Humans are responsible for spreading a deadly bee virus around the world through our movement of honeybee hives. The disease, called the Deformed Wing Virus, represents a threat not only to our hives, but to other pollinator species as well.
Bee populations across most of the world are in serious trouble, and while the causes are debated, it is clear they are being hit from multiple directions. Most public attention has focused on the contribution of neonicotinoid insecticides, but another important factor is the one-two punch of the Varroa mite and the Deformed Wing Virus (DWV).
Healthy bee populations can usually fight off either the mites or the virus, but when the two combine they are lethal. This now happens often, as the mite assists in the virus' transmission.
Consequently, learning about the virus' spread is an urgent priority. In Science, the University of Exeter's Dr. Lena Wilfert tracked the mite and virus's dissemination. The fact that the virus has multiple varieties allowed Wilfert to establish the source of new infestations.
“There are two main scenarios for DWV's origin,” the paper reports. “The first scenario is that Varroa introduced DWV to the European honeybee... The second scenario is that DWV is a reemerging disease whose current pandemic is promoted by Varroa.”
Using DWV sequences from 32 locations in 17 countries, Wilfert found that the most recent common ancestors of the DWV varieties existed in the mid-20th century, but she was unable to settle the viruses' ancestral host.
In the process, however, Wilfert gained important insight into how the virus is spreading. Every new infestation, even those in the Southern Hemisphere, involved a variety common in Europe, rather than something with a geographically closer foothold.
The Deformed Wing Virus as seen under a microscope. Professor Stephen Martin/University of Salford.
"This is the first study to conclude that Europe is the backbone of the global spread of the bee killing combination of Deformed Wing Virus and Varroa,” Wilfert said in a statement. “This demonstrates that the spread of this combination is largely manmade – if the spread was naturally occurring, we would expect to see transmission between countries that are close to each other, but we found that, for example, the New Zealand virus population originated in Europe.”
“This significantly strengthens the theory that human transportation of bees is responsible for the spread of this devastating disease,” she continued. “We must now maintain strict limits on the movement of bees, whether they are known to carry Varroa or not. It's also really important that beekeepers at all levels take steps to control Varroa in their hives, as this viral disease can also affect wild pollinators."
Co-author Professor Roger Butlin of the University of Sheffield said, "Domesticated honeybee colonies are hugely important for our agriculture systems, but this study shows the risks of moving animals and plants around the world. The consequences can be devastating, both for domestic animals and for wildlife.”
“To reduce the negative effects of DWV on beekeeping and wild pollinators," the authors argue, "tighter controls, such as mandatory health screenings and regulated movement of honeybees across borders, should be imposed, with every effort made to maintain the current Varroa-free refugia for the conservation of wild and managed pollinators.”