There are a number of products which will kill external parasites of chickens. Rotating between a variety of the ones that are effective against the bugs your particular birds are experiencing will help prevent development of resistance to the pesticide.
My experience and that of many others is that Sevin is quite effective; to the point that there is concern that it kills too many "good" insects, in particular honeybees. Use to control external parasites on animals is a per label use.
I will say that for on-animal application I prefer sprays to dusts. I do put sevin or permenthrin dust in nest boxes (especially when a hen is brooding eggs or chicks) and areas where the birds tend to dust themselves. I will put it underneath bedding, but am leery of putting any sort of chemical pesticide where they search for and sometimes find food. However, I spray all wooden parts of their coops with an alternating set of chemical pesticides, one of which is sevin. I heavily mix DE into bedding materials, which helps keep them dry and also helps prevent bugs. I regularly use ivermectin pour-on on my birds, and as necessary bathe them in flea/tick shampoo (which also kill lice and mites) or use spray.
) good luck!
