killing U.S. honeybees in large numbers.

The lawsuit, filed Monday in a Washington, D.C., federal court, accuses the U.S. Environmental Protection Agency of hiding the honeybee data.

The Natural Resources Defense Council sued after the EPA missed a deadline to respond to a Freedom of Information request the council submitted July 17.

It's not unheard of for federal regulators to take years to fulfill an FOIA request. But in the case of Bayer's pesticide chlothianidine, the Natural Resources Defense Council decided to push hard.

"It's an aggressive suit," said the group's spokesman, Josh Mogerman. "But in a scientific mystery that threatens the U.S. food supply, business as usual is not acceptable."

Dale Kemery, an EPA spokesman, declined comment until the agency fully reviews the lawsuit.

John Boyne, a spokesman at Bayer CropScience's U.S. headquarters in Research Triangle Park, said some of the data the Natural Resources Defense Council is seeking was published in a scientific journal a year ago.

"I don't know why they filed the lawsuit," Boyne said.

Chlothianidine is made to coat corn, sugar beets and sorghum seeds and protect them from pests. But the chemical has the potential to be very toxic for bees. Three months ago, German regulators banned chlothianidine and related chemicals after the family of pesticides was blamed for the destruction of about 11,000 bee colonies earlier this year.

The EPA approved the use of chlothianidine in 2003 on the condition that Bayer submit safety data, including how the use of the pesticide affects hives over the life of a honeybee.

"We met all the requirements of the conditional approval," Bayer spokesman Boyne said. Results of a field study that monitored the long-term effects of chlothianidine on honeybees were published in the June 2007 Journal of Economic Entomology, Boyne added.

The NRDC thinks the data from that study might show whether chlothianidine plays a role in the sudden loss of millions of U.S. honeybee colonies.

The phenomenon, also known as colony collapse disorder, threatens a significant portion of the U.S. food supply. About one out of every three mouthfuls in the U.S. diet stems from crops pollinated by bees.