Formic acid was approved for use in the U.S. wasn't it? My understanding is
that problems with packaging became a major hurdle for the company that was
going to sell it. As far as immunity to acids such as formic and oxalic,
wouldn't it be difficult for mites to become resistant to an acid. Simply
because bees survive the stuff doesn't mean it is harmless to them. Bees,
to my understanding, don't particularly like having formic acid in their
hives and I believe it does at cause at least some small amount of harm to
them at times.

Regards,
Dick