Mites in chickens are a pretty serious issue and left unchecked the parasite will cause hens to reduce their laying and the other hens in the flock to gang up and kill infected hens.
Mites like warm weather and in the winter months they are not much of a problem however the primary mechanism that chickens rely on for mite control, dust baths, is often limited in the wet months. I treat for mites year round as a preventative measure than than waiting for a problem to materialize that I need to react to.
Mites are parasite that lay eggs (lice) at the base of a chickens feathers. Adult mites will feed on the chicken and then very rapidly lay more eggs. They usually mass in vulnerable regions, like the vents and you can inspect hens for redness and swelling, as well as visual identification of the mites and eggs.
Hens that are not laying (and not molting), not going into the hen house at night, or look listless may have a mite problem. If the other hens in the flock are ganging up and pecking aggressively on a specific hen, it is very likely that bird has a mite problem. Left unattended to the other hens will kill the infected hen. It’s really very sad but a reminder that chickens can be savage to one another in the interests of the group.
I regularly employ two simple methods in my hen house and run areas to control mites.
Diatomaceous earth (DE) is a very fine-grained powder that is easily obtained and inexpensive. Ground up prehistoric fossils, DE under a microscope looks like a medieval weapon, each grain has sharp barbs that lacerate everything that comes in contact with it. For mammals, we’re not affected but insects are decimated when coming in contact with DE.
This is a completely natural product that is approved for organic farming. Affordable and widely available, you can order food-grade DE on Amazon in 5 lb. containers for about $20.
I dust the ground of my chicken run concentrating on the holes where the hens dust bathe themselves, nest boxes, and roosting poles with DE. These are the areas where the chicken will pickup DE on their body, which protects the chickens from mites.
In the summer months, usually beginning and then again in late summer, I will spray mixture of cooking oil and dish soap in the hen house to cover all the surfaces. This oil-soap mixture will disrupt the breeding cycle of mites.