Like many pollinating insects, honeybees have been in decline across the UK for some time. Many things have contributed. One is deformed wing virus (DWV), which most honeybee colonies have as it lurks in their food. DWV can leave bees with stubby wings and shortened, rounded abdomens. Their movements also become tottering and stunted. About 20 per cent of UK bee colonies are lost to DWV each winter.
But the disease has become much more severe in the UK in recent decades, ever since the arrival of a bloodsucking mite called Varroa destructor . "There is no factor which is more tangible, and for which the evidence is greater, than the introduction of Varroa mites in 1992 to the UK," says David Evans at the University of Warwick in the UK.
No mite, safe virus
To find out how the mites make the virus worse, Evans and his team took bees from an island called Colonsay, off the west coast of Scotland, which has no Varroa mites. The bees had low levels of the DWV virus, including many different strains, and showed no symptoms.
However, bees that had been in contact with the mites had 10,000 times more virus in their blood, all belonging to one highly virulent strain. That is odd, as the mites carry many strains. So Evans wondered if the mites' method of injecting the virus into the bees' blood was selectively transmitting the virulent strain.
To test this, they injected mite-free bees with many DWV strains, replicating what happens when Varroa feeds on them. Once injected, the bees ended up with the deadly strain.
Somehow the injection process benefits the virulent strain. It's not clear how, but it might be that by entering the bee's bloodstream the virus bypasses the insect's immune system.
It might be possible to create an antiviral agent to combat the virulent strain, says Ian Jones of the University of Reading in the UK. But one issue would be "whether you would completely eradicate the virus, or just knock it out for a season". Alternatively, we could make a virus that outcompetes the dangerous one.
Journal reference: PLoS Pathogens, DOI: 10.1371/journal.ppat.1004230
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