The aptly named Varroa destructor has been wreaking havoc on bee colonies around the world for several decades, but it's only now that scientists are coming to understand just why this mite's bite is so lethal.

The ectoparasite lives on a type of bee known as the European honeybee, which is found in many parts of the world, not just Europe, and kills its host by transmitting a fatal strain of deformed wing virus, or DWV.

Although not related to the more widely publicized colony collapse disorder that has killed off large numbers of bees in the U.S., the 1.5-mm mite is responsible for the deaths of billions of wild bees around the world.

'Beekeepers have had to change their beekeeping practices, and now, they all control for the mites, and if they don't, their colonies die.'— Stephen Martin, social-insect biologist

"The mite itself has caused the death of almost all the feral colonies in the world where it's moved into," said Stephen J. Martin, a social-insect biologist and senior research fellow at the University of Sheffield in Britain.

It is also an ever-present threat for people who cultivate bees, who can control Varroa through pesticides and other means but not eradicate it.

"The mites have been spreading around the world for the last 50 years," Martin said. "Beekeepers have had to change their beekeeping practices, and now, they all control for the mites, and if they don't, their colonies die."

Martin recently teamed up with Declan C. Schroeder, a virology expert at Britain's Marine Biological Association, and University of Hawaii entomologist Ethel M. Villalobos to track the precise progress of how the mites spread the lethal form of DWV to bees in Hawaii.

The key, he and his colleagues found, is in how the mite transmits the virus — directly to the bee's bloodstream — and the cyclical way the virus propagates from bee to mite to bee to mite.

Their findings were published this month in the journal Science.

Virus usually harmless

DWV has existed naturally in European honeybees for tens of thousands of years and is also found in other small insects such as ants and bumblebees.

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A microscopic image of a Varroa destructor, a 1.5-mm mite that lives on honeybees, feeding on their blood and passing on a deadly virus in the process. (Stephen J. Martin)

Normally, lots of different strains of the virus are present at low levels within the bees and don't do any harm, but when the Varroa mite enters the picture, most of the benign strains are weeded out, and what remains is one dominant strain that is both more prevalent and more deadly.

"There's a millionfold increase in the number of viruses per bee, and the strain diversity just disappears, and it almost goes down to one strain," Martin said.

The strain is made all the deadlier by the fact that when the mite feeds on the bee's blood, it transmits the virus directly into its bloodstream, which enables the virus to bypass the bee's natural immune barriers.

When the mite is not present, the virus is transmitted from bee to bee through feeding or mating and remains in the gut, never entering the blood.

"It's a similar case if you have people who've got E.coli,  just normal bacteria in their gut. If you take that out of the gut and directly inject it into your bloodstream, you die of septicemia," Martin said.

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The Varroa parasite originated in Asian bees but at some point jumped species to European honeybees like this one Pontevedra, Spain. (Miguel Vida/Reuters)

The mites probably don't mean to infect their hosts, he said, but pass the virus particles on accidentally through regurgitation of their stomach contents or contact with virus-coated mouth parts.

The mites are effective at spreading the virus quickly throughout a bee colony and to neighbouring colonies, and if there are enough of them, they wipe out the lot.

Even when mites are removed from a colony, the highly virulent strain of DWV they've passed on remains in the bees' system, but since bee-to-bee transmission is not as effective as the mite's lethal bite, the colony survives.

"It's the transmission cycle that's the key," Martin said. "Beekeepers have been living with Varroa now for decades, and as long as they keep the mite population down, that breaks the transmission cycle, and though the virus is present, it's not enough to kill the colony."

Hawaii offered unique snapshot of disease 

Just how the mites manage to produce the more lethal strain of DWV is still unclear.

"That's the next big question to work out — the actual precise mechanism," Martin said. "It could be that there's been a change in its ability to replicate, so it can replicate much faster. It could have been a change in the tissues that it can go into.

"It's very, very complex how the virus actually has to enter the cell, then it has to hijack all the cell's machinery to create itself. …There's a lot of really complex steps, and it could have been any one of these steps that has changed."

It only takes about 2,000 mites to kill a colony of 30,000 bees, but it doesn't happen overnight. It takes about two to three years from when the mites first appear in a colony to when the colony collapses from the lethal DWV strain.

'We had this perfect timeline to actually watch the disease as it emerged. It was the only place in the world where you could do this type of work.'— Stephen Martin, social-insect biologist

In that time, the proportion of bees who have the virus increases from a mere 10 per cent to 100 per cent, the researchers found.

"On one island on Hawaii, there was estimated to be over a 100,000 feral colonies, and they'll all be killed, basically, so it has been staggeringly horrendous," Martin said.

Hawaii was the perfect laboratory in which to track the deadly spread of DWV because the mites had only arrived on the archipelago in 2007, and the deadly strain of the virus was just beginning to emerge and spread from Oahu, where it first showed up, to Big Island when Martin and his colleagues began their work in 2010.

"We had islands where Varroa wasn't present (Kauai and Maui); there was an island where Varroa had been for three years (Oahu); and there was an island that had it for one year (Big Island), so, we had this perfect timeline to actually watch the disease as it emerged," Martin said. "It was the only place in the world where you could do this type of work."

Global industry helping spread virus

The fact that the mites appeared in Hawaii at all is an example of how the global nature of modern beekeeping and trade has helped spread the virus to pretty much all temperate regions of the world where the European honeybee is present — from Russia all the way to North America.

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A beekeeper checks on a colony of bees in Hawaii. The archipelago, which had been free of the Varroa mite until 2007, offered a unique opportunity to track the progress of the virus. (Ethel M. Villalobos)

Bees are an incredibly important resource for pollination in sectors such as the almond-growing industry and for honey production and are traded globally. They are shipped all over the world — both intentionally for commercial purposes and unintentionally through the international exchange of goods.  

"The mites can only live on honeybees; they can't travel in fruit or in clothing," Martin said. "They have to travel with honeybees, and honeybees are a global industry now, so you just needed one container crate to be left on a dock, and a swarm could have gone in there, and that could be carried to Hawaii."

Only Australia has been spared from the Varroa infestation, Martin said. In South America, the mite is present but has so far been tolerated by the local bees, which are a hybrid species known as the Africanized bee, an aggressive cross between the African and the European honeybee.

The incredible speed with which the mites have spread to colonies throughout the world in just five decades is entirely down to humans, Martin says. The mite originated in the Asian honeybee and must have jumped species to the European honeybee when the two were brought into contact at some point, he said.

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A lehua blossom from an ohi'a tree in Hawaii. The blossoms produce a highly prized specialty honey and are an abundant source of food for local bees. (Courtesy of Ethel M. Villalobos)

"The mites are not naturally occurring in the [European]

bee, so it's the three things: the mites, the virus and the bee — it's something that man's created. We've put them together by accident," Martin said.

Martin hopes the research he and his colleagues have done might have implications beyond bee colonies and help shed light on how other viruses work since DWV is what's known as an RNA virus and structurally resembles some viruses that occur in humans and other animals, such as polio and foot-and-mouth disease.

At the very least, it might get those still trying to solve mysteries such as colony collapse disorder to pay more attention to the diversity of strains of the particular virus they're investigating.

"All previous studies have just looked at the presence or the absence of the virus," Martin said. "What our study shows is that, actually, you need to look at the particular strains of the virus, because different strains have different effects. Some are completely benign; some can be actually quite lethal."