I have read that it is oxalic acid treatment that is mostly used during a broodless period, like late fall.
Powdered sugar dusting is the one treatment that can be used at almost any time. I wouldn't recomend it during a good honey flow or in cold weather when the bees are in cluster, or can't fly readily.
Sugar dusting is probably best used in spring to early summer when there would be brood and when the size of the hive [not stacked with supers] and colony population would be fairly low to medium, so that most of the bees get dusted. It has been established that brood is not affected by dusting unless it is "poofed", or blown directly into the cells.
Broodless period/time equals = late fall, winter, a package of bees, a recent swarm, a queenless hive after 21 days.