Permethrin (either powder or liquid) is an effective treatment against an active infestation, as long as the mites don't have a resistance to it and you follow the instructions on the packaging. If you're looking for a preventative then the only real option that I know of is elemental sulfur: https://www.backyardchickens.com/th...-for-the-chickens-coop.1351484/#post-22277802