There is an article that one of our sponsors posted about flys adapting after time Insecticide choices
Its a really interesting article. Since I read it, I rotate my fly spray. I have added a natural one to my rotation this year. I got some that came with this years fly predators from Arbico Organics. SO far its working pretty well, but the flys aren't out in full force yet although mosquitoes seem to be out in good numbers
The Holistic Fly Guard is what I got. It seems expensive but it mixes with water.