Honeybee death mystery deepens

SAN DIEGO, Calif. – A one-two punch by a gut parasite and viruses may help explain the mysterious decline in U.S. honeybees seen over the last four years.

Bees infected with both the fungal parasite Nosema ceranae and with any one of a handful of RNA viruses were much more likely to have come from hives on the decline than from healthy hives, researchers reported May 25 at a meeting of the American Society for Microbiology.

The finding represents a new twist in a complex and multifaceted scientific problem, termed colony collapse disorder, made urgent by the continuing and severe losses suffered by U.S. beekeepers beginning in 2006. About a quarter of beekeepers have been affected, according to the Apiary Inspectors of America, an industry group. These beekeepers, including honey producers as well as many who lease out their bees to pollinate food crops, have reported losing between 30 and 90 percent of their hives. The latest nationwide survey, of 2009-2010 winter losses, revealed more than 30 percent of hives were lost for a variety of reasons.

“We think that Nosema leaves the bees more open to infection by other organisms,” said bee researcher Jay Evans of the United States Department of Agriculture’s Agricultural Research Service in Beltsville, Md., who presented the new results. “Our current thinking is that the Nosema parasite is a precursor to infectious diseases” that lead to colony collapse disorder.

That view represents a shift: In 2007, many scientists were discussing the role Nosema ceranae might play in colony collapse, but Evans and his colleagues failed to find a clear link between the parasite and affected hives.

Only when scientists looked at the N. ceranae pathogen, which causes “bee diarrhea” among other symptoms, in combination with members of the dicistroviridae family of RNA viruses, did a strong correlation show up, Evans said. 

N. ceranae is a relative newcomer in the United States and Europe. It was first identified in domestic bee colonies just a few years before the bee declines began and has since become the dominant Nosema species, Evans said.

Eric Mussen, an apiculturalist at the University of California, Davis, says “it’s been awfully hard to tell whether a virus or Nosema or anything else is the culprit” in colony collapse.

“You find viruses in healthy colonies. Nosema ceranae is everywhere,” he says. “But I presume that if you have a parasite it would definitely affect the bees’ ability to deal with the virus. That makes sense to me.”

Although Nosema can be treated with an antibiotic “medicine,” Mussen says the parasite is particularly hard to get rid of entirely. So are viruses, although there are some new methods being tested for lowering overall levels of viruses in hives. The mites that spread viruses are also a problem, he says. “We’ve got a problem with all these stresses, all rolled in together.”