"Keith Malone"  asked why I understood that it was unlikely that varroa
would develop resistance to thymol and organic acids.

I am not a scientist, but I read - do not ask me where! - that pyrethroids
have long, complex molecules.  Resistance occurs when the the target
organism develops an enzyme which is able to split these molecules and
render them harmless to it.  Substances such as thymol or formic acid have a
much simpler structure which cannot be broken down in this way.

Perhaps others with more knowledge on the mechanism of resistance would care
to comment?

Peter Edwards
[log in to unmask]
www.stratford-upon-avon.freeserve.co.uk/

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