Of mites and men
Every year at about this time, new beekeepers overwhelm the social media with claims they can save the honey bee (and the world) with copious applications of powdered sugar. The onslaught is as predictable as the tide and just as insistent. I actually feel sorry for them. If they ever really studied the Varroa life cycle and how it complements the Apis life cycle, they would realize that powdered sugar is a great idea with very little promise as a long-term control.
So here, resurrected from an earlier post, is a quick rundown of the pros and cons in this contentious debate.
How it works
The theory behind powdered sugar dusting it that the bees will groom the sugar off themselves and the hapless mites will drop from the bees and fall through the screened bottom board and out of the hive. Mites on the ground soon become food for something else. Good riddance. Any inert powder applied to the bees will have a similar effect, but powdered sugar is a good choice because it is readily available, inexpensive, and harmless to the bees.
Most sources I consulted recommend one cup of powdered sugar per brood box. However, those who use bellows-type applicators use less. Those who use powdered sugar dropped through a food strainer use considerably more. As I said, the sugar is harmless to the bees, and even the small amount of corn starch found in commercial powdered sugar produces no ill effects.
Powdered sugar also has no harmful effect on mites. Some folks think the sugar makes it harder for the mites to hold onto the bee, others say it doesn’t affect the mite’s grip whatsoever. Most experts think it is simply the act of grooming that dislodges the mites: in an attempt to rid themselves of the sugar, they end up flicking away the mites. Consequently, most mite drop after a powdered sugar application occurs within the first few hours and decreases quickly after that.
What are the limitations?
Unfortunately, powdered sugar only affects phoretic mites. Phoretic mites are the ones riding around on the bodies of adult bees; they are the ones you can see. But by far the most mites, especially during spring and summer, are inside the cells of capped brood. These mites are protected by the comb and are unaffected by blizzards of powdered sugar.
That means that powdered sugar must be applied regularly to the hive in order to remove mites as they hatch along with brood. In his experiments, Randy Oliver found that the more treatments you applied, the more mites you “killed” (dropped out of the hive). Treatments every week killed more mites than treatments every two weeks, which killed more mites than treatments every month. Oliver found that the only treatment schedule that effectively suppressed mites over long periods was once per week.
But according to the University of Florida Institute of Food and Agricultural Sciences, when bee brood is present, a female mite may stay in the phoretic state from 4.5 to 11 days before she climbs back into a brood cell. That means that even with a once-per-week treatment schedule, many of the mites will evade the sugar dust completely.
Another factor is the method of application. Some experiments have shown the best coverage is achieved by removing every frame from the hive, treating both sides, and then replacing the frame. This is an egregious amount of work and a huge disruption of the colony, especially if done on a once-per-week schedule. Imagine the disruption to a triple deep hive if you were to tear down the colony once a week and dust both sides of each frame. In fact, except for hobbyists with just a few colonies, it would be virtually impossible.
Other experiments have shown that dusting the top bars only is fairly effective and more efficient, as long as at least some of the sugar immediately falls through to the bottom of the hive. But again, with multiple deeps, dusting the top bars only has very little effect on the lower boxes.
Another issue is humidity. Powdered sugar is amazingly hygroscopic, meaning it has an attraction for water. If you keep bees in a humid climate, or if your hives are full of moisture, the sugar will clump on the bees instead of dusting them, a situation that reduces the efficacy of the treatment. You end up with a hive full of bees and mites and clumps of wet sugar.
The good news
In spite of the negatives, sugar dusting has been found to significantly reduce adult mite populations at times when little brood is present. Colonies in a late summer nectar dearth or loosely clustered winter bees may be effectively treated with powdered sugar. Also packaged bees, swarms, and shook swarms—units without brood—may benefit from the treatment as well.
My own opinion is that powdered sugar dusting is effective only when little or no brood is present, and when it is used in combination with other Varroa management protocols such as drone trapping, queen sequestration, splitting, and the use of organic acids. Except under the most onerous treatment regimens that are highly disruptive to the colony, powdered sugar dusting by itself is insufficient for long-term mite management.