Honeybees are seen in a colony. U.S. beekeepers lost 38 percent of their bee colonies last winter, the most seen in 13 years of research, according to a University of Maryland-led study. (Baltimore Sun)

Honeybees are no longer disappearing suddenly and mysteriously. They’re dying persistently, in plain sight, and scientists often know why.

A parasitic mite is infesting commercial colonies, latching on to bees and sucking out the tissue that stores their energy and protein. The invasion is weakening the bees’ tolerance to pesticides, introducing new viruses and exacerbating a shortage of flowering plants they forage for nectar.

And the losses are mounting. A University of Maryland-led survey released Wednesday shows U.S. beekeepers lost 38 percent of their bee colonies last winter, the most seen in 13 years of research.

Those losses threaten an often overlooked but fundamental segment of the nation’s agriculture industry. Commercially raised honeybees pollinate $15 billion worth of the country’s food crops each year.

“We’re not worried about honeybees going extinct,” said Dennis vanEngelsdorp, a University of Maryland associate professor of entomology who helps lead the survey. “We’re worried about commercial beekeepers going extinct.”

It’s still not clear why, in 2006, bees began abandoning otherwise healthy colonies, a phenomenon that became known as colony collapse disorder. Scientists have been conducting annual surveys of bee losses ever since, in hopes of learning how they can better protect the pollinators.

But the latest survey results show the woes of the bees aren’t over.

On average since 2006, beekeepers have lost about 30 percent of colonies each winter, when hives must subsist on honey reserves to survive. But this winter, that figure jumped, surpassing a previous high of 36 percent in the winter of 2007-2008, the survey found. Over the past year, about 41 percent of commercial colonies have failed.

“We don’t seem to be making particularly great progress to reduce overall losses,” said Geoffrey Williams, an assistant professor of entomology at Auburn University and co-author of the survey.

There are some 2.69 million commercial honeybee colonies across the country. Many of them are actually mobile apiaries, moving around the continent throughout the growing season, depending on what is blooming when and where. As honeybees forage for nectar, they spread pollen, fertilizing flowers that then produce fruit and nuts. The bees produce honey to live on over the winter.

Varroa mites began attacking commercial honeybee colonies in the United States in the 1980s, jumping from an Asian variety of bees to the European breed that has been used commercially to make honey in this country since colonial times. The parasites latch on to honeybees and their brood, eventually sucking the life from a hive if not checked with a variety of countermeasures beekeepers use to stop their spread.

“If you control them, your bees are fine,” said Jim Fraser, president of the Maryland Beekeepers Association. “There are an awful lot of people who don’t control them.”

That has allowed wounds to fester in honeybee hives, vanEngelsdorp said.

The mites spread viruses that have mutated to become increasingly lethal to honeybees. The mite infestations also contribute to reducing the insects’ tolerance to pesticides that are ubiquitous in the environment.

In places like Maryland, where flowering plants are only abundant for a few months, honeybees already struggle to find nectar for much of the year. It doesn’t help that many residents kill or pull weeds that are actually prime nectar sources for bees, including white thistle and dandelion, or that farmers across the country plant massive swaths of corn and soybeans, which don’t require bees for pollination.

A Maryland law went into effect last year banning a class of pesticides known as neonicotinoids, which are particularly harmful to bees. While that could help reverse the losses, the pesticides are just part of that confluence of factors weakening colonies, biologists say.

The losses are most significant in the winter because bees must survive three to four months without foraging; if disease or pesticides cut their lives short by, say, 10 percent, that could be the difference preventing a colony from surviving to spring, vanEngelsdorp said. During the rest of the year, honeybees each only live about 35 days, making it easier for beekeepers to manage losses by splitting up the hives that are thriving and starting new ones.

As the University of Maryland shares the latest survey results, entomologists there are urging beekeepers to use three different strategies to fight the spread of varroa mites. Those can include chemical treatments, tweaks to colony management practices and use of essential oils that can keep the mites at bay.

Karen Rennich, executive director of the university’s Bee Informed Partnership, said many beekeepers she works with across the country are frustrated that many methods they’ve used to reduce varroa mites are no longer working. The nonprofit works to help apiaries troubleshoot such problems, while also collecting data that can lead to solutions.

“Think of it like antibiotics,” she said. “This is what keeps the wolves at bay.”

Don John, apiary manager at Baltimore-based Apex Bee Company, said he tried and failed at killing varroa mites using cardboard strips treated with chemicals and inserted into colonies. The treatments were expensive, and also hard on his bees, which are spread in colonies across Maryland. But he has found success using oxalic acid, a cheap organic substance sold in hardware stores as wood bleach.

“We’re optimistic this may turn the corner for us,” John said.

But it’s not always so easy. On a recent morning at a university apiary in Beltsville, where the Bee Informed Partnership routinely trains beekeepers in recommended management practices, vanEngelsdorp pulled a frame from a stack of wooden boxes containing a colony, shaking about 300 of its residents into a glass jar.

In a test performed routinely to estimate the colony’s varroa mite load, Mark Dykes, the partnership’s bee squad coordinator, tossed the bees in some powdered sugar and then shook the sweet dust out into a bowl through a lid of metal mesh. When dissolved in liquid, the sugar revealed an infestation: dozens of tiny reddish specks.

“That’s troubling,” vanEngelsdorp said. “This is a very high level.”

If the parasites remain so numerous come fall, he said, the colony will never survive the winter.

— Baltimore Sun