British scientists have worked out exactly how the Varroa mite has wiped out millions of bee colonies worldwide.
The scientists studied honeybees in Hawaii and found that the Varroa mite helped spread a particularly nasty strain of a disease called deformed wing virus, the BBC reports.
The mites act as tiny incubators of one deadly form of the disease, and inject it directly into the bees' blood. The findings are reported in the journal Science.
The team, led by Stephen Martin from the University of Sheffield, studied the honeybees in Hawaii, where Varroa was accidentally brought from California five years ago.
Some Hawaiian islands have honeybee colonies that are still Varroa-free and this provided the team with the ability to compare recently-infected colonies with those free from the parasite.
The team spent two years monitoring colonies - screening Varroa-infected and uninfected bees to see what viruses lived in their bodies.
Dr Martin told the BBC that most viruses were not normally harmful to the bees, but the mite selected one lethal strain of one specific virus.
"In an infected bee there can be more viral particles than there are people on the planet," Dr Martin said.
"There's a vast diversity of viral strains within a bee, and most of them are adapted to exist in their own little bit of the insect; they get on quite happily."
But the mite, he explained, "shifts something".
In Varroa-infected bees, over time, the vast majority of these innocuous virus strains disappear and the bees' bodies are filled with one lethal strain of deformed wing virus.
Although it is not clear exactly why this strain thrives in mite-infected bees, Dr Martin said that it could be the one virus best able to survive being repeatedly transmitted from the mites to the bees and back, as the mites feed on the bees' blood.
The effect appears to take once the mites have changed this "viral landscape" in the bees' bodies, the change is permanent.
"So the only way to control the virus is to control the levels of the mite," Dr Martin said.