WASHINGTON—Total losses of managed honeybee colonies from all causes were 21.9 percent nationwide for the 2011/2012 winter, according to an annual survey conducted by the U.S. Department of Agriculture, the Bee Informed Partnership and the Apiary Inspectors of America.

That represents a substantial drop compared to the previous five years, for which previous surveys found total colony losses of 29 percent to 34 percent.

Unusually mild weather could be a possible contributing factor, although no direct scientific investigation of the weather connection has been conducted. January 2012 ranks as the fourth-warmest January in U.S. history.

"A warm winter means less stress on bee colonies and may help them be more resistant to pathogens, parasites and other problems," said Dr. Jeffery Pettis, co-leader of the survey and research leader of the USDA Agricultural Research Service’s Bee Research Laboratory.

About 5,500 beekeepers, who manage nearly 15 percent of the country's estimated 2.49 million colonies, responded to the survey.

Among those who reported losing any colonies for any reason, 37 percent said they lost at least some of their colonies without finding any dead bees. The absence of dead bees is a defining symptom of colony collapse disorder, a serious problem that beekeepers began facing in 2006. Because the survey was interview-based, it was not possible to confirm that the colonies in question had CCD or whether the losses were the result of other causes.

Almost half of responding beekeepers reported losses greater than 13.6 percent, the level of loss that beekeepers have stated would be acceptable for their operations. Continued losses above that level threaten the economic sustainability of commercial beekeeping.

Contact Kim Kaplan, USDA ARS, at 301-504-1637.