In the UK where we have found varroa resistance to Apistan and Apiguard etc, then there has been more use of Oxalic Acid solution. This treatment must be done when there is no brood -e.g. March or October/November (the mites live, reproduce and incubate on the larvae)-and no honey on the hive that would be sold for public consumption.
The bees are unharmed and the varroa drop is very apparent within a day.
The formula is 20grams of Oxalic Acid mixed in 500mls 0f 1:1 sugar water solution.
Trickle, with a syringe, 5 mls of the formula over each line of bees between the frames in the brood box.This should use about 55mls per hive.
Wear a mask and protective gloves as fumes are nasty.
But I have treated my hives last winter and have seen no varroa this year -- so far