The age old DE divider. Most chicken owners seem to feel VERY strongly one way or the other when it comes to the effectiveness of DE. From my experience, DE can be an effective deterrent, but I've never had to count on it for mite treatment. I use it in my coop and run, but (touch wood) have never had mite/lice problem. That being said, if I did develop a mite problem, I would be using a chemical solution to the problem and would not count on the DE to clear it up. The advice about checking your birds at night is your best first step, be sure to check all your birds though! It helps if you're able to allow the birds to settle again once you're in the coop before turning your flashlight on their backsides, just remember to be patient.