Hello Peter and All,
I will try and give you a answer but you must realize there are many
different opinions on the subject . Below is only my opinion.

> So, how come the America's continue to have this enormous problem?

I believe the problem lies with those of us which use the Italian bee which
IS the most widely used bee in the U.S..  In my opinion the Italian bee in
the U.S. has a hard time dealing with the tracheal mite. Maybe you are
exactly right and it is a widespread gene in the Italian U.S. bee which is
the problem. Most beekeepers using Italians are very aware of the Italian
bees weakness with tracheal mites.  I have got Carniolans which have no
trouble dealing with TM.

> And why has it not been dealt with to arrive at bees that are "resistant".

I know I am going to get flak over what I am about to say but say I must.
My Italians out produce any other strain of bee I have ever used by at least
a full super on average every year.   Every once in a while you get a queen
of another race which produces a huge amount of honey  but If you are in the
honey production business Italians are my recommendation.   Year after year
they out produce all others.  One needs to treat for tracheal mites with
Italians.  In the south a crisco/sugar patty might be all that is needed to
control TM.

> I realise that there has been importations to try and deal with the
problem - but as mentioned, why the continued great  disparity?

When TM first hit I went to Buckfast bees for the resistance to TM.   Small
honey crops made me go back to Italians.   The truth is I was using Buckfast
Bees from the Weavers..  One day I went to get queens and they were out of
Buckfast so I tried some of the Weaver *All-American* Italian line.  They
outproduced the Buckfast by a couple supers on average so I said the heck
with resistance and went all Italians and started treating  for tracheal
mites.
Sincerely,
Bob Harrison
Odessa, Missouri
Ps.. Italian bees have got other traits which can cause beekeepers problems
if they do not understand their bees but lets only do one fault at a time..