I prefer a permethrin based "horse fly spray".. It's very effective, easy to use, affordable, labeled for use in chickens, does NOT require any egg withdrawal time, AND is safe for use on chicks!
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Found in the equine section under many names and ready to use or concentrated to mix in your own container. Make sure permethrin is the only active ingredient.
For adult birds.. a SMALL spritz directly to the skin with feathers parted, just below the vent, under each wing pit, and to the nape of the neck where the base of the head meets. Soaking is NOT required.
When treating chicks, I use a soaked cotton ball. Repeat in 7-10 days, use on nest, under roost, etc.
This product is considered not effective on scaly leg mites or depluming mites as those two species are systemic feeders and may never come to the surface.