I think there a host of items that all play a part in making russian bees mite resistent. Genetics, shutting down during dearth periods, swarming tendencies, overwintering characteristics, etc.
At the risk of once again being attacked for questioning the "establishment" and those far more educated than I, let me say one thing. I question the fact they brought russian bees to the states, and established them in Louisiana and other southern climate areas as a main point for research and breeding.
I think the russians are ideal bees for the northern beekeeper. But is breeding them in the deep south, and perhaps changing some of their natural cold climate survival traits a good thing?
I think we are taking a bee that has adapted well in cold climate areas, and we now want it to change. We want to breed it in the south. We want to make it less defensive. We want to make it like all the other bees that we have bred over the years. Is that good or bad?
Makes me wonder if years from now well just have another hole in our foot, and be surprised by the bleeding from another self inflicted wound by our own hands.