DE will not treat mites as they hatch in the wood, and you should never dust a bird with DE. Mites spend precious little time on the bird, what they do is devestating.
For mites, the best thiung to do is check both in the day and at night - when the birds are on the roost, and several times over two weeks. Also look for ashy substances in the eyes of the wood, in the cracks, etc. Or just do a preventative spray of Permethrin (not pyrethin, and certainly not carbaryl) on the wood of the coop, dust your birds with permethrin dust. Permethrin spray is simply goat lice spray (10%). Use it straight on the wood. Since industries use a diluted version directly on the bird by spray methods, you'll be using it safely and putting the spray where the mites usually are as well as their eggs. Redust the bird with permethrin dust every 7 days for three times total for best effect.
Lice are similar although they spend most of their time on the bird, laying on the bird.
And you'll get mites despite DE because sparrows and wild birds bring them.