A new national survey estimates honeybee deaths in the U.S. at 40 percent for the past year with Minnesota beekeepers reporting more than 50 percent losses.
The newest research shows that while winter bee deaths declined slightly, summer bee losses increased — a significant trend, said Dennis vanEngelsdorp, a University of Maryland researcher who led the Bee Informed Partnership survey.
"It's worrying that we've got this new norm, where just a continuous sort of hemorrhaging of losses seems to happen all year round," he said. "These bees are under incredible amounts of stress and we have to figure out what the drivers of those stresses are."
Summer losses are likely the result of poor nutrition and exposure to pesticides, he added.
More than 6,000 beekeepers responded to the survey; vanEngelsdorp says they represent about 15 percent of the nation's estimated 2.74 million managed honey bee colonies.
Beekeepers who responded to the survey reported average losses of 42.1 percent of their colonies over the course of the year.
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While winter loss rates declined from 23.7 percent last year to 23.1 percent this year, summer bee colony losses increased from 19.8 percent to 27.4 percent.
The rate of bee deaths is nearly double what beekeepers who were surveyed considered acceptable losses.
The survey provides broad trends, but it can be very difficult to pinpoint what causes a bee colony to die, said Katie Leem, a University of Minnesota graduate student who worked on the report.
"[For] a honeybee colony, sometimes something can happen to it months before the colony actually has issues," Lee said. "So they can have a nutrition issue earlier and then we may not see the effects until later."
The data provide researchers with some theories about why bees are dying, vanEngelsdorp said.
Among backyard beekeepers (defined as those who manage fewer than 50 colonies), a clear culprit in losses is the varroa mite, a lethal parasite that can easily spread between colonies.
Among commercial beekeepers, the causes of the majority of losses are not as clear.
"Backyard beekeepers were more prone to heavy mite infestations, but we believe that is because a majority of them are not taking appropriate steps to control mites," vanEngelsdorp said.
"Commercial keepers were particularly prone to summer losses," he added. "But they typically take more aggressive action against varroa mites, so there must be other factors at play."
The data can help researchers focus on possible causes, vanEngelsdorp said.
"I think you use that data to formulate some hypothesis," he said. "The hypothesis that strikes me is, what is the effect of poor nutrition and pesticides during the summer?"
This is the ninth year of the winter loss survey, and the fifth year it includes summer and annual losses in addition to winter loss data.
The slightly reduced winter losses are hopeful but growing summer losses are very troubling, said Jeffery Pettis, senior U.S. Department of Agriculture entomologist and survey co-coordinator.
"If beekeepers are going to meet the growing demand for pollination services," he said, "researchers need to find better answers to the host of stresses that lead to both winter and summer colony losses."