Domesticated honeybees, reeling from disease, are probably infecting wild bumblebees whose pollination is vital for farmers around the world, a study said Wednesday.
Bee populations, both wild and captive, are in decline in Europe, the Americas and Asia for reasons scientists are struggling to understand.
Reporting in the journal Nature, European researchers said they had found evidence to back a theory that bumblebees are being hit by viruses or parasites from honeybees in hives.
"There is some reason for concern here," Matthias Fuerst of the Royal Holloway University of London told journalists.
In a three-phase experiment, the team first exposed bumblebees in a lab to two pathogens, deformed wing virus and Nosema ceranae parasite, to see whether they can also catch diseases already known to affect honeybees.
"We found a significant reduction in their longevity, so these pathogens are really infective" for bumblebees, said Fuerst.
Bumblebee workers normally live about 21 days, but once infected lost between a third and a quarter of their lifespan.
Next, the team set off into the countryside, collecting bumblebees and honeybees from sites across Britain and testing them for infection.
"Honeybees and bumblebees have very similar levels of those pathogens at the same site, so that means there is some connection between honeybees and bumblebees at those sites that is highly indicative of a spillover," the biologist explained.
In the final step, the team determined that honeybees and bumblebees collected from the same site had more closely-related strains of the same virus than bees from elsewhere -- a sound indicator of inter-species infection.
While they could not definitively show that the pathogens crossed from honeybees to bumblebees, rather than the other way round, the team said this would be the likeliest and most logical conclusion.
More honeybees than bumblebees were infected, and infected honeybees had higher virus levels than infected bumblebees.
"It makes sense to model the flow from them (honeybees) to bumblebees," said Fuerst.
"The prevalence of the virus is much higher in honeybees so the logic for making that prediction is clear, but we don't have absolutely definitive evidence that the flow is in that direction."
The main route of infections was probably flower visits, said the team, with bees mingling on the same flower heads, leaving behind and picking up pathogens.
They may also spread disease when they rob each other's hives for honey or nectar.
Bee farmers can treat diseases in the hive, but wild insects cannot be medicated.
"We can't go out and find nests and treat the bees," said Fuerst's colleague Mark Brown.
"Treating wild populations of mammals where there are small numbers of them and they are big animals is already a challenge."
The answer, thus, lay in preventing the spread from honeybee hives.
"We need as clean honeybee hives as possible so that the spread into the environment is mitigated," said Fuerst.
A global bee population decline has variously been blamed on agricultural pesticides, single-crop farming practices that destroy the bees' food sources, a virus, fungus or mites -- or a combination.
A Food and Agricultural Organisation (FAO) report says pollinators contribute to the yield of at least 70 percent of the major human food crops.
The economic value of pollination services was estimated at 153 billion euros ($210 billion) in 2005.
Bees, mainly bumblebees, account for some 80 percent of pollination by insects.
"This is a real concern because they (bees) provide the ecosystem services of pollination without which we would lose a large proportion of our crops and also a large proportion of our natural biodiversity," said Brown.