Honeybee decline linked to deadly virus spread by mites

By Chelsea Whyte on June 11, 2012 7:20 PM EDT

Short-Haired Bumblebee
Declining bee populations linked to a virus-carrying mite (Photo: Creative Commons)

The large-scale death of honeybees, dubbed Colony Collapse Disorder, has been stumping scientists for years, but researchers at the University of Sheffield have now discovered that a parasitic mite which carries a wing-deforming virus exponentially increases the spread of the pathogen.

The study, published in the journal Science, showed how the Varroa mite caused Deformed Wing Virus (DWV) - a known viral pathogen - to increase its frequency among honey bee colonies from 10 per cent to 100 per cent, resulting in a million-fold increase in the number of virus parasites affecting each honeybee and leaving one dominant killer strain of the virus.

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"It is that strain that is now dominant around the world and seems to be killing bees," Stephen Martin of the University of Sheffield's Department of Animal and Plant Sciences told Reuters. "My money would be on this virus as being key."

The Varroa destructor mite spread from Asia across the entire globe over the past 50 years, reports The Guardian. It's been implicated in the decimation of bee colonies, but among other factors like pesticides and habitat destruction, researchers weren't able to determine the mighty mite's role. When it showed up five years ago on two of the Hawaiian islands, but not the other isles, scientists were offered a perfect chance to investigate the mite's impact on the spread of Deformed Wing Virus (DWV).

The mites act as a "viral reservoir and incubator," and inject the virus directly into the bees when they feed on their blood, "bypassing conventional, established oral and sexual routes of transmission," according to Agence-France Presse.

"Deformed Wing Virus is naturally transmitted in bees through feeding or sex but the mites change the disease so it becomes more deadly, shortening the bees' lives," the researchers said. By entering the bee's bloodstream directly, the virus bypasses its natural immune defenses.

The data showed that after the mites invaded bee hives, the usually inconsequential Deformed Wing Virus became more potent and stuck around even after a colony is cleared of the infestation.

"Just 2,000 mites can cause a colony containing 30,000 bees to die. The mite is the biggest problem worldwide for bee keepers; it's responsible for millions of colonies being killed," Martin said.

Honeybees are crucial to plant reproduction in the wild and to the production of many crops, with their monetary value as commercial pollinators estimated at $15 to $20 billion annually in the United States alone. And Hawaii is a significant bee-keeping location in the States, as almost all the queen bees used in the U.S. are bred on the islands, reports The Guardian.

"Understanding the changing viral landscape that honey bees and other pollinators face will help beekeepers and conservationists worldwide protect these important insects. We have discovered what happens at the start of an infection. The goal is to understand how the infection comes about so that we can control it," said Martin.

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