I have used a garden dust to treat mites previously. I treated each chicken and also the coop and nest boxes. Well I just took a look and they are back. I am using the deep litter method currently ( not sure if I'm doing it right or not???) . So what is the best advice on treatment and maintenance of these pesky little mites. They are the light brown ones seem to be mostly around vent. I will be treating each chicken tomorrow and also the house so advice is needed 
Also I have 80 chickens just FYI and 2 large coops with a lot of ventilation. wild birds ALWAYS decide to build in the gutters of the large shed/coop. Plus my birds free range and will pick them up from anywhere
My teenage birds don't seem to have them, they live in a different coop. So I'm wondering if it is just the older ones. They are about 9 months old now.

Also I have 80 chickens just FYI and 2 large coops with a lot of ventilation. wild birds ALWAYS decide to build in the gutters of the large shed/coop. Plus my birds free range and will pick them up from anywhere
My teenage birds don't seem to have them, they live in a different coop. So I'm wondering if it is just the older ones. They are about 9 months old now.