Varroa Mites Feed Primarily on Honeybee Fat Body Tissues, Not Hemolymph

Jan 15, 2019 by News Staff

A parasite called the varroa mite (Varroa destructor) is the most serious pest of European honeybees (Apis mellifera) worldwide. For decades, scientists have assumed that varroa mites feed on honeybee hemolymph (blood). But a new study suggests that these parasites instead have a voracious appetite for a honeybee 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.

In this electron micrograph, Varroa destructor (arrow) is wedged between the abdominal plates of a honey bee’s exoskeleton. Image credit: UMD / USDA / PNAS.

In this electron micrograph, Varroa destructor (arrow) is wedged between the abdominal plates of a honey bee’s exoskeleton. Image credit: UMD / USDA / PNAS.

“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 study lead author Dr. Samuel Ramsey, a researcher in the Department of Entomology at the University of Maryland.

“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 honeybee survival.”

In addition to breaking down toxins and storing nutrients, honeybee fat bodies produce antioxidants and help to manage the immune system.

Fat bodies also play an important role in the process of metamorphosis, regulating the timing and activity of key hormones. These organs also produce the wax that covers parts of bees’ exoskeletons, keeping water in and diseases out.

“The assumption that varroa mites consume honeybee hemolymph has persisted since the first paper on the topic was published in the 1960s,” Dr. Ramsey said.

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

Dr. 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 experiment Dr. Ramsey and co-authors performed was to observe where on the bees’ bodies the varroa mites tended to attach themselves for feeding.

“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,” Dr. Ramsey said.

“More than 90% 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 scientists then directly imaged the wound sites where varroa mites gnawed on the bees’ abdomens.

“The images gave us an excellent view into the wound sites and what the mites’ mouthparts were doing. We could see digested pieces of fat body cells. The mites were turning the bees into ‘cream of honeybee 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,” Dr. Ramsey said.

To further shore up their case, the researchers 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, they expected to see a bright yellow glow in the mites’ bellies after feeding. If they were feeding on fat bodies, on the other hand, they 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,” Dr. Ramsey said.

In a final experiment, the study authors 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, they fed the mites 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 University of Maryland’s Dr. Dennis vanEngelsdorp, co-author of the study.

“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 honeybee colonies.”

“Importantly, it also opens up so many new opportunities for more effective treatments and targeted approaches to control mites.”

The findings appear in the Proceedings of the National Academy of Sciences.

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Samuel Ramsey et al. Varroa destructor feeds primarily on honey bee fat body tissue and not hemolymph. PNAS, in press; doi: 10.1073/pnas.1818371116

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