Sugar dusting bees is a non-chemical, relatively harmless approach for dealing with varroa mites. The concept is that covering the bees with sugar will make things slippery for the mites and stimulate grooming behaviour in bees.
It's widely accepted that sugar dusting will successfully knock down a portion of the phoretic mites, that is those mites riding on the adult bees, but not those reproducing under capped brood. As such, this approach is most useful to employ during a queenless period after the old queens brood had emerged but before brood from the new queen is capped.
Generally sugar dusting is viewed as just one component of an Integrated pest management system, useful for keeping numbers down, but one shouldn't expect it to function as a magic bullet that will solve all your mite problems.
The effect upon the hive is immediate and obvious. Bees run for cover and once sugared, stop whatever else they were doing and proceed to clean the sugar off. I've typically tried this using about 1 cup of powdered sugar per deep brood box. In my experience, most of the mite drop will happen in the first 15 minutes. This photo is an example of what can be pulled out of the bottom of the hive in 15 minutes.
In the first hour I'm usually able to recover about 1/3 of the sugar from the bottom board. This technique works best in tandem with a screened bottom board, as it knocks off the mites but doesn't kill them. You can slide something like cardboard into the entrance of a solid bottom board when sugar dusting so the mite can be easily removed after the drop.
The main drawback to sugar dusting is that the bees do appear to devote a fair bit of energy cleaning out the sugar. The following video shows a colony actively in clean-up mode after sugar dusting. The video was shot at the end of November, there was little forage available this time of year, as such the hive entrance had been very quite prior to dusting.
Randy Oliver at scientificbeekeeping.com has experimented a fair bit about this form of mite control. His three part series is some of the most useful information I've been able to dig up on the subject. He's found weekly sugar dusting can slow mite reproduction, but it might not knock them back enough by itself. In the end his experiemnts may have raised as many questions as they answered. If nothing else, his experiments suggest that full colony sugar dusting might be one of the best ways of assessing a colonies infestation rate:
"The take-home message is that the results of this series of tests lead me to question the reliability of either natural mite fall or the alcohol wash (or ether roll) as monitors of mite infestation level! It appears to me that a whole-colony mite drop accelerated by sugar dust (or other mite dislodging agent) is likely the most accurate field-practical way to determine a colony’s mite level."