Varroa destructor (Varroidae) feed on honey bee fat body tissue not hemolymph

Authors: RAMSEY, SAMUEL - UNIVERSITY OF MARYLAND; Ochoa, Ronald - Ron; Bauchan, Gary; GULBRONSON, C. - U.S. DEPARTMENT OF AGRICULTURE (USDA); MOWERY, JOSEPH - U.S. DEPARTMENT OF AGRICULTURE (USDA); COHEN, ALLEN - UNIVERSITY OF NORTH CAROLINA; LIM, DAVID - UNIVERSITY OF MARYLAND; JOKLIK, JUDITH - UNIVERSITY OF MARYLAND; CIERCO, JOSEPH - UNIVERSITY OF FLORIDA; ELLIS, JAMES - UNIVERSITY OF FLORIDA; HATHORNE, D. - UNIVERSITY OF MARYLAND; VANENGELSDORP, DENNIS - UNIVERSITY OF MARYLAND

Submitted to: Proceedings of the National Academy of Sciences (PNAS)
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/6/2018
Publication Date: 1/29/2019
Citation: Ramsey, S., Ochoa, R., Bauchan, G.R., Gulbronson, C., Mowery, J., Cohen, A., Lim, D., Joklik, J., Cierco, J., Ellis, J., Hathorne, D.J., Vanengelsdorp, D. 2019. Varroa destructor (Varroidae) feed on honey bee fat body tissue not hemolymph. Proceedings of the National Academy of Sciences. 116(5):1792-1801.

Interpretive Summary: Varroa destructor is an external parasitic mite that feed on European honey bees and is one of the major causes of colony losses worldwide. It is commonly believed that these mites feed on the hemolymph of adult and immature honey bees, however, studies were conducted to determine if mites fed on hemolymph or fat bodies of the bee. Feeding studies of mites determined that they feed under the overlapping plates of the abdomen feeding primarily on the left side near large deposits of fat bodies. Low temperature scanning electron microscopy and transmission electron microscopy revealed feeding damage in the intervening abdomen membrane, providing conclusive evidence that Varroa feed on adult bees. Laser scanning confocal microscopy studies of biostain labeled hemolymph and fat body determined that Varroa allowed to feed on labeled bees ingested primarily fat body material. An in vitro system of rearing Varroa was developed using extracted bee hemolymph and fat body. Varroa fed on a diet composed exclusively of hemolymph tissue survived only a couple of days, whereas those fed entirely fat body tissue were able to survive the seven day duration of the trial and produce eggs. Collectively, these findings strongly suggest that Varroa are feeding on fat body, thus there is a need to update our understanding of the mite and management practices directed at this parasite. These results are important to bee keepers, honey producers, entomologists, virologist, biologists and agriculture scientists in the government, at universities, and at private industry who are interested in solving honey bee losses.

Technical Abstract: The parasitic mite Varroa destructor is the greatest single driver of the global honey bee health decline. Better understanding of the association of this parasite and its host is critical to developing sustainable management practices. Our work shows that this parasite is not consuming hemolymph as has been the accepted view, but damages host bees by consuming fat body, a tissue roughly analogous to the human liver. We captured the first images of Varroa feeding wounds on adult bees. Feeding occurs on the underside of the metasoma (abdominal region) where fat body is the immediate underlying tissue. Fat body at the wound site showed evidence of external digestion. Both hemolymph and fat body in honey bees were then marked with fluorescent biostains. The fluorescence profile in the guts of mites allowed to feed on these bees was very different from that of the hemolymph of the host bee but consistently matched the fluorescence profile of the fat body. Mites in their reproductive phase were then fed a diet composed of one or both tissues. Mites fed fat body tissue survived longer and produced more eggs than those fed hemolymph. Mites fed hemolymph showed fitness metrics no different than the starved control group suggesting that fat body is integral to their diet when feeding on brood as well. Collectively, these findings strongly suggest that Varroa are exploiting the fat body as their primary source of sustenance; a tissue integral to proper immune function, pesticide detoxification, overwinter survival and several other essential processes in healthy adult and immature bees. These findings fundamentally alter our understanding of the etiology of varroosis and underscore a need to revisit our understanding of this parasite and its impacts, both direct and indirect, on honey bee health.