I used to think that red mite just 'fall from the sky', but whilst that is true the main reason you get them is buying in infected stock. Red mite don't just live in the coop and come out at night, some stay on the birds so get transferred when the birds move. Any new stock must be quarantined for two weeks, to avoid contaminating the main coop. If you do find red mite in that period they must remain quarantined until they have been completely eradicated and that isn't easy. Chemicals simply breed resistance. The answer is a combination with a steam cleaner hitting the coop and chemicals on the underside of the perch. It will be at least two more weeks after repeated deep steam cleaning the coop, paying particular attention to all the joints in the assembly. Works for us- haven't had red mite in our flocks for 10 years. To give you an idea of how many red mite chickens can carry we steam cleaned a coop to the extent that nothing could possibly survive. The next morning I removed the perch that two Orpingtons we had bought had roosted on and counted an approximation of 2000 underneath it. The mites were killed and the perch replaced. Next morning 200 so the the process was repeated until after two weeks the count was zero.