>If resistance is defined as some kind of bee characteristic that influences mite levels so that colonies have a better chance of survival.  

Another recent research theme that seems to present the most promise for selective breeding in high-mite pressure environments without dismissing the other important contributions of colony and behavioral traits that support resistance is the role of Brood Effect (identified in both naturally-resistant and artificially selected populations):

'Host brood traits, independent of adult behaviours, reduce Varroa destructor mite reproduction in resistant honeybee populations':
https://www.sciencedirect.com/science/article/pii/S0020751923000929#s0055

‘Mite reproductive success rates did not significantly differ between treatment groups of either caged brood or brood exposed to adult bees and their possible removal behaviors, irrespective of the population’s genetic background (χ2 = 2.45, degrees of freedom (df) = 1, P > 0.11). The only variable that did influence Varroa mite reproductive success was the population’s genetic background, irrespective of treatment (χ2 = 44.51, df = 3, P < 0.005).’

‘Delayed egg laying was the most common reason for failed mite reproduction across all populations in this study, similar to a pan-European study assessing mite reproduction (Mondet et al., 2020). However, the absence of male mite offspring was significantly higher in the Swedish and French populations, which also have on average higher overall mite reproductive failure, compared with the Norwegian and control populations.’

And at least one cluster of studies that I am aware of (below and attached) has reported a single gene for resistance, exhibiting signs of Mendelian inheritance.

'A gene for resistance to the Varroa mite (Acari) in honey bee (Apis mellifera) pupae':
https://repository.up.ac.za/bitstream/handle/2263/73608/Conlon_Gene_2019.pdf;jsessionid=37BEA5AF617E41A9D88DF51CE11B8A45?sequence=1

'Selection for outbreeding in Varroa parasitising resistant honey bee (Apis mellifera) colonies':
https://onlinelibrary.wiley.com/doi/10.1002/ece3.6506



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