Dick said:

> Since it's obviously true that mites DO adapt, especially to our
> pesticides then they are not just clones of each other. 

and Bill:

> so the mites could easily be clones but get pesticide resistance
> from the bacteria!

There is evidence out there of Varroa adapting to pesticides, but that adaptation seems to be via old-fashioned mutation rather than anything more exotic.  Zach Huang's group's work for example:

http://www.cyberbee.net/huangpub/2002JAR.pdf

shows that there were six mutations which appear to have happened since the separation of the two main types of V destructor.  The assumption is that this variation has occurred since the jump to A. mellifera, and shows that there is some diversity reappearing in this mite since its massive bottleneck at the point of the switch from A. cerana.

One or more of the changes in this gene could have converted the sodium channel protein to a pyrethroid-insensitive version - simply by mutation and not involving any other means of affecting the expression of a gene.

There is a useful discussion of the multiple sites of change which can lead to resistance here:

http://lasi.group.shef.ac.uk/pdf/Martin%2057.pdf

The starting point for this discussion was the likelihood of Varroa adapting, in an isolated situation, to become less virulent.  I doubt very much that Varroa could achieve this for two reasons:

- Varroa may be accumulating mutations in genes under heavy selection, but it still seems unlikely that this kind of variation can easily yield less virulent forms

- less virulent Varroa will almost always be at a selective disadvantage when types that multiply more effectively are around.  In mixed colonies, as where such types arise through mutation, how can it out-compete the established type if it is less virulent?

all the best

Gavin

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