How you treat depends on what type of mite you have. Red roost mites will be crawling along the roosts at night. You can check for them with a flashlight right now. You will have to treat your coop if it's these.
Ivermectin pour on is a better choice than powder. Too easy to od with the powder. If you read the directions, you are supposed to use very little per bird. I also don't like breathing it in and it's almost impossible not to.
