According to a national survey of honey bee colony losses conducted by the Bee Informed Partnership, the Apiary Inspectors of America, and the U.S. Department of Agriculture, U.S. beekeepers lost more than one in five honey bee colonies in the winter of 2013-2014 — significantly fewer than the winter before. But tough times continue for commercial beekeepers, who are reporting substantial honey bee losses in summer as well. Beekeepers who tracked the health of their hives year-round reported year-to-year losses of more than one in three colonies between spring 2013 and spring 2014.

University of Maryland entomologist Dennis vanEngelsdorp, who directs the Bee Informed Partnership, led a team of 11 researchers who conducted the survey. A total of 7,183 beekeepers, who collectively manage about 22 percent of the country’s 2.6 million commercial honey bee colonies, took part.

No single culprit is responsible for all of the honey bee deaths, but their research shows mortality is much lower among beekeepers who carefully treat their hives to control a lethal parasite called the Varroa mite.

“If there is one thing beekeepers can do to help with this problem, it is to treat their bees for Varroa mites,” said vanEngelsdorp. “If all beekeepers were to aggressively control mites, we would have many fewer losses.”

The beekeepers were asked to list the probable causes of their losses, and in a separate survey some of them also described how they managed their hives. Those responses are still being analyzed, but an initial review, previous fieldwork, and past surveys all point to Varroa mites as a persistent and controllable problem.

“Every beekeeper needs to have an aggressive Varroa management plan in place,” vanEngelsdorp said. “Unfortunately, many small-scale beekeepers are not treating their bees, and are losing many colonies. And those colonies are potential sources of infection for other hives.”

Blue bars show the percentage of winter honey bee colony losses considered “acceptable” by U.S. commercial beekeepers participating in the annual national survey. Red bars show the percentage of colonies actually lost each winter by the participating beekeepers. Credit: Bee Informed Partnership.

The survey found that 23 percent of the managed honey bee colonies in the U.S. died between Oct. 1, 2013 and March 31, 2014. That’s well below the average loss of 29 percent over the survey’s history.

However, summer death rates were nearly as high as the winter ones. Over the summer, about 20 percent of survey respondents’ colonies died. Losses between spring 2013 and spring 2014 averaged 34 percent. (The winter and summer losses do not match up to the year-to-year losses because not all beekeepers filled out all sections of the survey.)

“Rising and falling rates of colony losses demonstrate how complicated the issue of honey bee health has become,” said Jeff Pettis, a survey co-author and research leader of the USDA’s Bee Research Laboratory in Beltsville, MD. Pettis pointed out that the causes of honey bee colony failures are complex, including Varroa mites, other parasites and viruses; poor nutrition, which can be due to a shortage of wildflowers in herbicide-sprayed or drought-stricken farm fields; and exposure to insecticides and fungicides.

Now in its eighth year, the survey originally focused on winter mortality in managed honey bee colonies. The generation of bees that lives through the winter must survive months longer than summer bees’ 30-day lifespan. In winter, honey bees don’t produce offspring and are confined in a hive where diseases can spread.

“We used to think winter was the critical period,” vanEngelsdorp said. “But during our field studies, beekeepers told us they were also losing colonies in the summer months. So we expanded the survey and found that in fact, colonies are dying all year round.”

About two-thirds of beekeepers surveyed, who ranged from hobbyists to large businesses, said their colonies suffered unacceptable losses –greater than the 19 percent mortality rate that, on average, they were willing to absorb. It was the second year in a row that most beekeepers reported unacceptably high mortality.

A summary of the 2013-2014 survey results is available at http://beeinformed.org/results-categories/winter-loss-2013-2014/.