Tobacco ringspot virus leaps from plants to threaten honeybees: study

A rapidly mutating virus has leaped from plants to honeybees, where it is reproducing and contributing to the collapse of colonies vital to the multibillion-US-dollar agricultural industry, according to a new study.

Tobacco ringspot virus (TRSV), a pollen-borne pathogen that causes blight in soy crops, was found during routine screening of commercial honeybees at a US Department of Agriculture laboratory, where further study revealed the RNA virus was replicating inside its Apis mellifera hosts and spreading to mites that travelled from bee to bee, according to the study published online on Tuesday in the journal mBio.

The discovery is the first report of honeybees becoming infected by a pollen-born RNA virus that spread systematically through the bees and hives. Traces of the virus were detected in every part of the bee examined, except its eyes, according to the study.

Commercially cultivated bees pollinate about 90 crops worldwide, a service valued at US$14 billion annually. But those colonies have been collapsing, and scientists have attributed that devastation to a deadly cocktail of pathogens, as well as pesticides and beekeeping practices that stress the insect's immune system.

In California, the US$3 billion almond industry spends US$239 million annually to rent more than one million beehives, and that cost is escalating.

Only about 5 per cent of plant viruses are known to be transmitted by pollen, and fewer still have been known to jump from the plant kingdom to insects. That added a complex layer to the forces driving colony collapse disorder, scientists warned.

The tobacco ringspot virus acts as a "quasi-species", replicating in a way that creates ample mutations that subvert the host's immune response. That phenomenon was believed to be the driving factor of recurring viral infections of avian and swine influenza and of the persistence of HIV, the study noted.

"They have a high mutation rate," said Yan Ping Chen, a bee pathologist at the USDA Agricultural Research Service laboratory in Maryland and lead author of the study. "Because of their genetic diversity, we see a lot of host jumping."

The relative role of the virus in the demise of colonies had not been measured, as it would be difficult to separate it from a cocktail of pathogens and stresses affecting bees, Chen said. "I want to be cautious," he 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."

Indeed, the new virus, along with the Israeli acute paralysis virus, was correlated with colonies deemed "weak" due to a variety of stresses. It also showed a similar seasonal fluctuation, infection rates rose to a 22.5 per cent high in winter.

Varroa mites, a "vampire" parasite, also were found to carry the virus but were not infected, leading researchers to conclude that they aided the spread of the virus within the colony. Whether the mites are more than a mechanical spreader of the virus, however, remains to be studied.