this came out a while back but gives a clue as to how or why some populations of honey bees can coexist with varroa for a long time.

> Here we show that despite Varroa feeding on a population of 20-40 colonies for over 30 years on the remote island of Fernando de Noronha, Brazil no activation has occurred and DWV loads have remained at borderline levels of detection. ... we predict that this honey bee population is a ticking time-bomb, protected by its isolated position and small population size. This unique association between mite and bee persists due to the evolution of low Varroa reproduction rates. So the population is not adapted to tolerate Varroa and DWV, rather the viral quasi-species has simply not yet evolved the necessary mutations to produce a virulent variant.

Brettell, Laura E., and Stephen J. Martin. "Oldest Varroa tolerant honey bee population provides insight into the origins of the global decline of honey bees." Scientific reports 7 (2017): 45953.

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