"An Inside Look at How the Varroa Mite’s True Diet Was Discovered" by Gary Bauchan, Ph.D., with Ron Ochoa, Ph.D., Joe Mowery, Chris Pooley, and Dennis vanEngelsdorp, Ph.D

Varroa destructor, a mite that feeds on honey bees, is the greatest single driver of the global honey bee health decline. For years, scientists and bee keepers were told that the Varroa mite feeds on hemolymph (the bee’s blood) based on scientific research done long ago. However, for our research team led by Samuel Ramsey, Ph.D., this did not make sense, as the mite’s digestive system and mouthparts just did not seem like they were structured properly for blood feeding. Their closest evolutionary relatives were not blood feeders either. As pointed out by insect-rearing expert Allen Cohen, Ph.D., even their excrement was all wrong for them to be blood feeders.

So, how and on what exactly do these mites feed? Our discovery is documented in a paper published in late January in Proceedings of the National Academy of Sciences.

Ramsey, then a Ph.D. student of Dennis vanEngelsdorp, Ph.D., at the University of Maryland, College Park, began his inquiry with a simple question: Are Varroa mites feeding in only one spot? If the mites can feed anywhere on honey bees (Apis mellifera) like a tick can on a person, then they are likely feeding on a tissue available all over the bee’s body (like hemolymph). However, if the mites feed only in one spot, maybe they are feeding on a tissue specific to that location. Out of 104 observations, mites on adult bees were found feeding only underneath the abdominal plates of their host. In other locations mites found, such as on the thorax, no signs of feeding were detected. Ramsey considered that this might be their feeding site but knew he would need some help to prove it.

This led Ramsey to the office of Ron Ochoa, Ph.D., research entomologist at the Systematic Entomology Lab and the Electron and Confocal Microscopy Unit (ECMU) of the U.S. Department of Agriculture’s Agricultural Research Service (USDA-ARS) in Beltsville, Maryland, and the leading mite expert at the USDA-ARS. In addition to the idea that Varroa mites only feed on the hemolymph of developing larvae and pupae, researchers were questioning whether the mites feed on adult bees at all. Some had already concluded that feeding only occurs on immature bees because we’ve never seen feeding wounds on adult bees and because we have long been referring to the life stage of mites found on adult bees as “phoretic,” a term that denotes a nonfeeding stage that uses another organism as a mode of transit.