Hello Rick & All,
Rick wrote:
 Why is it that mites have grown quickly resistant to Apistan but the
bacteria
 causing foulbrood have been slow to resist terra treatment?

Darwin couldn't have found a better example of natural selection than is
provided by the way the mechanism of resistance operates.  At this point we
are not quite sure why the resistant bacteria to foulbrood has happened and
several theories abound.
We do know why the resistance to Fluvalinate.
Quote from *Silent Spring* by Rachel Carson pg. 273
"Inevitably it follows that intensive spraying with powerful chemicals only
makes worse the problem it was designed to solve. After a few generations ,
instead of a mixed population of strong and weak insects, there results a
population consisting ENTIRELY of tough resistant strains."

The chemical industry is perhaps understandably loath to face up to the
unpleasant facts of insect resistance.  In 1959 ,with more than 100 major
insects showing definite resistance to chemicals, one of the LEADING
journals in the field of agriculture chemistry spoke of imagined insect
resistance.
A review of my resistance posts from last year quoting *The Varroa Handbook*
will show varroa is now resistant (in most areas of the world) to the most
effective chemicals we had in the world.  Over 100 chemicals had shown a
certain degree of Varroa control when the book was written in 1989. Varroa
will now (as Rachel Carson says) become harder to control with chemicals.
The stronger the chemical (98% control) the quicker the resistance.  Many
insects were resistant to DDT in 6 years.
Bob Harrison
Odessa, Missouri