How Do Varroa Mites Feed on Bees?

Western Apicultural Society FB Post  June 17, 2018

Sammy Ramsey, of the vanEngelsdorp Bee Lab, University of Maryland, spoke at the April meeting of the Alameda (CA) Beekeepers Association. This is a summary of his research findings, published in the most recent ABA newsletter.

HOW DO VARROA MITES FEED ON BEES?

Conventional wisdom is that Varroa mites feed on bees' hemolymph, which is like blood. When Sammy reviewed the research, he didn't think it actually supported that.

His PhD thesis was to determine how mites feed and what they feed on. He compared Varroa mites to other arthropods that feed of hemolymph or blood and found differences:
• Their digestive systems and excrement are quite different
• They are not closely related genetically

Next, he did an observational study of where on bees Varroa mites fed. Looking at mite placement, he found 99 percent of mites on the bottoms of the bees, wedging themselves under the plates called the metasomal sternites/tergites.

The mite pierce multiple layers of soft tissue in the membrane between sternites/tergites and then inserts its feeding tube to feed on the "fat body."

Bees typically have a long section of fat, the fat body, running along their undersides. This is an organ, not just a mass of tissue, with nine different functions, including growth and development; metamorphosis; metabolic activity; water and temperature regulation; protein synthesis; immune function; and synthesis of vitellogenin, the substance that allows some bees in the colony to overwinter.

When a mite has been feeding, it gets smaller and more dispersed. This indicates two things: The mite injects something into the bee to predigest tissue; and that fat is the mite's food.

Next, he stained the fat content and hemolymph of bees with specific stains that would fluoresce under a spectrophotometer. Then he put in mites and allowed them to feed. The mites consistently showed they had fed on fat.

Finally, he raised groups of mites off bees, feeding them on various combinations of fat body and/or hemolymph. Mites that were fed on fat body laid the most eggs. Mites fed only on hemolymph laid no eggs. Moreover, mites fed hemolymh died as quickly as those fed nothing. Those fed on fat body lived substantially longer.

His conclusion is that the mites feed on the fat body of the bee, not the hemolymph.

He has also found bacteria inside bees near the feeding wound, and the bee's immune system doesn't seem to attack them. The bacteria haven't been identified yet.

Mites that kill colonies the quickest are also the ones that have a better chance of dispersing into other colonies. He thinks beekeepers should treat or intervene in some way, for example, by removing drone comb right before the drones hatch. He advises using more than one type of treatment, so you reduce mites with different characteristics with the different methods.

Brood breaks are helpful, but there will still be mites on the adult bees.
His data shows that in four consecutive years, beekeepers who treated for Varroa lost fewer colonies than those who didn't treat.

Why does this matter? It shows:
• Need to update recommendations for treatment timing.
• Supplementing protein without controlling Varroa is not helpful.
• This info could lead to the development of systemic pesticides for Varroa.
• Important to make sure bees that will overwinter, which are developing in the cells in August, are not harmed by Varroa—so that's a good time to treat.

More about Sammy Ramsey: https://www.vanengelsdorpbeelab.com/samuel-ramsey.html