Varroa Mites Feed On The Fat Bodies Of Honey Bees, Not The Hemolymph. This Is Important!

Catch The Buzz By Dennis O’Brien January 30, 2019

cross section of honey bee abdomen.jpg

An image showing a cross section of a varroa mite feeding on a honey bee’s abdominal cavity is one of several ARS microscopy images changing what we know about how mites damage honey bees.

Research by scientists at the Agricultural Research Service (ARS) and the University of Maryland released today sheds new light — and reverses decades of scientific dogma — regarding a honey bee pest (Varroa destructor) that is considered the greatest single driver of the global honey bee colony losses. Managed honey bee colonies add at least $15 billion to the value of U.S. agriculture each year through increased yields and superior quality harvests.

The microscopy images are part of a major study showing that the Varroa mite (Varroa destructor) feeds on the honey bee’s fat body tissue (an organ similar to the human liver) rather than on its “blood,” (or hemolymph). This discovery holds broad implications for controlling the pest in honey bee colonies.

The study was published on-line Jan. 15 and in today’s print edition of the Proceedings of the National Academy of Sciences. An image produced by the ARS Electron and Confocal Microscopy Unit in Beltsville, Maryland is on the cover of today’s journal.

Varroa mites have been widely thought to feed on the hemolymph, of honey bees (Apis mellifera) because of studies conducted in the 1970’s which used outdated technology. But today’s collaborative study, by University of Maryland and ARS researchers at the ARS Electron and Confocal Microscopy Unit, offers proof of the mite’s true feeding behavior. Through the use of electron microscopy, the researchers were able to locate feeding wounds on the bee caused by the mites, which were located directly above the bee’s fat body tissue. The images represent the first direct evidence that Varroa mites feed on adult bees, not just the larvae and pupae.

In addition, University of Maryland researchers conducted feeding studies and found that Varroa mites that were fed a diet of fat body tissue survived significantly longer and produced more eggs than mites fed hemolymph. The results show, mites fed a hemolymph-only diet were comparable to those that were starved. Thus, proving conclusively that the Varroa mite feeds primarily on the fat body consumed from bees.

The results are expected to help scientists develop more effective pesticides and other treatments to help bees cope with a mite known to spread at least five viruses. They also help explain why Varroa mites have such detrimental effects on honey bees, weakening their immune systems, and making it harder for them to store protein from pollen and survive through the winter.

The study was part of the Ph.D. thesis of Samuel D. Ramsey from the University of Maryland and was conducted in collaboration with ARS researchers and study co-authors Gary Bauchan, Connor Gulbronson, Joseph Mowery, and Ronald Ochoa.

The study can be found here.

The Agricultural Research Service is the U.S. Department of Agriculture’s chief scientific in-house research agency. Daily, ARS focuses on solutions to agricultural problems affecting America. Each dollar invested in agricultural research results in $20 of economic impact.

Catch The Buzz: Varroa mites feed on the fat bodies of honey bees

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Honey Bee Parasites Feed on Fatty Organs, Not Blood University of Maryland January 14, 2019

In this electron micrograph, a parasitic mite,  Varroa destructor , is wedged between the abdominal plates of a honey bee's exoskeleton. Credit: UMD/USDA/PNAS

In this electron micrograph, a parasitic mite, Varroa destructor, is wedged between the abdominal plates of a honey bee's exoskeleton. Credit: UMD/USDA/PNAS

Honey bee colonies around the world are at risk from a variety of threats, including pesticides, diseases, poor nutrition and habitat loss. Recent research suggests that one threat stands well above the others: a parasitic mite, Varroa destructor, which specializes in attacking honey bees.

For decades, researchers have assumed that varroa mites feed on blood, like many of their mite and tick cousins. But new University of Maryland-led research suggests that varroa mites instead have a voracious appetite for a honey bee organ called the fat body, which serves many of the same vital functions carried out by the human liver, while also storing food and contributing to bees' immune systems.

The research, published in the Proceedings of the National Academy of Sciences on January 14, 2019, could transform researchers' understanding of the primary threats to honey bees while pointing the way toward more effective mite treatments in the future.

"Bee researchers often refer to three Ps: parasites, pesticides and poor nutrition. Many studies have shown that varroa is the biggest issue. But when compromised by varroa, colonies are also more susceptible to the other two," said UMD alumnus Samuel Ramsey (Ph.D. '18, entomology), the lead author of the paper. "Now that we know that the fat body is varroa's target, this connection is now much more obvious. Losing fat body tissue impairs a bee's ability to detoxify pesticides and robs them of vital food stores. The fat body is absolutely essential to honey bee survival."

In addition to breaking down toxins and storing nutrients, honey bee fat bodies produce antioxidants and help to manage the immune system. The fatty organs also play an important role in the process of metamorphosis, regulating the timing and activity of key hormones. Fat bodies also produce the wax that covers parts of bees' exoskeletons, keeping water in and diseases out.

According to Ramsey, the assumption that varroa mites consume honey bee blood (more accurately called hemolymph in insects) has persisted since the first paper on the topic was published in the 1960s. Because this paper was written in Russian, Ramsey said, many researchers opted to cite the first English-language papers that cited the original study.

In this cross-section of a honey bee's abdomen, a parasitic varroa mite (orange) can be seen lodged between the bee's abdominal plates, where the mite feeds on honey bee fat body tissue. Credit: UMD/USDA/PNAS

In this cross-section of a honey bee's abdomen, a parasitic varroa mite (orange) can be seen lodged between the bee's abdominal plates, where the mite feeds on honey bee fat body tissue. Credit: UMD/USDA/PNAS

"The initial work was only sufficient to show the total volume of a meal consumed by a mite," Ramsey added. "It can be a lot easier to cite a recent summary instead of the original work. Had the first paper been read more widely, many folks might have questioned these assumptions sooner."

Ramsey noted several observations that led him to question whether varroa mites were feeding on something other than hemolymph. First, insect hemolymph is very low in nutrients. To grow and reproduce at the rates they do, varroa mites would need to consume far more hemolymph than they would be able to acquire from a single bee.

Second, varroa mites' excrement is very dry—contrary to what one would expect from an entirely liquid blood diet. Lastly, varroa mites' mouthparts appear to be adapted for digesting soft tissues with enzymes then consuming the resulting mush. By contrast, blood-feeding mites have very different mouthparts, specifically adapted for piercing membranes and sucking fluid.

The first and most straightforward experiment Ramsey and his collaborators performed was to observe where on the bees' bodies the varroa mites tended to attach themselves for feeding. If the mites grabbed on to random locations, Ramsey reasoned, that would suggest that they were in fact feeding on hemolymph, which is distributed evenly throughout the body. On the other hand, if they had a preferred site on the body, that could provide an important clue to their preferred meal.

"When they feed on immature bees, mites will eat anywhere. But in adult bees, we found a very strong preference for the underside of the bees' abdomen," Ramsey said. "More than 90 percent of mites we found on adults fed there. As it happens, fat body tissue is spread throughout the bodies of immature bees. As the bees mature, the tissue migrates to the underside of the abdomen. The connection was hard to ignore, but we needed more evidence."

Ramsey and his team then directly imaged the wound sites where varroa mites gnawed on the bees' abdomens. Using a technique called freeze fracturing, the researchers used liquid nitrogen to freeze the mites and their bee hosts, essentially taking a physical "snapshot" of the mites' feeding habits in action. Using powerful scanning electron microscopes to visualize the wound sites, Ramsey saw clear evidence that the mites were feeding on fat body tissue.

This microscopic image shows a varroa mite that has consumed honey bee fat body tissue tagged with Nile red, a fat-soluble fluorescent dye. Observing this red fluorescence in the mites' digestive systems helped researchers determine that varroa mites feed on honey bee fat body tissue--not blood, as previously assumed. Credit: UMD/USDA/ PNAS

This microscopic image shows a varroa mite that has consumed honey bee fat body tissue tagged with Nile red, a fat-soluble fluorescent dye. Observing this red fluorescence in the mites' digestive systems helped researchers determine that varroa mites feed on honey bee fat body tissue--not blood, as previously assumed. Credit: UMD/USDA/PNAS

"The images gave us an excellent view into the wound sites and what the mites' mouthparts were doing," Ramsey said. "We could see digested pieces of fat body cells. The mites were turning the bees into 'cream of honey bee soup.' An organism the size of a bee's face is climbing on and eating an organ. It's scary stuff. But we couldn't yet verify that blood wasn't also being consumed."

To further shore up their case, Ramsey and his colleagues fed bees with one of two fluorescent dyes: uranine, a water-soluble dye that glows yellow, and Nile red, a fat-soluble dye that glows red. If the mites were consuming hemolymph, Ramsey expected to see a bright yellow glow in the mites' bellies after feeding. If they were feeding on fat bodies, on the other hand, Ramsey predicted a telltale red glow.

"When we saw the first mite's gut, it was glowing bright red like the sun. This was proof positive that the fat body was being consumed," Ramsey said. "We've been talking about these mites like they're vampires, but they're not. They're more like werewolves. We've been trying to drive a stake through them, but turns out we needed a silver bullet."

To drive the proverbial final nail into the coffin of the idea that mites feed on hemolymph, Ramsey performed one last experiment. First, he painstakingly perfected the ability to raise varroa mites on an artificial dietary regimen—hardly an easy task for a parasite that prefers meals from a live host. Then, he fed them diets composed of hemolymph or fat body tissue, with a few mixtures of the two for good measure.

The results were striking: mites fed a diet of pure hemolymph starved, while those fed fat body tissue thrived and even produced eggs.

"These results have the potential to revolutionize our understanding of the damage done to bees by mites," said Dennis vanEngelsdorp, a professor of entomology at UMD and a co-author of the study, who also served as Ramsey's advisor. "Fat bodies serve so many crucial functions for bees. It makes so much more sense now to see how the harm to individual bees plays out in the ways that we already know varroa does damage to honey bee colonies. Importantly, it also opens up so many new opportunities for more effective treatments and targeted approaches to control mites."

Read more at:

More information: Samuel D. Ramsey el al., "Varroa destructor feeds primarily on honey bee fat body tissue and not hemolymph," PNAS (2018). 

Journal reference: Proceedings of the National Academy of Sciences 

Provided by: University of Maryland

Saving the Mighty Honey Bee

University of Maryland    November 15, 2015

Massive honey bee die-offs threaten our food production, but entomology Assistant Professor Dennis vanEngelsdorp is leading a group of UMD and national researchers to find a way to save these important pollinators.

Report Says Fewer Bees Perished Over the Winter, but the Reason is a Mystery

The New York Times   By John Schwartz   May 15, 2014

Honeybees could be on their way back, according to a new federal report.

The collapse of bee populations around the country in recent years has led to warnings of a crisis in foods grown with the help of pollination. Over the past eight years, beekeepers have reported winter losses of nearly 30 percent of their bees on average.

The new survey, published on Thursday, found that the loss of managed honeybee colonies from all causes dropped to 23.2 percent nationwide over the winter that just ended, down from 30.5 percent the year before. Losses reported by some individual beekeepers were even higher. Colony losses reached a peak of 36 percent in 2007 to 2008.

The survey of thousands of beekeepers was conducted by the Department of Agriculture and the Bee Informed Partnership, an organization that studies apian health and management.

“It’s better than some of the years we’ve suffered,” said Dennis vanEngelsdorp, a director of the partnership and an entomologist at the University of Maryland. Still, he noted, a 23 percent loss “is not a good number.” He continued, “We’ve gone from horrible to bad.”

He said there was no way to say at this point why the bees did better this year...


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Chemical & Engineering News    By Puneet Kollipara   3/14/14 

A neonicotinoid compound affects fruit flies’ ability to have offspring even at low doses.

For at least one member of a controversial class of insecticides, low doses may cause as much harm to nontarget insects as high doses do, according to a new study. The number of offspring that fruit flies produce drops significantly when the insects are chronically exposed to nanomolar concentrations of imidacloprid, a member of the neonicotinoid insecticide group (Environ. Sci. Technol.2014, DOI: 10.1021/es405331c).

In Europe, policymakers have moved to temporarily ban some of the neonicotinoids based in part on previous studies that showed that honeybees exposed to low doses of the compounds developed significant behavioral changes. The new study builds on these low-dose findings and suggests regulators need to rethink how they assess the safety of chemicals applied in the environment, says Dennis vanEngelsdorp, an entomologist at the University of Maryland, College Park, who wasn’t involved in the work.

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