http://www.beeswax.co.nz/oxalicacid.htm

"A system is also used where oxalic acid crystals are placed on a heated
metal plate which is slid into the entrance of a hive.  The heat causes the
crystals to volatilise and the vapours kill the varroa."

No references given but sounds reasonable for the heat method. I thought
being tidy little housekeepers the bees licked up the residues around the
hive and in turn this gave either the systemic action or else the acid
burned the mite shell.

Still looking for reference papers.

Kevin Gibbs
Masterton
NZ

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