Me anyway, I don't think anything undesirable was introduced with the carniolan semen, other than carniolan genetics. There is a theory that the semen introduced DWV. But before then I would very occasionally see a bee with deformed wings, I think the virus has always been with us, but just came into prominence when varroa arrived and helped it multiply.
About breeding varroa resistant bees from our own genetic base, it has been tried for many years, including by a program the government spent 12 million $$ on. Results have been zero.
Compared to America we have very limited genetics here, America, which was populated by Europeans hundreds of years before we were, brought in bees from all over the world, as for a long time there were no restrictions. As a result, out of the several million feral hives that existed when varroa arrived, there were some that had just the right re combination of genetics to resist varroa. As other hives died, these multiplied. So there are genuinely treatment free bees in America now. Also in Britain which also imported bees from every corner of the globe. Here in NZ, we have only had a few importations, and nowhere near the genetic pool of some other countries.
