Can anyone tell me how old chicks have to be before you can apply DE directly to them and to use in their bedding? I have three baby chicks, all 3-weeks old. Two look fine; one looks ratty and itches a lot. Acts fine in all other ways, and I don't see or feel anything on her. I want to rule out mites/lice and thought I'd apply DE as a preventative, but I want to make sure they're old enough? Can anyone help?