Why would untreated bees be more likely to be resistant to mites than bees that have been treated? If the mechanism of resistance works for colonies with large mite loads wouldn't it also appear in colonies with fewer mites?
This is a good example of selective pressure where the small effects of several genes are in play. So long as susceptible colonies are propped up with miticides, there is no selective pressure to bring the mite tolerant colonies to the forefront. Varroa mites sans treatments are an incredibly intense form of selective pressure. By treating bees, we remove this selective pressure and allow the susceptible colonies to thrive. Susceptible colonies produce most of the flying drones in an area therefore the queens don't get mated to drones carrying tolerance genes.
It is a vicious cycle, the only way off the treatment merry-go-round is for someone to bite the bullet and let a lot of susceptible colonies die, then breed from the survivors. Once survivors are established, introgress those traits into larger pools of susceptible bees until the entire population can be taken off treatments. The beekeeper can do a lot to speed up this process. For example, allogrooming traits are relatively easy to select by counting total mite drop vs mauled mites and breeding only from colonies with the highest percentage of damaged mites. Hygienic behavior is also easily selected using various brood removal tests.
It is not one magic bullet that will stop the mites, but several small effects added together can do the job. At some point, susceptible colonies have to be allowed to die and resistant colonies have to be the only colonies left to propagate. Brother Adam said so aptly that this is the inevitable fight and that beekeepers have to win it with genetics. In his day, it was trachea mites. That battle was fought and won in very short order because there were no huge efforts to treat bees. Breeding worked then and it will work now.
I've posted this elsewhere. This list is not exhaustive, it is the list of traits I know of that affect varroa tolerance.
The known mechanisms for varroa tolerance include:
Varroa Selective Hygiene - disrupts the reproductive cycle of the varroa mite
a. Detect infested larvae
b. Uncap infested larvae
c. Remove infested larvae
d. selection involves testing for hygienic behavior and removal of infested larvae
Allogrooming - bees grooming each other to remove mites
a. Varroa mauling - chewing and biting the mites which kills them
b. Selection involves monitoring for chewed mites on the bottom board
Breaks in brood rearing - during brood breaks, varroa cannot reproduce.
a. Heavy pollen collection - bees that collect pollen heavily are more sensitive to lack of pollen and shut down brood rearing earlier.
b. Sensitive to nectar dearth - bees that react to nectar shortage by breaking the brood cycle
c. Selection involves monitoring for bees that reduce brood rearing when pollen is unavailable
Reduced days to worker maturity - fewer days gives mites less time to reproduce
a. some worker bees mature in 19 days vs standard 21
b. using small cell foundation and timing brood emergence
c. Selection involves identifying the small percentage of colonies that mature workers in fewer days
Mite entombment - trap and kill mites in the cell
a. pupating larvae kill mites by trapping them between the cocoon and cell base
b. selection involves measuring and selecting for number of entombed mites
It's always difficult, leaning towards impossible with so many parameters to consider. I often think that those promoting their single silver bullet may be missing other factors just as or more important. Coincidence is unfortunately often interpreted as consequence.
