A multi-microscopy approach to discover the feeding site and host tissue consumed by Varroa destructor on host honey bees

Authors: RAMSEY, SAMUEL - University Of Maryland; RULBRONSON, CONNOR - Oak Ridge Institute For Science And Education (ORISE); Mowery, Joseph; Ochoa, Ronald - Ron; VAN ENGLEDORP, DENNIS - University Of Maryland; Bauchan, Gary

Submitted to: Microscopy and Microanalysis
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/1/2018
Publication Date: 8/1/2018
Citation: Ramsey, S., Rulbronson, C., Mowery, J.D., Ochoa, R., Van Engledorp, D., Bauchan, G.R. 2018. A multi-microscopy approach to discover the feeding site and host tissue consumed by Varroa destructor on host honey bees. Microscopy and Microanalysis. 24(S1):1258-1259. https://doi.org/10.1017/S1431927618006773.
DOI: https://doi.org/10.1017/S1431927618006773

Interpretive Summary: The parasitic bee mite, Varroa destructor, causes considerable damage to honey bees. For 5 decades, researchers and beekeepers have operated under the notion that Varroa feed exclusively on the hemolymph (blood) of honey bees. To test this, we conducted studies using unique microscopy protocols for Low Temperature Scanning Electron Microscopy (LT-SEM), Transmission electron microscopy (TEM), Fluorescence Microscopy (FL) and Confocal Laser Scanning Microscopy (CLSM) to determine where the mites feed and what tissue they ingest. Scanning electron microscopy and TEM revealed that the mites feed on adult bees by puncturing the membrane between the ventral abdominal plates. The immediate underlying tissue in this region is the fat body. Fat body and hemolymph tissue were differentially stained in host bees and FL and CLSM studies were used to determine the contents of the host meal consumed by the mite. Fat body tissue was detected as the primary tissue in the mite’s digestive system while only negligible amounts of hemolymph were detected. Collectively, the usage of multiple microscopy technologies strongly suggests that Varroa destructor feeds on fat body tissue rather than hemolymph. Thus, there is a need to update our understanding of this economically significant parasite. These results are important to beekeepers, entomologists, virologists, and agriculture scientists in the government, at universities, and in private industry who are interested in mitigating the heightened trend of honey bee colony losses.

Technical Abstract: The parasitic bee mite, Varroa destructor is considered the primary health stressor implicated in the continuing global trend of heightened honey bee colony losses. For 5 decades, researchers and beekeepers have operated under the conclusion that Varroa feed exclusively on the hemolymph of honey bees though experimental evidence proving this is lacking. Studies using microscopy techniques were conducted to determine if the mites feed on hemolymph or the proposed alternative tissue, fat body. Four different microscopy systems including Low Temperature Scanning Electron Microscopy (LT-SEM), Transmission Electron Microscopy (TEM), Confocal Laser Scanning Microscopy (CLSM) and Fluorescence Microscopy (FL) were used to determine where the mites feed and what tissue they ingest. LT-SEM studies revealed feeding holes in the intervening membrane between the overlapping plates that make up the honey bees abdomen. In addition, LT-SEM freeze fractures coupled with separate observations via light microscopy revealed that the immediate underlying tissue in this region is primary fat body. TEM studies showed a hole with irregular edges where the mouth parts of the mite had penetrated the host bee’s membrane and the presence of degraded fat body cells at the feeding site; clear evidence of extra-oral digestion in this mite. Honey bees were then fed fluorescent biostains which differentially marked the fat body and hemolymph tissue. The gut contents of mites fed on these bees were analyzed via CLSM and FL. Fluorescence associated with honey bee fat body tissue was consistently detected inside the gut of the mites with very little fluorescence from hemolymph tissue. Collectively, these multiple microscopy technologies provide evidence that Varroa primarily consume fat body tissue rather than hemolymph exposing a need to update our understanding of this economically significant parasite and the management practices directed at it.