This article, lately in the American Bee Journal, talks about untreated bee colonies (feral) developing resistance to DMVirus and varroa mites, and surviving into winter as well or better than treated colonies. Interesting.
What Can We Learn From Feral Honey Bees? - American Bee Journal
https://americanbeejournal.com/what-can-we-learn-from-feral-honey-bees/