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The mysterious mass die-offs of honeybees that have wiped out roughly a third of commercial colonies each year since 2006 may be linked to a rapidly mutating virus that jumped from tobacco plants to soy plants to bees, according to a new study.

The research, reported Tuesday in the online version of the academic journal mBio, found that the increase in honeybee deaths that generally starts in autumn and peaks in winter was correlated with increasing infections by a variant of the tobacco ringspot virus.

The virus is found in pollen that bees pick up while foraging, and it may be spread as the bees mix saliva and nectar with pollen to make “bee bread” for larvae to eat. Mites that feed on the bees may also be involved in transmitting the virus, the researchers said.

Among the study’s authors are leading researchers investigating the bee deaths at the Agriculture Department’s laboratories in Beltsville, Md., as well as experts at U.S. universities and at the Chinese Academy of Agricultural Sciences in Beijing.

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Bees have died at more than twice the usual rate since it was identified seven years ago.

But most researchers, including the study’s authors, suspect that a host of viruses, parasites and, perhaps, other factors like pesticides are working in combination to weaken colonies and increase the death rate.

The infection of bees by the tobacco ringspot virus, spotted by chance during a screening of bees and pollen for rare viruses, is the first known instance in which a virus jumped from pollen to bees. About one in 20 plant viruses is found in pollen, the researchers wrote.

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