>Reduced virulence of mites has nothing to do with hb stock; rather it is an adaptation on the part of the mites.

Not sure we have the evidence to make this claim. What we do have:

1. There appears to be a global rise in DWV-B on landscape scales: In 'Deformed wing virus variant shift from 2010 to 2016 in managed and feral UK honey bee colonies', Dr. Martin's team documents the rise of DWV-B as the dominant variant in the UK and presages its dominance on a global scale (which in-fact is currently happening). This study is a continuation of work that began with the so-called 'Swindon Bees' and the question of whether DWV-B will in fact prove to be both: a reliably dominant DWV variant and; less virulent than the other identified variants:

'So far, DWV-B dominance in honey bee colonies may be beneficial, as DWV-B-infected colonies appear to be healthier than those infected with DWV-A [23], indicating that the disease induced by DWV-B may be lessening in severity naturally. The exact mechanism underlying the advantageous nature of DWV-B over DWV-A has yet to be identified; however, we demonstrate that DWV-A prevalence was dramatically reduced from 2009/10 to 2016, which coincided with an increase in DWV-B. This may be explained by genetic variations between the DWV master variants that favor host cell invasion and replication, as well as a potential tradeoff in DWV virulence initiated by the Varroa vectoring cycle.'

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8421296/

2. There have been a few recent studies which have examined the genetic make-up of varroa mites in two resistant populations (Gotland and Toulouse) in an attempt to understand whether:

The genetic signature of the mite populations in resistant colonies differ from susceptible colonies and;

The relative diversity of the mite population impacts their virulence.

In the study 'Population genetics of ectoparasitic mites suggest arms race with honeybee hosts' (2019 Beaurepaire et al):

'The mites from the mite-resistant population on Gotland were compared with a geographically neighboring mite population from managed mite-susceptible honeybees that have not experienced natural selection pressures. The changes in the genetic diversity and genetic structure was compared over time using microsatellites to compare historic mite samples collected in 2009 with current mite samples collected from the same populations in 2017 and 2018.'

'… these … analyses clearly indicate that V. destructor populations have changed within the eight years separating the collection of the current and historical samples. These changes led to a higher level of differentiation and an increased diversity of mite genotypes in the resistant colonies compared to the susceptible ones.'

'This observation suggests that a strong selection against the most common mite genotypes is occurring in the resistant colonies. To conclude, the above-mentioned evidences strongly suggest that the main factor responsible for the temporal changes in the genetic structure of V. destructor measured here are caused by the strong selection pressures induced through the co-evolution of the mite with its host, rapidly leading to observable genetic changes in the parasite population, potentially aiding their survival in adapted mite-resistant A. mellifera colonies.'

'In conclusion, the observed changes over time in the genetic structure of V. destructor suggest adaptations of the parasite, in a host-parasite coevolutionary arms race, most likely in response to selection pressures applied by the adapted resistant traits of the host. The magnitude of these changes between the historic and current samples of this study demonstrate a relatively fast response.'

https://www.nature.com/articles/s41598-019-47801-5.pdf

Building on this work, research entitled, 'Selection for outbreeding in Varroa parasitising resistant honey bee (Apis mellifera) colonies' was published by Conlon et al which:

'… investigated the potential for the evolution of counter resistance traits and selection for outbreeding in Varroa infesting resistant honey bee colonies near Toulouse, France.'

'Having identified colonies which exhibited unusually high rates of nonreproducing Varroa, we investigated the reproductive success of Varroa between singly and multiply infested cells. We then used genetic analyses to test if reproductive success was linked to genetic polymorphism in founding females. Selection for increased recombination, via outbreeding, could increase evolutionary rates in this Varroa population and indicate it is engaged in a co-evolutionary arms race with its host.'

'We screened four honey bee colonies, three (Mother; Daughter 1; and Daughter 2) resistant and one (Daughter 3) susceptible. The mother colony was produced in 2015 while the 3 daughters came from 2016. None of the resistant colonies varied significantly from a 50:50 distribution of resistant:susceptible pupae.'

'…only the nonresistant colony exhibited significant variation from a 50:50 ratio of Resistant:Susceptible pupae, with no intermediate levels of resistance, supported the previous identification of a single resistance-linked locus in this population and suggests Mendelian inheritance of the resistance trait.'

'Our results show that, when Varroa resistance is allowed to develop by natural selection, it is possible for a host–parasite relationship to evolve. The increased reproductive success we identify when Varroa co-infests the drone pupae of resistant honey bee colonies means that, in contrast to acaricide-treated colonies, there may be selection for outbred offspring. This, combined with a small proportion of Varroa reproducing in each generation, could reduce the selective pressure for the evolution of more virulent counter resistance traits and result in a more stable host–parasite relationship.'

https://www.researchgate.net/publication/342637280_Selection_for_outbreeding_in_Varroa_parasitising_resistant_honey_bee_Apis_mellifera_colonies

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