I scrub the coop each week with a weak solution washing up liquid and jeyes fluid and use DE in all the nooks and crannies, in the nest box and brush under the slats on the roof as much as I can. I've tried liquid Total Mite killer too. Last Tuesday and yesterday I sulphur bombed the coop and power washed it. I'm just concerned by power washing that I may be spreading the mites far an wide over the allotment as I know red mites live for about 8 months.
The problem seems to be the red mites are getting between the roof plywood and the wooden slats that are attached to it. It was a cheapish coop as I was new to chicken keeping. The three chickens are dusted as well and they have gone off lay but combs etc look red, so they are not anaemic. I'm feeling quite despondent about the problem.