Oh ok , so mites are very hard to see it could be lice that you have seen, either way they will make your chickens very ill. They have a tendency to suck the life out of them, cause anemia and play havoc with the immune system. Check just below the vent for egg sacks, like these.
You need to pay particular attention to the vent area, under the wings and behind the neck when dusting. It's also important to re-apply in 10 days, and clean the coop out from top to bottom. Sprinkle dust in the clean nest boxes and add some dust to the dust bath area.
http://parasitipedia.net/index.php?option=com_content&view=article&id=2540&Itemid=2816
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I love the wonderful advice and references that FancyChookLady posts - always a great help!
In our case for lice/mite health maintenance we don't use powders - our climate is very hot here in SoCal so I use an organice OMRI Poultry Protector liquid spray for parasite maintenance. The reason I use the liquid spray other than it being organic is because chickens are prone to respiratory issues so dusts and powders can be an irritant. In fact we had a couple Leghorns that would sneeze for an hour straight after taking ordinary dust baths. We certainly didn't want to add pesticide powders, DE, baking soda, or powdered vitamins to their coops or feed to keep the flying powders down so we tried the Poultry Protector per very thorough label instructions for both the coop and on the chickens. Our hens have been parasite free for 3 years. We had 9 and 12 week old juveniles ordered and shipped to us that were crawling with lice and we immediately treated with the Poultry Protector while they were in quarantine and it works wonders - no flying pesticide powders or dusts around them. We treat at roost time to give the Poultry Protector time to do it's magic on the chickens before they wake up in the morning for their dawn dust baths. This is a routine on coop and chickens we do about once a month during very hot weather and maybe once every two months during cooler seasons. I once had to clean a broken soft-shell egg in a nestbox crawling with ants. A couple short spurts of the Protector on the ants/broken egg and it immobilized them from crawling up my arm during the clean-up and I didn't have to breath flying pesticide powder in the air.