I have found one chemical solution extremely effective, but not cheap. Avid is
the miticide Abamectin from Merck. It is totally lethal to mites in very low
dosage and has very low human/animal toxicity. Alas it costs $80 - $200 per
quart. One quart will last a home grower a lifetime.
Note: I use it on all my orchids and my small collection of aroids only if
mites are actually present. I don't recommend any pesticide on a prophylactic
basis. If you have any doubts as to its effect on a particular species, test a
few plants first. It is listed as harmful to certain ferns, for instance.
Clark T. Riley