> 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