Colony Collapse Disorder affected 23 percent of U.S. beekeepers last year. Affected beekeepers lost an average 45 percent of their bees to the phenomenon -- the bees simply disappeared, leaving empty or nearly empty hives.
The disorder threatens many crops that rely on bees for pollination, and could have a $75 billion impact, according to the U.S. Department of Agriculture.
Scientists and beekeepers have been puzzled by the disappearances, suggesting causes as disparate as parasites, pesticides, environmental stressors and cell phone towers. Many saw the new study as a breakthrough.
"This is a very significant finding," University of Delaware entomologist Dewey Caron, who was not involved in the study, told Technology Review magazine.
Still, Caron and others cautioned that the new finding do not rule out other possible causes, because bees weakened by parasites or environmental stressors could be more susceptible to the virus, called Israeli Acute Paralysis Virus.
Scientists identified IAPV as a culprit using new gene-sequencing methods. The researchers mapped DNA taken from both healthy and infected bee colonies around the country, as well as colonies in Australia and jelly produced by bees in China. After they subtracted the DNA that came from the bees themselves -- which was possible because scientists recently finished mapping the bee genome -- they were left with DNA from the bacteria, fungi and viruses that infected the bees.
They found that of the 30 colonies affected by Colony Collapse Disorder, all but one showed IAPV, while the 21 healthy colonies did not.
"The authors themselves recognize that it's not a slam dunk, it's correlative," entomologist May Berenbaum of the University of Illinois told the Associated Press. "But it's certainly more than a smoking gun -- more like a smoking arsenal. It's very compelling."
The researchers' next step will be to infect previously healthy colonies, as well as colonies with mites and other stressors, with IAPV, to find out what happens.
"At least we have a lead we can now begin to follow," study co-author Ian Lipkin, and entomologist at Columbia University, told the AP. "We can use it as a marker and we can use it to investigate whether it does in fact cause the disease."
IAPV had previously been detected in bee colonies in Australia and Israel, however, it hasn't caused colony collapse disorder in those countries. That bolsters the idea that the virus may work in concert with other causes, the researchers say.
The study also suggests that the virus may have entered the United States from Australia -- all of the diseased hives were either imported from Australia or had contact with Australian bees. And the first signs of Colony Collapse Disorder appeared soon after beekeepers started importing Australian bees in 2004.
The researchers suggested that it's possible that the virus, harmless in Australia, might have mutated to a more dangerous form after arriving in the United States.
"We know from other viruses like West Nile that very small genetic changes can turn a benign virus into virulent ones," biologist Edward Holmes, who was involved in the study, told Technology Review.
The researchers say it's also possible that the virus affected U.S. bees already stressed by mites or other factors.
The Australian government did not respond to requests for a comment on the study, according to Bloomberg News, but a bee industry group there rejected the findings.
"We unequivocally reject claims that Australia caused the introduction of Colony Collapse Disorder in the U.S.," Stephen Ware, of the Australian Honey Bee Industry Council, told Bloomberg News.