In a message dated 16/03/02 05:01:35 GMT Standard Time, [log in to unmask] writes: << The down side of this is you may select for resistance to both at the same time and wake up to a surprise that both treatments no longer work. Cross resistance may also be obtained quicker. >> It depends how similar the treatments are. For example, although Apistan and Bayvarol are different formulations of artificial pyrethroid, they are so close that if the mites are resistant to one they will be resistant to the other. Although the mechanics of resistance can take many paths the one, in Europe at least, that the mites use is to detoxify themselves. Putting in in human terms, the treatment selects for the mites with the most efficient kidneys. Mites with this form of resistance will therefore have a head start in coming to terms with the next chemical treatment to come along. Alternating between chemical and bio-technical (physical) treatments would seem to be a likely way of postponing resistance. Chris