Good questions. One problem is, that the number of races used for professional (and hobby) beekeeping is already very small. Mostly queens are selected for laying eggs for peaceful and prodcutive workers, not so much for hygiene (removing infected sisters) or defense-oriented (beekeepers don't like that) bees.
As a consequence the current races might not be able to evolve resistance so easily. Earlier efforts to breed European bees in combination with (I think) Japanese bees both imported to the Philippines led to the import of varroa destructor mites.
Man is not so lucky with his manipulation of nature, it seems. Or choose another word.
Interesting. If the fly has evolved to infect bees, then it might take a while before the bees evolve resistance to it. Honey in America might be about to go up in price.
And how is the rest of the world making sure these American flies don't make it out of America?