I would think they would be all right now, but do they need it? I wouldn't do it unless they need it. I had a chick that was 1 or 2 days old that I dusted once, the poor thing was covered with them from the nest its mama chose to 'roost' in. It was almost dead, and I put some DE in my hand and rubbed the baby with it real good. It's outside doing fine now
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They don't need it...actually their coop is on the side of the property farthest away from the wild bird population. The older girls' coop is close to area with high wild bird population, and I've noticed they are more prone to mites...which is why I got into habit of dusting monthly (keeps the numbers of mites under control).
I agree, permethrin is very toxic. Used in appropriate amounts it can be helpful treating mites but I wouldn't use it unless you need to get a handle on an outbreak. If you just happen to notice some minor mite symptoms, I'd work through the less extreme options first like DE and some of the mite spray that you can find in pet stores before I would use Permethrin.
Not saying it doesn't have it's place, just know that it's powerful stuff. A little goes a long way.