I am no expert and have been doing this for only 4 years, but Bjorn has it right. Hit the problem from multiple directions. I do believe that some lines are better than others - but not for every location. I have done well by splitting off my most successful hives. Some of my less successful hives I have split to prevent swarming, and often as not they are less successful too. My son has a hive at my place he calls the "derp" hive, they struggle every spring with tracheal mites, and then shake them off later. The split we did - same problem. Oddly, those hives do very well with varroa. In horticulture or animal breeding, adapting to local conditions tends to have advantages. Any living thing will react to any challenge worse if they are already stressed. A cow adapted to the tropics is going to have more respiratory problems in Iowa, not because they are prone to respiratory trouble, but because they are so stressed from the cold. I have trouble believing bees would be any different. I also use screened bottom boards, and let the bees build their own comb on empty, wired frames. I also have drone comb frames that I have yet to use because I haven't needed to yet. But I am prepared to go to that if I have to. So far I have not had trouble with varroa. It is probably about time for individuals from cold foreign climes to chime in and tell us we are all idiots, but so far this is what has worked for me.