varroa destructor introduction and evolution

I heard on the CBC not long ago that bee colonies throughout North
America were collapsing inexplicably.     More recently, I read in the
newspaper about Varroa Destructor, which Wikipedia indicates was known
throughout the world for quite some time.  Why would it be a mystery
in North America not too long ago?

As well, the article said that the mother varroa enters the brood cell
and lays one male and upto 5 females.  The male then mates with his
sisters.  Isn't this inbreeding of the most intense kind?  Why haven't
they inbred themselves into dysfunctionality and unsurvivability by
now?