Varroa mites aren’t vampires, they are actually werewolves

Across the beekeeping community, the long-held belief was that varroa destructor mites feed on the blood of bees, officially called «hemolymph». We too have been preaching this for years, whenever we had to describe the catastrophic effects of varroa we told that varroa sucked the life out of our honeybees. In the beginning of 2019 however, a ground-breaking study was published by a group of American researchers which showed that varroa mites do not suck hemolymph, but actually eat their fat body tissue. So, about time we set the record straight and look into what these findings actually mean for honeybees and the struggle with varroa.

The idea that varroa fed on the hemolymph did not seem to make sense for lead Author Dr. Samuel Ramsey from the University of Maryland. When he started digging into the literature on varroa feeding behaviour, he was surprised to find a lot of assumptions instead of hard evidence. Historically, varroa first became a problem in China and Russia, so the first articles where published in Chinese and Russian. Many European and American scholars adopted their assumptions that varroa fed on hemolymph without checking adequately themselves. Hence, the idea of varroa mites as blood-suckers was born.

Varroa destructor is one of the biggest causes of bee mortality.
Varroa destructor is one of the biggest causes of bee mortality.

However, the digestive systems of other mites and small insects who feed on bodily fluids look pretty different compared to varroa mites. Furthermore, the excrements of varroa mites are solid, which indicates that they cannot be feeding on fluids alone.

This pointed the researchers into the direction of fat body tissue. This would also explain why varroa mites can be so devastating. Many vital functions such as immune response, detoxification (for example from pesticides) and thermoregulation depends on the fat bodies of the bees. One could describe the fat bodies as the bees liver and fat storage. Feeding on fat bodies makes sense from the mite’s point of view, as this is a very nutrient-rich diet.

Experiments

Changing a scientific belief that is dominant for decades is no easy task (which isn’t a bad thing). Ramsey and his colleagues conducted several experiments to be sure to exclude any other possible explanations. First, they looked where the mites actually were located on the bees. Hemolypmh is distributed all over the bees body, so if mites feed on hemolymph, it would not matter where the mites would position themselves on the bees. However 92.5% of the mites where located at the underside of the bees abdomen. Squeezing between their abdominal segments as you can see in the image below.

A varroa mite squeezed himself between the bees abdominal segments (arrow). [Photo credit: USDA-ARS, Electron and Confocal Microscopy Unit, Beltsville, Maryland, [1]]
A varroa mite squeezed himself between the bees abdominal segments (arrow). [Photo credit: USDA-ARS, Electron and Confocal Microscopy Unit, Beltsville, Maryland, [1]]

The few varroa mites that weren’t located on the bees abdomen, were found on the thorax and appeared not to be feeding but instead looking to hop onto a different bee.

But hold on, what then about bee brood? Because varroa mites can be found all over honey bee larvae and pupae, would that not disprove this theory? Well no, as the fat body tissue of developing brood is spread throughout their body.

However, to be sure that the mites where actually feeding when they are between the bees abdominal segments as seen in the picture, the researchers looked into the wounds of the bees. By freezing bees that were plagued by mites, they found digested pieces of fat body cells in the mites guts. This showed it was evident that fat body tissue is part of the diet, the next step was looking at the ratio of fat body tissue and hemolymph that was being consumed by the mites.

The bees where fed two different dyes, yellow-colored «Uranine», which is water soluble and red-colored «Nile-red» which is soluble in fat. If mites were feeding mostly on hemolymph, their digestive system would turn yellow. When fat bodies was the main source, it would turn red. When the researchers analysed the guts of the mites (as shown in the figure below), they found the were predominantly red. A clear indication that the fat body tissue made up the bulk of the diet of the mites

Mean levels of Uranine and Nile red detected in Varroa after 24 h of exposure to stained honeybees. It is evident that the level of fat bodies (Nile red) are higher than that of the hemolymph (Uranine) [1].
Mean levels of Uranine and Nile red detected in Varroa after 24 h of exposure to stained honeybees. It is evident that the level of fat bodies (Nile red) are higher than that of the hemolymph (Uranine) [1].

In a final experiment varroa mites were fed exclusively hemolymph or exclusively fat body tissue. The mites feeding of fat body tissue were able to produce eggs, while the ones consuming hemolymph starved. Leaving little doubt over the mite’s feeding habits.

Impact

These results fundamentally changes the way researchers understand the varroa mite, which is very valuable for any further research on this dangerous parasites.

«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.» (Dr. S. Ramsey [2])

Further, it explains why the mite can do so much harm, as the fat body tissue is so important for honeybees. For example, a while ago it was reported that varroa and pesticides cause even more harm when combined. This combined effect sounds logical, when knowing that bees ability to detoxify pesticides can be affected by varroa feeding on their fat bodies. This whole research project shows how harmfull varroa mites are and action has to be taken to protect our honeybees. Our chemical-free varroa solution has proven to be an effective treatment against varroa (killing 95% of the mite offsprings). Regardless of their feeding habits, no varroa mite is safe from our Varroa solution!

Further info

Interview with Dr. Samuel Ramsey.

Want to learn more about this study? You can find the full article here or watch the interview of lead author Dr. S. Ramsey here.

If you want to check out our varroa solution, you can find all the information here or contact us directly

Literature

[1] Samuel D. Ramsey, Ronald Ochoa, Gary Bauchan, Connor Gulbronson, Joseph D. Mowery, Allen Cohen, David Lim, Judith Joklik, Joseph M. Cicero, James D. Ellis, David Hawthorne, Dennis vanEngelsdorp. (2019). Varroa destructor feeds primarily on honey bee fat body tissue and not hemolymph. Proceedings of the National Academy of Sciences, 116(5), 1792-1801.

[2] University of Maryland. (2019, January 15). Honey bee parasites, varroa mites, feed on fatty organs, not blood. ScienceDaily. Retrieved October 28, 2019 from www.sciencedaily.com/releases/2019/01/190114161137.htm

Vatorex, Felix Poelsma
28 October, 2019
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