Good to hear that their good breeding work is still alive in bees around US. Queens are like babies to a breeder. They make you immortal.
I don´t know much about their operation, but I remember once reading that they considered 50% losses to be the best level in varroa resistance breeding. My own idea was around 30%. The idea being that with too small losses breeding is not going forward, and with too big losses you will lose also valuable hives, which just were unlucky to get an extra load of mites somehow.
Well, as we say in Europe, everything is bigger in US.