Resistance (tolerance) to the presence of Oxalic acid by Varroa mites 
should be an interesting phenomena to study.

AFAIK, most resistance mechanisms are due to the inherent weakness of 
interfering with a metabolic process. The target organism shows a 
mutation to an involved enzyme pathway and Bob's your uncle!

Compounds that attack by physically causing damage may become less 
effective (due to few things being impossible) but I would be pleased to 
hear of such an example.

The talk of future OA tolerance is due to comparison with previous 
treatments.

Since the mode of attack by OA on VD is not clear - I suggest that this 
would be one position to clarify.

Does OA have an efficacy due to metabolic interference or an ability to 
  physical damage the mites?

Once the above questions are sorted the discussion on future resistance 
would have more of a solid footing than at present.

Regards,

Peter

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