This winter during broodless period (Mid December - Early January ) will use oxalic acid as dribble.
That is OK. Further more you should treat the colonies in late summer when they rear winter bees. Stuffs are thymol or formic acid. They need about 17C day temps.
Oxalic is only one which works in under 15C temps
With my 10 y experience with trickling I can say that if that trickling is only treatment, mites will kill quite big part of winter cluster bees before autumn. Late summer treatment is much more important than winter trickling.
Is it ok to treat also with oxalic acid after summer harvest in Mid July and the again next December ? I often read a limitation of only 1 time per year. The bees will be completely different. What do you think ?
I do not think, but I know. Nowadays there are no limitation how many times a year you treat.
Oxalic trickling makes big damages to the (winter )brood and it will be as bad as mite itself. Furthermore, it does not affect on mites under brood cappings.
The idea, that you need mite treatment after honey harvesting.... that is why is trickling. It gives the last hit on mites that they are very few when brood season begins.
In Finland most guys use thymol pads during winter feeding. It kills about 80% of mites and the rest you should kill with trickling.
This is a way how thymol pad works