The mystery of colony collapse disorder in honeybees has a remarkable new suspect – a plant virus that has made a spectacular jump across 1.6 billion years of evolution to infect insects.
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The potential hive killer is tobacco ringspot virus, named for the discoloured circles it forms on infected leaves. It has at least 90 different plant hosts and is so difficult to get rid of that some farmers have stopped raising susceptible crops.
It often travels on pollen from one host to another by thumbing a ride in insects, including varroa mites, aphids and bees. But such viral hitchhikers usually stay in the gut or salivary glands, ready to make a quick jump to the next plant host.
So when a team of scientists from the US Department of Agriculture and China’s Academy of Agricultural Science spotted it in honey bees they were not expecting to find that it had spread throughout the animals’ bodies, and was doing particularly well in wings, antennae, trachea, hemolymph (insect blood) and nerves.
Such jumps are not unheard of. Rhabodoviridae, the family of viruses that includes rabies, has members that have both plant and animal hosts. Shorter hops are routinely made between species by influenza and HIV famously transferred to humans from apes.
Common to all these viruses is the use of RNA rather than DNA to encode their genetic templates. RNA is the messenger molecule that tells cells how to build proteins. It is not as rigorously policed as DNA, and so is far more likely to have copying errors. As a result, viruses that rely on RNA mutate more often.
Tobacco ringworm virus was also found in varroa mites, which parasitize honeybees, and may play a role in spreading the disease.
The prime suspect in colony collapse disorder remains neonicotinoid pesticides, which were banned in the European Union in November 2013.
However, the case against them is far from proven, and researchers continue to hunt for other candidates, including viruses.
The US-China team screened six strong and four weak colonies over a year for tobacco ringspot and other viruses, deformed wing bee virus, black queen cell virus and Israel acute paralysis virus and found that higher concentrations presaged colony collapse, although no apparent disease symptoms were spotted in individual bees. The four weak colonies studied had collapsed by February.
What remains unclear is whether the viruses are causing the decline, contributing to it or just taking advantage of it.
Honeybees pollinate 90 commercial crops worldwide and their services in the US alone are valued at $14.6bn a year.