Bee populations have been falling rapidly in many countries, fuelled by a phenomenon known as colony collapse disorder. Its cause is unclear but the Varroa mite is a prime suspect, since it spreads viruses while feeding on hemolymph, or bee's "blood".

To clarify the link between mites and viruses, a team led by Stephen Martin of Britain's University of Sheffield studied the impact of Varroa in Hawaii, which the mites have only recently invaded.

They found the arrival of Varroa increased the prevalence of a single type of virus, deformed wing virus (DWV), in honey bees from around 10 percent to 100 percent.

At the same time the amount of DWV virus in the bees' bodies rocketed by a millionfold and there was a huge reduction in virus diversity, with a single strain of DWV crowding out others.

"It is that strain that is now dominant around the world and seems to be killing bees," Martin said in a telephone interview. "My money would be on this virus as being key."

Other factors - including fungi, pesticides and decreased plant diversity - are thought to play a role in colony collapse, but Ian Jones of the University of Reading said the latest findings pointed to the virus and mite combination as being the main culprit.

"This data provides clear evidence that, of all the suggested mechanisms of honey bee loss, virus infection brought in by mite infestation is a major player in the decline," he said.

Jones, who was not involved the research, said the findings published in the journal Science reinforced the need for beekeepers to control Varroa infestation in colonies.

The threat to bee populations extends across much of Europe and the United States to Asia, South America and the Middle East, experts say.

Bees are important pollinators of flowering plants, including many fruit and vegetable crops. A 2011 United Nations report estimated that bees and other pollinators such as butterflies, beetles or birds do work worth 153 billion euros ($191 billion) a year for the human economy.

(Editing by Robert Woodward)

• A Permanent Economic Emergency