By Carey Gillam

(Reuters) - Honey bees, crucial in the pollination of many U.S. crops, are still dying off at an worrisome rate, even though fewer were lost over the past winter, according to a government report issued on Thursday.

Total losses of managed honey bee colonies was 23.2 percent nationwide for the 2013-2014 winter, according to the annual report issued by the U.S. Department of Agriculture (USDA) and the "Bee Informed Partnership," a group of honeybee industry participants.

The death rate for the most recent winter, October 2013 through April 2014, was better than the 30.5 percent loss reported for the winter of 2012-2013, but worse than the 21.9 percent in 2011-2012, the report said. Previous surveys found total colony losses averaged 29.6 percent over the last eight-year span.

Over the past few years, bee populations have been dying at a rate the U.S. government says is economically unsustainable. Honey bees pollinate plants that produce about a quarter of the food consumed by Americans, including apples, almonds, watermelons and beans, according to government reports.

Scientists, consumer groups and bee keepers say the devastating rate of bee deaths is due at least in part to the growing use of pesticides sold by agrichemical companies to boost yields of staple crops such as corn.

They pointed to a study issued on May 9 by the Harvard School of Public Health that found two widely used neonicotinoids — a class of insecticide — appear to significantly harm honey bee colonies over the winter, particularly during colder winters.

"With the damning evidence mounting, pesticide companies can no longer spin their way out of this crisis," said Michele Simon, a public health lawyer who specializes in food issues.

Monsanto Co, DuPont, Syngenta AG, Bayer AG and other agrichemical companies say the bees are being killed by other factors, such as mites. Bayer and Syngenta make the pesticides in question, while Monsanto and DuPont have used them as coatings for the seed they sell.

Monsanto-owned BeeLogics, a bee health company, is one of the collaborators in the partnership with USDA that issued the report on Thursday, which appeared to lay much of the blame for die-offs on the "varroa mite," an Asian bee parasite first found in the United States in 1987.

"Yearly fluctuations in the rate of losses like these only demonstrate how complicated the whole issue of honey bee heath has become," said Jeff Pettis, research leader at the USDA's agricultural research service.

Pettis said viruses, parasites, nutrition problems and pesticides are all factors.

Last year, the European Union said it would ban neonicotinoids used for corn and other crops, as well as on home lawns and gardens. Similar constraints in the United States could cost manufacturers millions of dollars.

The survey results reported are based on information self-reported by U.S. bee keepers. About 7,200 bee keepers who managed 564,522 colonies in October 2013, responded to the survey. Those bee keepers represent 21.7 percent of the country’s 2.6 million colonies.

In January, the Environmental Protection Agency said it would fund more than $450,000 in research projects to reduce the use of pesticides that may harm honeybees

(Reporting by Carey Gillam in Kansas City. Editing by Andre Grenon)