Virus linked to bee colony collapse
8 October 2009, by Sara Coelho
A change in the behaviour of a bee virus may be responsible for honeybee colony collapses in Britain. The results are worrying but open the way to new strategies to protect bee colonies from disease.
Bee on clover.
Bee populations are declining all over the world thanks to colony collapses caused by viruses, parasites, bacteria, fungi or combinations of these. Because of the bees' crucial ecological role as pollinators and economic value as honey-producers, colony collapse has attracted the attention of many scientists, politicians and the general public alike. But a clear cause for bee decline is still elusive.
The tiny Varroa mite is a known bee parasite and is responsible for the transmission of many bee viruses. 'When a virus is spread by this mite, it's usually associated with a specific symptom, which gives the name to the virus,' explains Dr Declan Schroeder, a virologist at the Marine Biological Association in Plymouth.
So, the acute bee paralysis virus leaves bee workers paralysed, while the black queen cell virus kills young queen larvae. The deformed wing virus interferes with the development of workers who are born with malfunctioning wings.
Varroa mite.
'We set out to look at the relationship, if any, between the viruses, the mites and honeybee colony health,' says Schroeder. To do that, the team sampled 15 bee colonies from England, all infected with several viruses and Varroa mites, as well as three colonies from the Scilly Isles, so far a disease-free area.
Schroeder and his colleagues collected twenty honeybees from each colony at different times during 2007 and 2008 and looked for genetic traces of viruses, while taking note of the degree of mite infestation.
Throughout the period of one year, four England-based colonies collapsed during winter, with no obvious trigger for their decline. 'The bees from these colonies showed no clear symptoms of disease,' says Schroeder. The collapsed colonies were also not infested by an unusually large number of mites.
So what happened? To figure out, the team looked at the amount of viruses in bees during the previous year. The results, reported online last week in the Applied and Environmental Microbiology journal, show that the bees from the four collapsed colonies had higher deformed wing virus loads during winter than the others.
'This is the first observed link between the abundance of a virus during a specific time of the year and over winter colony collapse, without Varroa mites being the cause,' says Schroeder.
Schroeder believes his team found evidence for a change in the behaviour of the deformed wing virus, possibly caused by a mutation.
'We are used to observe the deformed wing virus being transmitted by mites and causing clear symptoms,' he explains. 'Now the virus is implicated in colony collapse, with no obvious signs of disease and without the mites.'
It's not clear what is happening, but Schroeder suggests that the virus is interfering with the bees' behaviour, changing flight ranges, life span or even the queen's fertility, without the hallmark deformed wings. 'The individual bees may look healthy, but the colony as a whole is not,' he says.
'Now that we are aware that this virus is able to change behaviour, we can think of new strategies to protect colonies,' adds Schroeder.
A.C. Highfield, A. El Nagar, L.C.M. Mackinder, L.M-L.J. Noël, M.J. Hall, S.J. Martin and D.C. Schroeder. Deformed wing virus implicated in over-wintering honeybee colony losses. Appl. Environ. Microbiol. doi:10.1128/AEM.02227-09
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