I have heard of two "origins" of Varroa which are present in the US:
varroa that came into the states from Europe and varroa that came into
the states from Japan.  This is hearsay, second hand from a beekeeper
who got it from a researcher who also keeps bees.  In other words, I
CANNOT document it.  But my understanding is that there are two distinct
strains of Varroa (European and Japanese) both with a common ancestry
(SE Asia).  It may be perhaps that one strain is more prone to developing
resistense to Apistan than the other strain, possibly the influence of
genetic selection in an island environment vs a mainland environment.
I do not know the geography of Italy enough to know exactly where the
resistant strain first originated, but might "islands" or "oceans"
be a common denominator rather than "product misuse"?
 
I don't think that assertions that resistence has not taken hold in
Northern climes are valid.  Pockets of resistence have been documented
in the US in both South Dakota and Pennsylvania, with a common bond
that the bees had been in Florida.  And again there's the possibity
of an ocean influenced environment.  Admittedly this is stretching it,
but this functional geographic illiterate pictures the ocean's (sea's)
influence on the climates of Japan, Italy and Florida to be at least
a remote possibility.  Hey, why not?
 
Anyway I don't think, no, I'm sure no one really knows.  In the meantime
look for resistent mites coming to a neighborhood near YOU!
 
Aaron Morris - thinking better answers only come from better questions!