I've given up on store bought fly spray, plus with several horses it's much too expensive when you go through a whole bottle in a few days.
I started making my own mix:
2 cups vinegar
1/2 cup lemon juice
1 tbsp lemon dish soap
2 tbsp garlic powder
1 tbsp tea tree oil
Avon skin so soft (can't remember how much)
I put it in an empty fly spray bottle and fill the rest of the bottle with water. I find the bugs don't even land on them, and the horse flies and deer flies around here are the size of birds I swear...
They don't smell all that great but hey, it works! I also cut an old flannel sheet and nailed 3" wide pieces hanging down from our shelter and sprayed them with diluted citronella oil. They like to camp out in there when the bugs are really bad, and I lie to think walking through the sheets rubs some citronella off on them