So we had a visit from our regional beekeeping advisor from the state.
I asked him if treating varroa with sugar powder helps. He tried not to laugh, and said studies show continiously that the bees clean themselves but do not kill any varroa they may remove. so unless you are catching them and killing them as they fall the effect is almost nil.

I have never tried it out myself. My Varroa management regime is :
removal of all capped drone brood (I supply empty frames specially for this).
60% formic acid 2ml per frame applied from above every 5-7 days in july/august 6-7 repatitions
oxalic acid sugar water in Dec. 30-50ml per colony.

So what do you do?
What verified research results do you know of that prove or disprove a treatment method?