Mite treatments need to put on the hive by around Sept 10th.
Waiting to treat can jeopardize the bees ability to survive. Mites build up with time. The mites can cause irreparable harm to the bees as time goes on. Most bee researchers put around Sept 10th as the latest date to treat for mites.
Join the mite a thon, that is put on by pollinator.org. This is a North American effort to see what the mite load on a massive scale.
Nature's Nectar LLC does sell the Univ of MN mite test kits.
http://pollinator.org/miteathon