Sevin dust is used regularly by folks with mite problems. I used to use it myself, but switched over to Eprinex. It's applied topically and beats the heck out of having to dust birds.
Have you done a mite check? You generally see mites after dark. Sneak into the coop at night with a flashlight and pluck a bird off the roost. Check them over by the light of the flashlight paying close attention to the vent area, which is where mites like to hang out. Mites are so tiny they can be hard to see. They look like tiny moving dirt particles. If you can't find any evidence of them on the birds and the birds don't seem to be ailing, then I would just assume that they are molting.
If you have mites you will need to clean out the coop and treat all the birds and then repeat the whole procedure in 7-10 days. Mites live in the coop as well as on the birds.
Good luck.