I use Sevin dust in my coop & have never had a problem with mites or fleas. I never dusted my hens, just the floor of the coop, roosts & nest boxes beneath the pine shavings I use in the coop & boxes. I just added 2 baby chicks (going on 3 weeks old) and have sectioned off part of the coop for them. I have a plywood floor in the baby coop with shavings on top of that so there was no contact with the Sevin I used in there. How old should chicks be before I dust Sevin under the shavings in their coop?