Humans and honeybees go way back, but like any long-term relationship, this dynamic can be sour as well as sweet. Sometimes our bees sting us, for example. And sometimes we accidentally trigger a vast, international collapse of bee colonies so complex it takes us years to figure out what we're doing wrong.

That's the case with colony collapse disorder (CCD), a strange plague that's been obliterating honeybee colonies for at least a decade. Although we rely on bees to pollinate our food crops, a new study suggests we've royally screwed up this deal in recent decades — and it'll take more than a bouquet of flowers to patch things up.

It's well-known that many insecticides, including popular neonicotinoids like imidacloprid, leave a toxic residue in flowers of certain plants. But while that can hurt domesticated honeybees and wild pollinators, it's only part of the CCD puzzle.

At least two other scourges share the blame: Varroa mites and deformed wing virus (DWV). Neither is considered a major threat to bee populations on its own, but when Varroa mites carry DWV, the combo can be catastrophic. (Not only do the mites eat bee larvae; they also inject the virus into bodies of adult bees.) And according to the new study, this deadly duo didn't traverse the planet on its own. Humans helped, it concludes, by recklessly shipping honeybee colonies and queens across oceans.

"Domesticated honeybee colonies are hugely important for our agriculture systems, but this study shows the risks of moving animals and plants around the world," says co-author and University of Sheffield biologist Roger Butlin in a statement. "The consequences can be devastating, both for domestic animals and for wildlife. The risk of introducing viruses or other pathogens is just one of many potential dangers."

Varroa mite on honeybeeA Varroa mite on a honeybee host, captured by scanning electron microscope. (Photo: Wikimedia Commons)

honeybee with deformed wing virusA Carniolan honeybee shows signs of deformed wing virus, or DWV. (Photo: Wikimedia Commons)

To trace the pandemic's path, the researchers collected 246 DWV samples from both honeybees and Varroa mites at 32 locations around the world, supplemented with pre-existing studies of the virus. Their phylogeographic analysis showed "a recent global radiation and pandemic of DWV," with the most recent common ancestor dating back to the mid-20th century — roughly the same time when Varroa mites expanded from their native Asia to become a widespread honeybee parasite.

The simplest explanation, the authors write, is that an endemic honeybee virus has re-emerged "through ecological change and the spread of Varroa as a vector, alongside increased global movement of infected bees or other contaminated material, such as pollen." They found that DWV spread largely from Europe to North America, Australia and New Zealand, with some two-way movement between Europe and Asia, but none between Asia and Australasia despite their closer proximity.

"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," says Lena Wilfert, an evolutionary geneticist at the University of Exeter and lead author of the study.

"This demonstrates that the spread of this combination is largely man-made," she adds, because "if the spread was naturally occurring, we would expect to see transmission between countries that are close to each other."

beekeeper in FranceBeekeepers, like this one in France, carry on a tradition that dates back millennia. But in recent decades, global shipping of bees has reportedly unleashed a disastrous honeybee pandemic. (Photo: Georges Gobet/AFP/Getty Images)

The key transmitters are European honeybees, the researchers conclude, enabled by unwitting human handlers. The outbreak is now wreaking havoc with managed beehives, some of which may already be weakened from pesticide exposure. But on top of domesticated honeybees, the spread of DWV and Varroa mites also threatens many native bees whose pollination services are vital to their ecosystems.

"This significantly strengthens the theory that human transportation of bees is responsible for the spread of this devastating disease," Wilfert says. "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."

It may be discouraging to know humans made this mess, but as co-author Mike Boots points out, at least that means we might be able to do something about it. "The key insight of our work is that the global virus pandemic in honeybees is man-made, not natural," says Boots, a biologist at the University of California-Berkeley. "It's therefore within our hands to mitigate this and future disease problems."

Russell McLendon ( @russmclendon ) writes about humans and other wildlife.