The permethrin worked great on the old girl. We're using Elector so I feel like the concentration will be appropriate. I think the heavy infestation was due to her age and illness. We are also moving the coop and going to put some more biosecurity safeguards in place (we weren't careful about keeping food away from wild birds, we may not have always fed them the optimal diet). We're newbies and these were rescue chickens, our neighbors said they were going to kill them or give them to us. I think they had a good life for their 10 months with us.