The mite compounded a problem that some European honeybee colonies had learned to live with for centuries, a virus that deformed their wings, now known as DMV. Before the introduction of the mite, the virus existed on the body surface of the European honeybees. But the biting mite picked up the virus and injected it into the bees' bloodstream, making the problem far worse, said Lena Wilfert, a lecturer at the University of Exeter and the lead author of the study.
When European honeybees were introduced to the Americas and other parts of Asia in subsequent years, a localized endemic in Europe evolved into a global pandemic that led to bee colony collapse disorder and is threatening agriculture that relies on pollinating honeybees to grow food crops.
Making matters worse, honeybees are spreading the virus through their saliva and feces to plants used by other pollinators, such as bumblebees and other solitary bees. "DWV has been detected in various insect groups that play dramatically different ecological roles, including insect predators and scavengers, pollinators, and pest species that live inside the colony," according to a Science article that announced the study.
"We really see this as a multi-host problem," Wilfert said. "It’s really up to the beekeepers. When they keep their bees healthy, they also keep the wild pollinators healthy. The virus can be transmitted by a plant indirectly." Mites that leap from bee to bee quickly die when they slip or fall, but "the virus can contaminate flowers through pollen," Wilfert said. "Bees don’t have toilet, so there's fecal transmission and oral transmission."
To better understand how the mites spread the virus, the researchers used molecular sequencing of the virus and mites from 32 locations in 17 countries. By studying how the host behaved in different geographic regions, they determined the major routes of the virus’s spread .
The mite is a natural pest called Varroa that's native to Southeast Asia. The virus is a strain that emerged in Europe. Both existed separately as manageable local problems likely for eons. But human trade transformed them into invasive species that are wreaking havoc from the United States to Chile to New Zealand. Massive honeybee die-offs were first detected in the United States in 2006.
"People didn’t on purpose do this," said Wilfert, who authored her paper with seven colleagues. "People don’t go to the trouble of sending bee queens to the States for stupid reasons. They do it to get better hives or honey, to get more pollination. Until recently we didn’t understand how common it is to spread diseases that way."
But humans wade into complicated ecosystems and disrupt them with little knowledge of how they work. Wilfert said the spread of deformed wing virus is "a man-made thing" mostly done "without evil intent." However, she said, "somewhere we have messed up the ecology. We need to be careful with this stuff. The more complex the systems are, the more unpredictable."
The spread of the deformed wing virus fits with a familiar global trade narrative involving animals and disease. There are invasive pythons from Burma destroying native species in the Everglades, invasive Asian stinkbugs laying waste to crops in the Mid-Atlantic, invasive zebra mussels and dozens of species of fish such as the Northern snakehead and Lionfish muscling out native fish in the Atlantic Ocean, the Great Lakes and the Chesapeake Bay.
Wilfert said her team has moved on to another study of islands around Great Britain and France to compare those where the Varroa mite exists and where they do not to determine how they affect different habitats.
The next step for the honeybee trade is clear, she said. Trade should be thoroughly policed at national ports to keep the problem from worsening.
"If there are any mites around, there’s no question they shouldn’t be traded at all," Wilfert said. "In general only trade queen bees. They need to all be screened for known diseases. Something that’s perfectly fine in Europe might not be perfectly fine in Chile."