This is a question I’d rather sidestep, but it keeps insinuating itself into my inbox. So I’ve finally decided to take a quick stab at this complex (and oftentimes heated) debate.
The theory behind powdered sugar dusting is 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 has 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 makes no difference. Most think it is just the act of grooming that dislodges the mites. Consequently, most mite drop after a powdered sugar application occurs within the first few hours and decreases quickly after that.
So far, I’ve given you all the good news. Now, the rest:
Powdered sugar only affects phoretic mites—those mites that are riding around on the bodies of honey bees. By far the most mites, especially during spring and summer, are inside the cells of capped brood. These mites are oblivious to 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.
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. In fact, except for hobbyists with just a few colonies, it would be virtually impossible. Other experiments have shown that dusting the top bars is more effective, as long as at least some of the sugar immediately falls through to the bottom of the hive.
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.
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 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 used in combination with other Varroa management protocols such as drone trapping, queen removal, splitting, and the use of organic acids. Except under the most onerous treatment regimens, powdered sugar dusting by itself is probably insufficient for long-term Varroa management.