Bee Collapse: The Varroa Mite Is More Destructive Than Scientists Ever Knew

"Now we understand why the pesticides the bees have been exposed to for decades are killing them."

The drastic decline in global honeybee populations is no secret. The phenomenon has been named “colony collapse syndrome,” and though it’s not clear what factors led up to it, entomologist Samuel Ramsey, Ph.D., explains that the culprits have been narrowed down to a triad of contributing factors: pesticides, poor nutrition, and parasites.

Out of these factors, he says, parasites hurt honeybee populations the most. And out of all the parasites, Ramsey shows in a new PNAS paper, the ominously named Varroa destructor is the very worst.

The varroa mite, a tiny parasitic arachnid that hitches a ride on honeybees and feeds on their innards, has menaced beekeepers for a long time. But for decades, they assumed it just sipped on bee blood (hemolymph) and spread diseases. The paper written by Ramsey and his colleagues reveals the varroa mite is far more dangerous. Rather than being one part of a dangerous triad of bee threats, parasites like the varroa mite might be at the top of a hierarchy.

“I was very excited, specifically because this is something they’ve believed about these arachnids for more than half a century now, and it’s gone unquestioned for years and years and years,” Ramsey, who worked on this research as a Ph.D. candidate at the University of Maryland, College Park and is the study’s first author, tells Inverse. He is now an entomologist at the Bee Research Laboratory with the USDA’s Agricultural Research Service.

The implications of the new paper for curbing colony collapse syndrome are profound. Not only does it explain why varroa mites are so deadly, but it also explains why pesticides and poor nutrition have seemed to play such a big role in bee population decline. But perhaps most importantly, it resurfaces a long-ignored scientist’s theory about the mites that might have helped us save the bees sooner.

This low-temperature scanning electron microscope image shows a Varroa destructor mite attached to a honeybee.

PNAS/ Ramsey et al

In the paper, Ramsey and his colleagues show that the varroa mite doesn’t just suck blood but feeds on a vital organ in the honeybee called the fat body, which stores nutrients and filters toxins — sort of like a human’s liver. “It’s less like having a mosquito land on you and drain out your blood, and more like having a mosquito land on you, liquefy your liver, suck that out, and fly away,” Ramsey says.

This brutal observation underlies the effects of the triad of bee killers. “Now we understand why the pesticides the bees have been exposed to for decades are killing them,” Ramsey says. The same thing goes for nutrition, since not only is the fat body a toxin-filtering organ, it’s also a nutrient storage organ: “They’re not able to store nutrients when the tissue is constantly being decimated by these mites,” he adds.

This revelation is a major one for beekeepers and entomologists, who have been laboring for decades to figure out how to deal with varroa mites.

“I genuinely hope this research will be used to create new methods for reducing mite levels,” says Ramsey. A major obstacle to developing effective defenses against the mites has been this fundamental misunderstanding of how they feed.

But the problem might have been solved much sooner had entomologists paid attention to the work of North Carolina State University entomology researcher Allen Cohen, Ph.D., whose work has gone mostly unnoticed, until now.

The varroa mite has been doing more damage to bees than scientists once believed.

Unsplash / Massimiliano Latella

“Someone had published a paper somewhere along the line that varroa mites feed on the hemolymph of bees, and it just stuck there without verification,” Cohen tells Inverse.

As Ramsey and his team outline in PNAS, the assumption that varroa mites feed on bees’ blood comes from three papers from the late 1970s, all written in the Soviet Union. American scientists had worked with mediocre English translations of these papers for years, and the scientific community had cited them over and over, but none of these studies actually offered solid evidence that the mites were drinking bee blood.

Ramsey refers to this issue as “chain citation.” Scientists cite a source, then someone else cites the second source that cited the first source, and so on until eventually the origin of a piece of information (or misinformation) is irrelevant because it’s just all over the scientific literature.

“Since people were often citing a citation of a citation, people never looked at the details of the study,” he says.

Cohen, however, had questioned this assumption from the start. In a litany of research papers, he and his longtime collaborator Eric Erickson, Ph.D., laid out the evidence that varroa mites and other parasites must be eating something more nutritious than insect blood — a notoriously nutrient-poor substance — most likely by injecting digestive juices to dissolve tissue then sucking it back up. In the video below, recorded in 2006 or 2007, he explains this idea. “We’ve long known spiders feed that way,” he says. “For mites, that just wasn’t known.”

Cohen — and the bees — have been unfortunate victims of chain citation, but Ramsey’s paper redeems his nearly 40 years of research on parasitic and predatory insects and arachnids. When Ramsey and his then-Ph.D. adviser Dennis VanEngelsdorp, Ph.D., realized their work lined up with what Cohen had been saying all along, they contacted him to discuss their new work. Cohen was thrilled.

“He was really happy I could take it up and move some of this dogma out of the way,” recalls Ramsey. Cohen, who was pleased to finally have his idea publicly confirmed, eventually ended up as a co-author on the paper. He expressed his admiration and gratitude that they’d brought him in on the project.

“Samuel Ramsey is a class act. A lot of people will get ideas started from somewhere else, then cover it up, and make it look like they invented the wheel,” Cohen says. “He has been very honest and gracious about sharing responsibility, so I really do appreciate that.”

Scanning electron microscope images of the wounds left by the Varroa destructor mites show clear evidence that they're feeding on bees' fat bodies, not just on blood.

PNAS/ Ramsey et al

With this hypothesis confirmed once and for all, in a major scientific journal, Ramsey says he hopes scientists can use the information to come up with better systemic insecticides — a chemical that won’t harm the bee but will kill a mite who bites the bee, sort of like a dog’s flea pill.

Ramsey’s next steps will be to investigate another genus of bee parasites called the Tropilaelaps mite, which has recently expanded beyond its original range of Korea and China and been found in the Middle East. He and his colleagues hope that by studying this mite, US beekeepers and entomologists won’t be caught off-guard like they were with Varroa destructor.

“I’m trying to make sure that the fundamentals of this organism’s biology are accessible for us as researchers before it gets to the US,” he says. “Part of my work now is to make sure we don’t end up in the same situation that we did with varroa.”

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