Wild birds bring poultry mites and lice, and it can happen anytime, unless your flock is never outside or visited by those miserable house wrens. Checking monthly at night with a flashlight (around the vents) is best, and only treat if needed. I have used permethrin powder, but recently discovered permethrin spray, and will never go back! Spinosad is also approved here, but is harder to get, and more expensive. I'll save it as an option in future years. Carbaryl is no longer approved in the USA for chickens, so I'm not using it. DE has limited usefulness in the face of an outbreak, and is nasty to inhale, so I also don't use it, in spite of the many online testimonials. I have lost nice birds to mites, and still feel very guilty about it. Never again! Mary
Totally agree about the Permethrin spray! It's so easy to use compared to dusting! (Although, spray isn't a good thing to use in the middle of winter if you live somewhere cold, so we still have to dust half the year. lol)