A rapidly mutating virus may be partially responsible for the massive bee die-offs known as colony collapse disorder (CCD), which has wiped out a third of commercial bee colonies annually for the past seven years, a group of U.S. and Chinese researchers reports . Most scientists, including the study’s authors, believe CCD is triggered when colonies are weakened by a combination of factors , such as viruses, parasites, and perhaps pesticides. The study, published in the journal mBio, found in bees a variant of the tobacco ringspot virus, an RNA virus that likely jumped from tobacco plants, to soy plants, to bees. Weak bee colonies began succumbing to massive die-offs in autumn, and the researchers found those bees were heavily infected with tobacco ringspot — which is believed to affect honeybees’ nervous systems — and other viruses. Strong colonies that made it through the winter showed no evidence of infection by tobacco ringspot. The researchers believe the virus jumped from plants to bees through “bee bread,” a concoction of pollen, nectar, and saliva they feed their larvae. Bee infection by tobacco ringspot is the first known instance of a virus from pollen jumping to bees.