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.