FRESNO, Calif. (AP) -- The U.S. Department of Agriculture says fewer honeybee colonies are being lost, suggesting bees' health may be improving.

A survey made public Thursday shows that about 22 percent of U.S. honeybee colonies were lost this winter. That's a lower mortality than in the previous five years when colonies were decimated at a rate of about 30 percent a year.

Bees are essential pollinators of about a third of the United States' food supply.

Some of the devastation is attributed to colony collapse disorder, in which all the adult honey bees in a colony disappear or die. Prior to 2006, when the disorder was recognized, losses were about 15 percent a year from a variety of pests and diseases.

Researchers say an unusually warm winter could have impacted colony survival this year.

5,543 beekeepers completed the survey.

Copyright 2012 The AP.