It is my understanding that here in the USA a small amount of cornstarch is
often added to powdered sugar as an anti-caking agent (and the bag still
says 100% pure cane sugar on the front).  I prefer brands with a little corn
starch because the dust spreads more easily and therefore I feel it does a
better job in humid weather.  I would be cautious about using sugar with
cornstarch in cold weather when the bees do not have frequent flight
opportunities.
Disclaimer: sugar dusting is not a major part of my mite control strategy.

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