Do you apply oxalic acid as an anti-varroa treatment to your hives during winter?
For us there's usually a warm period shortly before Christmas when one can apply the acid and then the next day or two the bees can fly out and clear their bowels. The oxalic does wonders to keep varroa down. Compared to it, formic acid is just something you use over the warm part of the year until you can apply the real thing again
That's what I use - the two organic acids, nothing else. Would prefer to use nothing and let the population try and sort out some resilience but I don't want to be a varroa factory for my neighborhood.
Our weather forecast is currently very promising, it seems I'll be able to do the treatment over the weekend or in the first days of the next week.