Max,
Michael Bush has been keeping bees for a very long time. When varroa hit, like everyone else he started treating with all of the poison and chemicals. He then decided to let the bees figure it out. He has not been treating his bees for, I think, over ten years now. He has done a few things to help the bees out.
He shaved his brood frames down by an eighth of an inch and puts 11 frames in a 10 foot and box. This allows each bee in the brood area to do the work of two bees. The other thing he does is use fountain less frames to allow the bees to make smaller bees to allow them to hatch out a day or two earlier. This reduces the number of mites that can mature to adulthood. Mite originally developed on apis Cerana drone brood only. Apis Cerana is a smaller bee that hatches sooner than our bees. Loosing a few drone brood is not a problem. The foundation in our hives was designed to make larger bees, to bee able to carry more nectar. Larger bees take more time to develop.
Maybe Michael will provided more information.
I think when Varroa hit Italy, may have been another country, they decided to not treat their bees. The first couple of years they lost a large percentage of their hives. Then the bees changed and survived with the mites.
Jim Altmiller