What bedding are you using?

You may have to move these guys inside or inside your garage for a few days/weeks and disinfect the WHOLE enclosure. Pain in the rear I know but you may have to take those necessary steps.
I've never dealt with mites but I remember growing up with lice and ticks from our dogs--once we comepletly disinfected EVERYTHING from inside/outside we were lice/tick free for life.