Vickie....the permethrin is great stuff. I have used it religiously this summer, in the coop, on the roost bars, in the nest boxes, around the perimeter of the coop/house. I even sprayed the outdoor runs and it seems to have helped keep down not only the mites, but it seems the flies, mosquitoes, gnats all seem to be fewer this yr.Have you sprayed your roosts with liquid Permethrin ? It gets into all the nooks and crannies that the powder doesn't. DE is vastly over rated. It does NOT grind up worms in the digestive tract of chickens. It may supply a few minerals, depending on the source.I don't think your mites are coming in on straw. More likely on wild birds.
My feed store keeps it in stock in a concentrate and its pretty cheap....$6-7 and if I recall, makes 10 gallons which goes a LONG way