The culprit responsible for the mass collapse of commercial honeybee colonies for the past few years may be a mutating virus that transferred from plants to bees.

The tobacco ringspot virus (TRSV) was accidentally found during a routine screening of honeybees for viruses at a U.S. Department of Agriculture laboratory. TRSV is an RNA virus which means it mutates faster than other viruses and comes up with workarounds to its host's defense system. Human diseases caused by RNA virus include influenza and AIDS.

The discovery of the TRSV in bees prompted the study "Systemic Spread and Propagation of a Plant-Pathogenic Virus in European Honeybees, Apis mellifera," which has been published in the academic journal mBio. "They have a high mutation rate," study author Yan Ping (Judy) Chen, a bee pathologist at the USDA Agricultural Research Service laboratory told the Los Angeles Times. "Because of their genetic diversity, we see a lot of host jumping."

For the study, the researchers classified bee colonies as either "strong" or "weak" and observed that the TRSV and other viruses are more prevalent in weak colonies. The researchers found that mass die-offs of honeybees coincide with an increased TRSV infection. "The results of our study provide the first evidence that honeybees exposed to virus-contaminated pollen can also be infected and that the infection becomes widespread in their bodies," said author Ji Lian Li, at the Chinese Academy of Agricultural Science in Beijing.

The researchers, however, pointed out TRSV may not be the culprit behind colony collapse. "I want to be cautious," Chen said. "The cause of colony collapse disorder remains unclear. But we do have evidence that TRSV along with other viruses that we screen on a regular basis are associated with lower rates of over-winter survival."

Still, the researchers suggested that pollens be monitored as a transmission medium for host-jumping infections.

"While intracellular life cycle, species-level genetic variation, and pathogenesis of the virus in honeybee hosts remain to be determined, the increasing prevalence of TRSV in conjunction with other bee viruses from spring toward winter in infected colonies was associated with gradual decline of host populations and winter colony collapse, suggesting the negative impact of the virus on colony survival," the researchers concluded.