Parasitic mite spreads lethal virus to honeybees

27 June 2014 by Tamera Jones

A parasitic mite has helped spread a particularly nasty strain of a virus to countless honeybees, helping to wipe out hundreds of colonies, according to the latest study.

Honey beeA UK team of researchers found that when the Varroa mite gets into hives, it massively amplifies a disease-causing strain of deformed wing virus (DWV), rendering bees unable to forage properly and leading to high colony losses.

The findings explain why the bloodsucking parasite, which has long been considered one of the biggest threats facing honeybees, is such bad news for bees and beekeepers.

DWV is common in UK honeybees. Lots of different strains of the virus exist, but only at low levels which means it's usually harmless. Mites feed on young, healthy developing bees' blood, taking in this mixture of virus strains. When they move to another developing bee they inject the mixture and the virulent strain of DWV out-competes all of the others and replicates to very high levels.

The findings, reported in PLoS Pathogens, explain why colonies infested with Varroa suffer most severely, and could lead to new ways to help protect the pollinators.

We need to confirm that this virulent strain of DWV is distributed globally. If it is, our results mean that future antiviral therapies only need be developed for one strain of DWV.

- Professor David Evans of the University of Warwick

"We find deformed wing virus in almost all colonies - it's one of the most common viruses that affects honeybees," says Professor David Evans of the University of Warwick, who led the study.

"But we only find high levels of this virulent strain of the virus when the Varroa mite has fed on the developing bee. In this case it accounts for more than 99.9% of the DWV present."

Honeybees are an important pollinator, contributing an estimated $40bn globally to crop value each year. But over recent years, parasites and the viruses they transmit have spread widely, resulting in many colonies dying out over the winter.

This, on top of other threats to insect pollinators, is putting huge pressure on farmers who are already struggling to meet the demands of a growing global population.

Until now, it wasn't clear why Varroa-infested colonies showed much higher levels of DWV disease than uninfected ones.

"We had two ideas - either the mite amplifies and transmits virulent strains of DWV, or the route of transmission - injection - favours this virulent strain of the virus," says Evans.

Varroa mite

Varroa mite

To find out if the differences in Varroa-infested and non-infested colonies were down to Varroa amplifying the lethal strains of virus or because only this virus grows better when injected, his team used a syringe to inject honeybee pupae in the lab with a mix of DWV strains to see how the pupae would fare.

They found that the same virulent strain of DWV was amplified up to 10,000 times in the pupae that had been injected and that it was identical to the DWV strain transmitted by Varroa.

"Our results show that the most virulent strain of DWV is actually advantaged by this direct transmission route," says Evans, who is also an enthusiastic beekeeper.

Varroa is thought to have spread from the Asiatic honeybee to the European honeybee during the first half of the 20th century and has since spread to all beekeeping regions of the world, apart from Australia.

"We need to confirm that this virulent strain of DWV is distributed globally. If it is, our results mean that future antiviral therapies only need be developed for one strain of DWV," adds Evans.

The project is part of the Insect Pollinators Initiative, jointly funded by the Biotechnology & Biological Sciences Research Council (BBSRC), Defra, NERC, the Scottish Government and the Wellcome Trust under the auspices of the Living with Environmental Change (LWEC) partnership.

Eugene V Ryabov, Graham R Wood, Jessica M Fannon, Jonathan D Moore, James C Bull, Dave Chandler, Andrew Mead, Nigel Burroughs and David J Evans, 'A Virulent Strain of Deformed Wing Virus (DWV) of Honeybees (Apis mellifera) Prevails after Varroa destructor-Mediated, or In Vitro, Transmission', PLOS Pathogens, published 26 June 2014.