I know what you mean, horse flys would be real bad one year and the next year there would hardly be any, all my horses wanted their butts scratched especial when the dear flys were bad.Years ago I raised Arabs and in summer I pastured them all out together. The horse flies were awful and it seemed like no matter what I tried it didn't last long and I would spend hours out with them killing the flies. It was amazing how upset people that came to see them would get when my stallion would come running up to us and present his rear when we entered the pasture.