>  Why is it that mites have grown quickly resistant to Apistan but the
> bacteria causing foulbrood have been slow to resist terra treatment?

Resistance is a complex topic.  There are a number of mechanisms by which
resistance or tolerance can develop, a number of scenarios under which
resistance is likely to happen fairly quickly, and conditions under which it
cannot occur at all.  There are also apparently mechanisms -- both natural
and artificial -- by which resistance to a molecule or class of molecules
can be transferred between organisms which are only distantly related.

The mechanisms and conditions are different in each case.  There are some
obvious similarities between the resistance to OTC we see in some strain(s?)
of AFB and the tolerance we see in varroa exposed over time to fluvalinate,
however I doubt that there is a simple explanation.  And I doubt that anyone
has fully studied either phenomenon.

allen