A European honey bee (Apis mellifera) with a Varroa mite on its back

Many fruit, nut and berry crops depend on honey bees for pollination, but more and more bee colonies are dying each year. Parasitic mites, viruses, and pesticides may all play a role, but as Véronique LaCapra reports, researchers are still looking for a way to stop the die-off.

Honey bees are the most valuable pollinators of agricultural crops worldwide. In the United States, approximately 130 crops depend on honey bees for pollination. Their work is worth about $15 billion a year.  

Dennis VanEnglesdorp is Pennsylvania's acting state apiarist. He's responsible for tracking the health of the state's commercial honey bee colonies, and he estimates that one in every three bites of food we eat are directly or indirectly pollinated by honeybees. "Honeybees are the moveable pollination force in modern agriculture."
    
Almonds, blueberries, and apples; carrots, onions, and squash – all of these fruits and vegetables grow in different parts of the country and bloom at different times of the year. So, to meet the pollination demand, commercial beekeepers truck their hives around the country. A single beekeeper may move tens of millions of bees, covering thousands of kilometers in a single trip.

"So all your fruits and vegetables, all your flowering plants require insect pollination," says VanEnglesdorp, "and honey bees do a majority of that pollination."

Parasitic Varroa mites are a major cause of U.S. bee colony declines

In the 1940s and '50s, there were approximately five million managed bee colonies in the United States. Today, that number has dropped to less than half that. Severe declines began in the 1980s, with the accidental introduction of a new parasite called the varroa mite.

"It's actually an amazingly large parasite," describes VanEnglesdorp. "If we were a bee, it would be like a dinner plate feeding on us." The mite has very sharp mouthparts, which it uses to pierce the skin – or exoskeleton – of the bee.  Van Englesdorp says that varroa actually spits inside the bee, "and in that spit we believe that there's a protein which acts a lot like AIDS virus does in the fact that it breaks down the insect's immune system."

The mite can also transmit viruses and other pathogens from bee to bee, and wipe out entire colonies. According to VanEnglesdorp, varroa is the biggest challenge facing commercial beekeepers: "it still kills most of the colonies in the country."

Jeff Pettis, lead researcher at the U.S. Department of Agriculture's Bee Research Laboratory, holds a frame of honeybees

Jeff Pettis is the lead scientist at the U.S. Department of Agriculture's Bee Research Lab, just outside of Washington, D.C. Pettis and his team have been studying a more recent and mysterious threat to honey bee colonies.  Honeybee colonies usually suffer a loss of about 15 to 20 percent each year. "Our last two years we've been over 30 percent losses," says Pettis, "and so this is what we're calling this phenomenon of CCD, colony collapse disorder."

CCD looks very different from other causes of bee death, and it happens much more quickly: within just a few weeks, most of the adult worker bees disappear from the hive, leaving the queen and all the young bees behind.

Since CCD was first reported, researchers have been scrambling to find a cause. They've looked at parasites, viruses, pesticides, and even colony management problems like poor nutrition and transportation-related stresses.

Experimental beehives at the USDA Bee Research Laboratory. A single commercial beehive can house as many as 50,000 honey bees

Pettis says that researchers have already done enough testing that if one new pathogen or other problem were causing the bee deaths, they would already have identified it. "We think it's a complex, maybe even a syndrome – things that are coming together to cause the losses of bees."

The lack of clear answers worries Dennis VanEnglesdorp. "What's really frightening about this new condition is we don't know what causes it, so we don't know how to stop it." He says that the mobile, commercial honeybee operations that we rely on to pollinate different crops across the country are in real danger.  In a single year, "they can lose 30 to 50, sometimes 80 percent of their colonies." Van Englesdorp says beekeepers can absorb that kind of loss for a year or two, "but they can't do it three years in a row and stay in business."

And bee declines are not limited to the United States. "We're hearing reports from Europe, from Canada, and from South America, even some from Asia – where honeybee populations are collapsing and decreasing."

There are still enough bees in the U.S. to meet demand, but VanEnglesdorp warns that continued colony losses could threaten the production of some crops, and drive more and more beekeepers out of business.