As I wrote nearly a month ago, two of our hives have an absurdly low number of varroa mites, but a three-day-sticky-board count averaged out to 29 mites per day in the third hive.  Even that relatively-high mite fall isn't so bad, but I've regretted it before when I've let borderline hives go into winter --- they often perish.  So I decided to use one of safest methods of mite control, treating the hive with powdered sugar.

Scientificbeekeeping.com has a fascinating series of articles on the efficacy of using powdered sugar to lower varroa mite numbers.  The author, Randy Oliver, found that 50% of the mites in a hive can be removed by adding half a cup of powdered sugar per shallow (or a whole cup per deep) to the top box.  The sugar falls down through the cracks to coat all of the bees present, making it tough for mites to keep their footing.  The bees do have to work hard that first day, grooming off powdered sugar (and mites), but the intrusion is worth it if you're worried about your mite population since so many mites fall through the screen in the bottom of the hive and perish..

In addition to the intrusiveness of the powdered sugar technique, there is another downside to using powdered sugar as your sole mite-control measure.  Oliver explains that powdered sugar only removes the mites currently on bees, and since mites carry out part of their life cycle inside capped brood, you won't really do much good if the bees are actively raising young at the time of treatment.  However, when colonies are letting populations drop to get ready for winter, powdered sugar can really decimate mite populations.  Oliver concludes that properly-timed powdered sugar applications (plus screened bottom boards) might be all the treatment you need if you're raising bees that have been bred to be mite-resistant.

To return to my own experiment, I smoked our bees gently just to make sure they wouldn't fly up into the powdered sugar, then dusted the top of the hive with 2.5 cups (half a cup per Warre hive box).  The sugar that built up on the top bars was easy to push back into the hive using the bee brush.

Immediately after dusting the hive, I could see bees out on the porch licking up powdered sugar.  Although powdered sugar is harder for bees to consume than sugar water is, our ladies are still able to convert some into food, and, eventually, honey.  So the varroa mite treatment doubles as a quick feeding boost to lightly prop up fall stores.

The other bonus of the sugar treatment is that it gives you a more accurate portrayal of probable mite levels in the hive, compared to the potentially-problematic sticky-board counts.  Just put the bottom board in before treatment, take it out a timed interval later, and count mites.  The chart to the left comes from Oliver's second article about powdered sugar, and the second column shows what fraction of the total daily mite drop comes during various intervals right after treatment.  I counted 26 mites on the bottom board of our hive 30 minutes after dusting with sugar, so if we assume 39% of all the mites that were going to fall that day had already hit the bottom, and that the treatment will remove 50% of the mites in the hive, I get a population estimate of 133 mites in the hive pre-treatment.

Actually, I think I probably didn't use quite enough sugar, since Warre hives are so tall compared to Langstroth hives.  So, perhaps my sugar treatment is only going to remove 25% of the mites from the hive, providing a pre-treatment estimate of 267 mites in the hive instead of 133.  Either way, that's pretty low, suggesting I probably didn't need to dust the bees with sugar, but the treatment is unlikely to have done much harm, and potentially will help the bees survive the winter since they won't have so many parasites sucking their blood.

If I remember, I'll run another sticky-board count in a few days to see what varroa mite levels are like post-treatment.  I haven't decided yet whether I should treat the low-mite hives, or leave them alone to go into winter without the day of trauma.