I had my first experience with mites this wet wet summer here in Colorado.
What I did that worked well for me...
I took a large tote and put 3 of the large coffee cans filled with DE in it mixed with about 1/4 can of sevin dust 5%. I grabbed a hen and set her in the tub then worked the mixture all through her feathers paying close attention to the vent area and under the wings. Grab another hen and repeat.
It is very messy and they do not like it at all. ( Wear safety goggles and a dust mask )
I then removed all bedding from the coop and set to dusting the entire coop with the same mixture. I payed special attention to anywhere that wood meets wood.
I did treat the nest boxes with straight sevin since that is where the mites were found.
I did not allow the hens back into the nest boxes for 5 days so I could leave any bedding out and watch for mites.
I repeated the treatment one week later and continue to add DE to all the shavings I put in the coop.
It has been about 2 months of continued wet weather and no sign of more mites.
Side note the mites in the nest boxes stopped moving almost instantly when I put the sevin dust in.
The only area I did not coat in powder was the ceiling since I cannot reach it and was very tired after scrubbing powder into 5 coops.
I hope this helps.