I'm going to get some DE to dust them and maybe put in the coop under the shavings.
https://www.backyardchickens.com/forum/viewtopic.php?pid=2717718
DE will not treat an infestation. Use Poultry dust. Here is a copy from the above thread:
Poultry dust is "permethrin" in its active ingredient. (Always read the active ingredient). It used to be made of flowers - ground up petals. But these days it's made chemically into the same chemical. The reason I don't use Sevin is that it's devestating to waterfowl, of which I have a lot. It also kills honeybees (which are already having enough troubles with diseases) and it's just very very long lasting and dreadful. (Unfortunately I used it for years before and had no idea the toll it was taking.)
permethrin comes in dust and sprays. Dust is usually something like "Poultry Dust", "Poultry and Garden dust", "Livestock and poultry lice dust", etc. The active ingredient is the key. If you use it to prevent or kill mites, you can get liquid. Then it's still permethrin (generally under 10% solution) and called "Ectiban EC", or "Goat Lice Spray". I understand you can use some permethrin sprays on the birds too, but I just use the dust.
You can also use Adams Flea and Tick mist on crested birds, incidentally. It's handy for spot-treating heads, too. Roll a papertowel into a tightly pointed cone - spray the cone end well, use to wipe into their "beards" and around their face (tho not by their eyes). You can cover their face with your hand and spray forwards (from behind their head towards their beak) and ruffle that into their neck feathers in a pinch.
But permethrin dusts for the bird and sprays for the wood are awesome. They do have quick knock down, actually.