Since the title of my original thread started in October was not appropriate for the below summary/recap, I am reposting my results here with a more appropriate title:
After losing my first hive to PMS this summer, I had a second hive showing signs of advanced PMS in the fall. Very high 24 natural drop mite counts (100's), bees with DWV outside the entrance, spotty brood pattern. This hive had been a strong double deep hive coming out of the summer, but by late October had started to dwindle and was down to about 1 deep (2 half-deeps).
I had never tried powdered sugar dusting before but decided to see if I could save the hive by dusting extensively (daily / whenever possible) over a several week period.
Between October 26th (when I started) December 3rd (the last time I have dusted), I dusted this hive a total of 25 times.
My other posts report the detailed mite drop data, so I will just recap a summary here:
Date(s) . . starting 1-hr dusting drop . ending-1-hr drop . average over period
10/26-11/1 . . >250 mites . . . . . . . . . . . 15 mites . . . . . 685/7=98 mites
11/6-11/14 . . . 110 mites . . . . . . . . . . . 30 mites . . . . . 207/7=30 mites
11/23-12/3 . . . . 25 mites . . . . . . . . . . . . 1 mite . . . . . 123/10=12 mites
The last three times I dusted the hive, there were only 2,3 and 1 mite, so this is when I stopped.
So one conclusion is that by dusting 24 times over a 38 day period, I virtually eliminated mites from the hive.
Another conclusion is that daily dusting did not appear to harm the bees in any way. The brood cluster was always cleaned of sugar by the next day (though as the hive dwindled from the effects of PMS, the empty frames towards the outside began to show signs of residual sugar from one day to the next).
On October 26th, when I started, the hive had 2 frames of open and capped brood in the upper deep, on top of 4 frames of open and capped brood in the lower deep. There were probably about 10 frames of bees total at that time and the brrod pattern did not look good (spotty).
By November 14th, the hive had dwindled down to 3 frames of brood and less than 5 frames of bees but the brood pattern had improved on one frame to a small, solid cluster.
On December 1st, the hive had dwindled to 2-3 frames of bees, no open brood, only a single frame 25% full of capped brood. I spotted the queen and she looked fine. At this point I was down to a small nuc and compacted the hive/nuc into a single deep with the cluster in the middle and stores on either side.
On December 24th, I did a quick inspection and the hive/nuc now has a softball-sized cluster spanning three frames with open and capped brood and a good pattern. Quick inspections of the bottom boards have not shown any signs of mites, but I will try to do a more careful 24-drop measurement or another dusting when we next get a clear day.
In a colder climate, this hive would be dead by now, but because I am in California where our winters are relatively mild, I think this hive/nuc has a good chance to make it now. The bees are already bringing in tons of pollen and the nuc is expanding, so unless we get a severe cold snap and the bees do not survive because of lack of critical mass to maintain the temperature of the brood cluster, I believe this hive which was severely stricken with PMS has been saved with nothing but powdered sugar.
If I were to try this experiment again, I think I would not have tried to save the brood at the beginning. If I had removed all open and capped brood (brushing the bees back into the cluster) and confined the queen to my Nicot cage for a few days, I believe I could have eliminated all of the mites in the first few days of dusting between 10/26 and 11/1. Then releasing the queen she could have immediately started to establish fresh uninfested brood while she had a larger mass of bees to establish a larger cluster and I would not have had to keep dusting to clean out the newly-emerging mites from the infested capped brood.
The theory about dusting is that it knocks off approximately 50% of the phoretic mites in a single 24-hour dusting. My results generally support this, and assuming that this is correct, sacrificing all open and capped brood, confining the queen, and dusting 5-7 times as quickly as possible should reduce the phoretic mite count to between 1/32nd and 1/128th of the starting level. Strictly speaking, the queen can be freed 4 days before the dusting is finished, since the mites should not be able to enter open brood that is less than 5 days old.
Another option if one has a spare queen or is treating two infested hives would be to split the hive and use one split/hive with a confined queen to emerge all infested open and capped brood over a full 21-day period, only dusting down the phoretic mites once the brood has all emerged, and use the other split/hive to begin establishing uninfested brood after first dusting out the phoretic mites over a 5-7 day period. 4 weeks after this treatment, one uninfested split will have uninfested open and capped brood, many foragers and few nurse bees, and the other uninfested split wll have no brood but many uninfested nurse bees - recombining the splits (or rebalancing the hives) should result in a stronger hive(s) than if an entire generation of open and capped brood is lost.
I hope I never allow one of my hives to become this infested with Varroa again, but I have learned a great deal from this experiment and it has made me a believer of the powdered sugar treatment approach if it can be applied properly. Dusting once a week for three weeks is pretty much of a complete waste of time but dusting daily for 5-7 days without brood should be an effective way to all but elimiate mites from a hive. This technique would probably not be practical for a commercial operation, but for any hobbiest searching for ways to help his bees fight off the mites without the use of chemicals, I believe that intensive broodless dusting is an option to strongly consider.