I have 2 hens that have irritatated skin near their behinds. I suspect external parasites. When I first got them I dusted them w/ DE. Then the other day I decided that the DE didn't appear to be working and need something a little more harsh, so used approved chicken poultry powder w/ permethrin in it. How long will it take before I no longer see red/irritated skin? How often should I dust them? They are currently isolated from the rest of my flock?