Keith Malone posed the question about mite resistance:
I believe the varroa mite IS less likely to develop resistance to formic
acid or thymol in comparison to pyrethroids (Apistan, Bayvarol etc),
organophosphates (Checkmite, Perizin) or amidines (amitraz - Apivar) but I
don't think it's to do with different levels of efficacy against the varroa
mite. Many essential oil/organic acid treatments have given very high
efficacy, often as good as or higher than Apistan or Checkmite (before
resistance).

The three classes of pesticide listed in common use as varroa treatments
have very specific modes of action, the different pesticide classes working
on separate, precise processes during neuronal transmission in the mite.
Thickening of the cuticle is one mode of resistance to a pesticide but it is
very much a secondary method of resistance (common in flies for example).
The primary modes of resistance in insects and mites are detoxification of
the substance and modification of the site of action of that substance.
If the selection pressure, in the form of a pesticide has one specific site
of action, and the selection pressure is intense (eg same pesticide applied
over many generations) a population of insects/mites may develop a method to
block that substance or detoxify it within a (relatively) short space of
time. That resultant population would be called resistant.

Thymol and I believe formic acid too, are not specific in their mode of
action. Thymol is a protein denaturant (it is believed to break down cell
walls and disrupt general cellular processes. It would appear to be more
difficult for a target pest to develop resistance to such wide-acting
substances. The development of resistant populations also usually coincides
with extensive use of the one chemical class. Essential oils and organic
acids have been used successfully for quite a while in Europe but have not
yet taken over the world as varroa treatments. Even as they do become more
popular and necessary I don't see resistance looming.
I hope not anyway; I'm trying to sell Apiguard here. It would be a little
frustrating, after 8 years of product R&D and registration to now find
varroa resistance to thymol. All I can say is that when I started the
development of Apiguard in Italy back in 1993/4 pyrethroid resistant mites
were being decimated in the Apiguard trials. Today, Apiguard in Italy, for
example, is having exactly the same effect with no sign of resistance.
Let's hope the theory holds.

Max

Dr Max Watkins
Vita (Europe) Limited
21/23 Wote Street
Basingstoke
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UK

Tel.: +44 (0) 1256 473177
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web: http://www.vita-europe.com

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