Have you tried mixing the permethrin with white vinigar? I found that on line just a few days ago. Its a 50/50 mix. I also use permethrin and it works ok, but Im starting to see some great results mixed with vinigar. And as Im sure you know, this is a super cheap combo. Im not positive on how it works for deer flies but it is working great right now for stable flies and horse flies. My horses normally spend the majority of thier day in the barn (free stall) since spraying them with this mixture, they are out grazing all day long and very little tail swishing. Im loving it cause that's less poo I have to scoop!!!