FRESNO, Calif. -- Commercial beekeepers and environmental organizations filed a lawsuit Thursday against federal regulators for not banning the use of two pesticides they say harm honeybees.

In the suit, filed by the Center for Food Safety in the U.S. District Court in San Francisco, the group asks the U.S. Environmental Protection Agency to suspend the use of insecticides clothianidin and thiamethoxam - known as "neonicotinoids," a class of chemicals that act on the central nervous system of insects.

The chemicals are used to treat corn, cotton and other crops against a variety of pests. Research shows that the chemicals build up over time in the soil, plants and trees. They are used widely in the Midwest, where many bees used for California's annual almond pollination are located. Each February, more than half of the country's honeybees - about 1.5 million hives - are trucked to California's almond orchards, the nation's biggest pollination event.

Beekeepers and some scientists have for years blamed the pesticides for higher bee die-offs. Bees are exposed to the insecticides via residues in nectar and pollen and in contaminated dust from planting of treated seeds. Critics of neonicotinoids say they are toxic to bees, making them more susceptible to pathogens, and could be a significant factor in colony collapse disorder, in which all the adult honeybees in a colony suddenly disappear or die.

Since it was recognized in 2006, colony collapse disorder has destroyed colonies at a rate of about 30 percent a year, according to the U.S. Department of Agriculture. Before that, losses were about 15 percent a year from pests and diseases. No one has determined its cause, but most researchers point to a combination of factors, including pesticide contamination, poor nutrition and bee diseases.

France, Germany and Italy have limited or banned the use of neonicotinoids to protect honeybees. In January 2013, the European Food Safety Authority found that neonicotinoids pose an unacceptably high risk to bees and that industry science may be flawed. Experts say the pesticides are also harmful to other bee species and other beneficial insects such as butterflies and ladybugs.

The lawsuit comes on the heels of a shortage of healthy bees in this year's almond pollination and a higher than usual bee die-off. Experts say some beekeepers lost up to 50 percent of their colonies. The shortage had some growers scrambling for bees - even sub-performing bees - driving bee prices to an all-time high.

Bees' health is a great concern, farmers say, because with California's almond acreage increasing steadily in recent years to 760,000 acres, the number of healthy bees needed is expected to increase. California grows 80 percent of the world's almond supply. Bees also pollinate about a third of other U.S. crops.

A year ago, the beekeepers and environmental groups filed a legal petition urging the EPA to ban clothianidin. The EPA denied that suspension request in July. The agency has been conducting a re-evaluation of clothianidin and other neonicotinoids, but those re-evaluations won't be finished for several years.

The lawsuit also challenges the EPA's use of so-called "conditional registrations" for the pesticides, which expedites commercialization by bypassing review. And, the suit says existing pesticide labels do little to prevent harmful exposures.