Permethrin 10 spray is very good to treat for lice or mites on poultry. Lice will be killed, and if the permethrin is reapplied at 10 day intervals, it will kill the newly hatched lice. There is also a permethrin poultry dust, and both are sold at TSC and other feed stores. That would be much better than dipping the chickens, and it is approved for poultry.
One bottle of this mixed with water 37 ml per gallon of water, can be used in spray bottles, and also in a garden sprayer to treat coop and nests when beeding is removed:
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The dust is applied as it is:
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