Beekeepers across the country lost 44 percent of bee colonies from April 2015 to April 2016.
That's 3.5 percent higher than the previous year, according to the results of an annual survey done by the Bee Informed Partnership, a group of university, industry and government researchers.
It's normal to have some bee losses in the winter, said the partnership's project director Dennis VanEngelsdorp, but for the second year in a row, summer bee losses increased.
"What's really surprising is that in the summer when things are usually good for bees, they're dying at a higher rate than they are during the winter which is a stressful time. So, that's something we really have to try to understand," said VanEngelsdorp, who is also an assistant entomology professor at the University of Maryland.
A pest called the varroa mite is a significant cause of bee deaths. But researchers suspect bees are more susceptible to the mites because they are under stress from environmental factors other than mites.
"Some of the other stressors we're looking at are pesticides of course and we also are looking at poor nutrition. What we see is the landscape has changed dramatically over the last ten years. There's less and less bee forage out there, especially in the Midwest," said VanEngelsdorp.
Because of all the attention paid to bees in recent years, a growing number of small beekeepers are raising bees to help declining populations. But the survey found backyard beekeepers are doing a poor job of managing varroa mites in their bee colonies.
Despite good intentions, VanEngelsdorp says, some may actually be harming bees by spreading the mites to other bee colonies.
"We think we're seeing that in the environment where people have taken precautions, but a month later they have really high mite levels, probably because they've had mites move in from neighboring colonies," he said.
Annual bee colony losses near 50 percent force commercial beekeepers to keep spending time and money to rebuild their colonies.
"More importantly, the industry needs these bees to meet the growing demand for pollination services. We urgently need solutions to slow the rate of both winter and summer colony losses," said USDA senior entomologist and survey co-coordinator Jeffery Pettis.
This is the tenth year of the winter loss survey, and the sixth year to include summer and annual losses. More than 5,700 beekeepers from 48 states responded to this year's survey. They represent about about 15 percent of the estimated 2.6 million managed honey bee colonies across the country.
Bee loss numbers for individual states will be released later.
The USDA is also conducting an expanded survey of commercial bee operations this year to provide more information about the state of the commercial bee industry.