The Beekeeper’s Companion Since 1861
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Beekeeping Basics

Time to Control Mites

- September 1, 2021 - Dewey Caron - (excerpt)

honey bee and varroa mites

You have undoubtedly read or discussed or listened to (one or more) YouTubes on varroa mites — perhaps you have digested a great deal about mites? August and September represent our last best chance to obtain varroa mite control. High mite numbers can lead to high deformed wing virus (DWV) levels and sudden colony death. Control of varroa mites in the fall is critical to avoid a colony collapsing or not overwintering successfully.

We love our bees — unfortunately so do mites. The varroa mite is a formidable foe for our European honey bee. Beekeepers have to step up their efforts in the fall to control varroa. There are some bees and some stock selections that are capable of keeping mite numbers at lower levels. Beekeepers however often need to intervene in September when there are other close colonies and/or close beekeeping neighbors.

Varroa mites have two phases in the host bee colony: phoretic — now being labeled as dispersing — the stage when varroa mites are found on the adult bees; and a second, reproductive phase when mites are raising young within capped drone and worker brood cells. Individual varroa spend several days on the adult host, feeding on fat body between ventral abdominal segments, and then two weeks inside capped brood cells where they reproduce more female mites. It is the female mite we are discussing, as male varroa are not of pathogenic concern.

Using more than one mite control is now the “norm.” There is no “magic bullet” — no one control method alone that is satisfactory if we hope to keep our losses below the average 40% for small-scale individuals — about double the level for larger-scale beekeepers. Techniques to control both mite phases are needed as we unfortunately lack weapons to control the virus. Varroa control means a concerted effort to understand and use Integrated Pest Management.

IPM is the concept of seeking to use a variety of methods to keep a pest below an injury level. The Food and Agriculture Organization (FAO) of the United Nations defines IPM as “the careful consideration of all available pest control techniques and subsequent integration of appropriate measures that discourage the development of pest populations. IPM keeps pesticides and other interventions to levels that are economically justified to reduce or minimize risks to human health and the environment.” IPM is not anti-chemical but rather implies a judicious use of chemicals, integrated with other approaches, to keep pest populations below economically damaging levels.

As beekeepers we are well aware that to be successful, we manage the population of our bees. IPM is managing the population of a pest species. We might seek to flatten the growth curve of our bees if we find they are preparing to swarm (i.e., we see developing queen cells in spring buildup phase); with the pest (varroa mites) we want to flatten their growth curve in May-August. By September/October we often need a chemical intervention to enable colonies to rear the “fat” (diutinus bees) bees to survive six months of the coming clustering season.

 Mite Sampling

Sampling is key to an IPM approach to mite control. Sampling allows us to monitor mite buildup. The question is not “Does my colony have mites?” but rather, “How many mites does my colony have?” Looking for mites on adult bees or in drone brood is not an adequate means to get that answer. Sticky sample (debris) boards are not very precise. The best method is to take a sample of 200-300 adult bees and “wash” the mites from the bee bodies. We then count how many are found per 100 bees — percent infestation.

Using alcohol, grease-cutting soap or window-washing fluid are the best ways to wash adult bees. Powdered sugar, engine-starting aerosol and CO2 work also but not as well. Sampling in April or May should show low mite numbers as most mites are in their reproductive phase at this time and mite populations are low (especially if oxalic acid was used during a broodless winter period). But sampling is not a one and done — it is a continuous process. The benefit will be that sampling gets easier and practice will improve precision. It is best to have a record spanning a longer time period.

It is recommended to sample bees from a brood frame, or from a frame on the edge of the brood area, to reduce chances of queen capture. Instead of collecting bees directly into a sample jar, shake bees from a frame into a plastic dishpan or 5-gallon bucket to increase the chance of spotting a queen to make sure she doesn’t accidently get into the sample jar. Three hundred bees is one-half cup filled with adult bees. Count bees a few times to practice and mark the sample jar — it will become easier to estimate with practice.

One potential advantage of going into the brood area to take a sample is that it will enable you to look at the queen brood pattern. A spotty brood pattern, while it may indicate several things, is a possible sign of high varroa populations. When spotty, irregular patterns are observed, you need to look further to potentially see if there is a problem. Spotty brood patterns may also occur if you have stock that is hygienic.

The most difficult sampling and control period in colony (and in mite) buildup is when colonies have filled the brood boxes and supers are on colonies. It is important that counts be made during this period. Make monthly counts as a minimum. It is far better to have a record of mite buildup in a colony than to wait until September, after supers are removed, to begin mite sampling.

Knowing how many mites are in a hive, even when sampling is not entirely accurate, allows the determination of a risk threshold to target treatment decisions. We need to adopt a mite level we feel comfortable with — a level we believe our colony can tolerate. This number has been steadily dropping as we understand the dynamics of mites and transmission of viruses. Consider anything over 3% (3 mites per 100 bees washed) in September sampling to mean there is some risk that your colony will not successfully overwinter. The higher the number the greater the risk.

Cultural control

Flattening the mite growth curve begins with what we term “cultural control.” This involves the beekeeper doing things like spacing of colonies, using different hive color schemes, reducing the number of colonies in one location and using sunny versus shaded colony sites. Stepping up would be to incorporate more mite-resistant queen stock with annual requeening. A further step-up is encouraging those around you to join in mite control to avoid development of mite hive bombs. One individual can have ….