My aim is becoming a treatment-free beekeeper. Unfortunately the density of bee colonies and beekeepers in Germany is rather high and varroa reinvasion is a common problem. Things seem to be different in some parts of the US, UK or Scandinavia, where some folks seem to cope well with no treatments at all.
If I determine the need for reducing varroa levels, I would recommend using oxalic gas vaporization.
During summer I would carry out two to four vaporizations in a row every 5 (4 vapors) or 10 (2 vapors) days.
In brood-free December I would recommend one or two vaporizations in a row of 7 days.