Simple answer is that OA dribble/vapor doesn't penetrate the cappings, of
either honey or brood.

OA is basically a dust, not necessarily a vapor.  I'm not a chemist, but
I'm guessing that formic acid has a much different vapor pressure than
oxalic acid.  Formic's a liquid and it vaporizes and manages to pass
through wax cappings, kill brood, etc etc., but OA basically stays as a
solid even after being boiled and injected into a colony.

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