By Richard Black
Environment correspondent, BBC News website
Before and after... a collapsed and deserted honeybee colony
A virus has emerged as a strong suspect in the hunt for the mystery disease killing off North American honeybees.
Genetic research showed that Israeli Acute Paralysis Virus (IAPV) turned up regularly in hives affected by Colony Collapse Disorder (CCD).
Over the last three years, between 50% and 90% of commercial bee colonies in the US have been affected by CCD.
The hives are transported around the country to pollinate important crops, notably to California for almonds.
The state produces about 80% of the world's almonds in an industry worth $2.5bn per year.
"This really highlights the value of pollinators," said Jeff Pettis, research leader of the US government's Bee Research Laboratory.
"We're operating under a limited number of colonies - we had five million in the 1950s, now we have half of that number."
Dr Pettis is one of the CCD research team that reports its initial findings in the journal Science.
The honeybee decline can be traced back at least 20 years, and the introduction of the parasitic varroa mite is one of the principal causes.
But in 2004, beekeepers began seeing and reporting a new and serious phenomenon, in which entire colonies would desert their hives, leaving behind their brood and stocks of food - a syndrome that was later labelled Colony Collapse Disorder.
Theories on what is causing it have ranged from mobile phone radiation to pesticides, from genetically modified crops to climate change.
Disease remained a strong contender though, particularly in the light of the known impact of mites such as the varroa. And genetics offered the opportunity to analyse what organisms were living with and on the bees.
"The genome of the honeybee had just been completed," noted Diana Cox-Foster, an entomologist from Penn State University. "So it was possible to do the (genetic) sequencing and then eliminate the genetic material of the bees."
The scientists' trawl revealed a diverse cargo even in healthy colonies. Eight types of bacteria appeared to be present in all bees, suggesting they perform some function useful to their hosts.
The researchers also found genes from parasites, fungi, and viruses, in both healthy hives and in those which had undergone collapse. But IAPV only appeared in samples from CCD populations.
"This virus appears to be strongly associated with CCD," commented Dr Cox-Foster, "but whether it's the causative agent or just a very good marker (of the syndrome) is the next question we need to address."
And if it is a cause, it might not be the only one.
"I still believe that multiple factors are involved in CCD," said Jeff Pettis, "and what we need to do is look at combinations such as parasites, stress and nutrition (together with the virus)."
Collapse of the bee industry would be grave for US agriculture
Meanwhile, theories connected with mobile phones, climate change and GM crops can probably be discounted, the researcher suggested.
As its name would suggest, IAPV was first identified in Israel, but the symptoms it produces in bees there are quite different.
Whether this is down to a small genetic difference in the virus between continents, or whether IAPV is acting in concert with different environmental factors, is an open question.
Also open is the question of how the virus arrived in the US. One finger of suspicion points to Australia, from where the US began importing honeybees in 2004 - the very year that CCD appeared in US hives.
The researchers found IAPV in Australian bees, and they are now planning to go back through historical US samples to see if the Antipodean imports really were the first carriers.
If they were, the US might consider closing its borders to Australian bees.
If IAPV does turn out to be a major factor causing CCD, there may be little that scientists or beekeepers can do about it.
"We're unlikely to come up with a treatment for viruses in bees," said Dr Pettis, "and so beekeepers are likely just to have to keep the other things that might affect CCD, such as mites, under control."
With commercial honeybees worth an estimated $14bn to US agriculture, the political pressure on scientists to come up with some answers is considerable.