The parasitic fly Apocephalus borealis has been found to invade, disorientate and finally kill honeybees, according to a paper in the journal PLoS ONE. As well as being lethal to bees directly, the flies harbour deformed wing virus and other pathogens, the study found.
"We don't fully understand the web of interactions," said Professor John Hafernik of San Francisco State University, one of the research team.
"The parasite could be another stressor, enough to push the bee over the tipping point. Or it could play a primary role in causing the disease," he added.
Debate continues as to the cause or causes of CCD, including mites, fungi, viruses, crop chemicals, loss of food sources and even mobile phone masts. While the existence of CCD in the UK is disputed, the British Beekeepers' Association has reported four years of "unacceptably high" colony losses.
Agriculture's role in the decline of bee and other insect populations - and in reversing it - will be the subject of a Society of Chemical Industry conference at Rothamsted Research in Harpenden on 25 April.