Well, if oxalic acid solutions are absorbed through the mite exoskeleton then it might be directly analogous to formic vapor in terms of surface area to volume.  I did a little google-fu but only found one reference to a paper (below Nanetti 1999 which I havent read because I can't get it via the net) that suggested formic acid acted through respiratory inhibition, which would suggest that OA and lactic acids would be acting through some other mechanism.  It's really hard to say, studies have shown that other low molecular weight acid such as acetic, citric and propionic acids are not effective at all as miticides.  Could be that OA is better absorbed through the exoskeleton, clearly formic is much more volatile than all the others so respiration could be a significant pathway for entry into the mite.  OA is somewhat unique since it is a di-acid and the first proton is approximately one thousand fold more acidic than acetic acid and a hundred times more acidic than formic, lactic, or citric acids.  There might be a corrolation with acidity and effectiveness of these acids and that due to formic's high volatility it can get to higher levels in the mites and compensate for the lower inherent acidity by achieving a higher concentration.  tough to say... the link after the reference is to an excellent new zealand ministry of agriculture and forestry paper (in pdf) that discusses all the available mite treatments with a lot of good references. 

Mark

Nanetti, A. 1999. Oxalic acid for mite control – results and reviews. Coordination in Europe of research on integrated control of Varroa mites in honey bee colonies. Merelbeke, Belgium, Commission of the European Communities, 6-11.

http://www.delta-business.com/CalgaryBeekeepers/Bee-Club-Library/Bee%20Disease%20&%20Treatments/Review%20of%20varroa-treatment-options%20NZ..pdf

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