Pyrethrum is the product made from chrysanthemum flowers. It is natural, yes, but it is also toxic. I think that food grade diatomaceous earth would be the best thing for them. You might want to check what you are using for bedding and see if that might be contributing to the problem. It may need to be changed.
Can you get chickens to eat garlic? I wonder if that would help.
I just did a search and found a video on YouTube! http://www.youtube.com/watch?v=hgFHJzXTk5c
I do not know if I would use ash, though. Spraying eucalyptus on chickens seems like it might be irritating to them, but maybe all right if diluted well enough. Just don't get it in their eyes, or yours! After changing the bedding I would recommend taking your clothes off outside (or in the garage), and spray with orange oil & water mixture before bringing them inside to wash.