The Beekeeper’s Companion Since 1861
icon of list

Cover Story

Keeping colonies Safe from Varroa Mites

- September 1, 2016 - Meghan Milbrath, PH.D. - Michigan State University Extension - (excerpt)

Varroa mite graph

We are in the middle of an epidemic of varroa mites (Varroa destructor). Varroa mites damage developing honey bees and transmit a number of deadly viruses, often leading to colony death. This deadly pest has become so widespread in the United States that they are in virtually every honey bee colony, and can re-infest colonies quickly. Every beekeeper in the United States in 2016 should be prepared to deal with varroa mites, but not every colony is doomed to die from the viruses that these mites transmit. If the varroa population is kept in check, the honey bees can remain healthy. When the varroa population gets out of control, the colony becomes profoundly sick from injury and disease, and is at a high risk of dying. A lot of beekeepers lose their colonies to varroa-transmitted viruses every year, and many of these deaths could be prevented if the mite populations were managed. The information below is designed to explain the different tools that we have to manage varroa populations. Your goal as a beekeeper should be to develop a strategy at the beginning of the season that will use a variety of these tools, making sure that varroa mites never take over your colonies, and your bees stay healthy.

Varroa mite populations, when left unchecked, can grow quickly. Each female mite reproduces multiple times in her life, and each time she reproduces, she lays multiple daughters (and they all reproduce multiple times, and they all produce multiple daughters, and those daughters reproduce…). All of this reproduction is occurring under capped brood cells, which means two things 1) the more capped brood we have, the faster varroa can reproduce, and 2) we can’t see the population of varroa as it grows out of control. A honey bee colony can look very healthy and large, even when the population of varroa, hidden from our view, is about to explode.

Even with exponential growth it takes some time for varroa populations to get to dangerous levels. In the real world, they often grow all summer and peak right when winter bees are being raised (late summer / early fall). These winter bees have to survive a period of high stress, and can’t handle the extra challenge of being bitten and filled with viruses. This is one of the reasons that varroa mites kill colonies so often in the fall/ early winter.

A colony that is heavily infested with varroa can act as a reservoir, and put other colonies around it at risk. When varroa mites take over a colony, bees will often drift or abscond, and neighboring bees will rob the weakened colony. When this happens, the mites very quickly get transmitted to other neighboring colonies. This means that your infested colony can affect the bees and beekeepers around you, or that your otherwise healthy colony can become reinfested quite quickly from a neighboring colony. Not only are the bees in an overwhelmed colony profoundly unhealthy, but they are a risk to other bees in their neighborhood.

So how do we keep the varroa population from getting out of control? Unfortunately, most of our bees don’t naturally have strategies to keep mite populations down on their own (yet) – varroa mites are relatively new to our honey bees (they jumped over from a different bee species), and our bees haven’t had enough time to evolve natural defenses. While breeders are working tirelessly to find bees that do have defensive strategies, as of 2016, most of the colonies in the United States don’t have the ability to manage varroa populations by themselves. Left alone, the mite populations grow uncontrolled, and our bees get overwhelmed, very sick, and die. We don’t have a silver bullet for managing varroa mites (if we had a perfect strategy, they wouldn’t be a problem, and we could all go back to happy, easier beekeeping). However, even though we don’t have a one-size-fits-all strategy, we aren’t helpless. We do have a variety of tools that, if employed well, can help …