When I went to clean out the coop this morning I found more mites. These ones were all smaller and bright red (I think that means they are younger than the larger black ones). I took all of the bedding out of the nest box, sprayed the mite spray that I have been using and put down some more newspaper. When I cam back about an hour later there were quite a few dead mites on the newspaper.
I think this is going to be a problem for a long time…