It's not necessarily strange that only some seem to be affected. Some birds will naturally resist mites/worms better than others. Also, I have heard that mites prefer younger birds (although that was not my experience), so they may be more heavily affected. It's kind of like people - you know...
Yes, permethrin. I just looked at the container. It is "Y•Tex GardStar Garden and Poultry Dust ...with permethrin"
I didn't think I put it on that thick on any of them, but since it's been almost a year, and half the time I can't remember yesterday....
I do remember, however, that I paid...
Good luck with your hens. Was just browsing here while waiting (hoping) someone will answer my question. I wanted to mention though - I used "garden and poultry dust" on my chickens last year for mites. It seemed to kill the mites.... but it also killed my favorite chicken.
She was a 4 yr old...