I was surprised to learn that since import of live bees into the US has been forbidden since 1922, our gene pool of bees in America has been dwindling - and part of that is varroa resistance:
http://www.npr.org/sections/thesalt/2017/07/13/536884827/no-offense-american-bees-but-your-sperm-isnt-cutting-it?utm_medium=RSS&utm_campaign=news
Part of the problem is certainly a reliance upon a relatively small gene-pool from initial importations - but perhaps the activities of commercial queen-breeders should also be examined. For example: Steve Sheppard discovered that 900,000 queens were raised from just 600 breeders - that's 1500 daughter queens from each queen mother, an activity which - year on year - must contribute towards a serious reduction in genetic diversity.
http://www.meamcneil.com/SheppardABJms.pdf (part one)
http://www.meamcneil.com/SheppardMs2ABJfinal.pdf (part two)
LJ
LJ,
That does sound like trouble. To make it worse most of those queens that they used probably came from the same source.
Jim