Cider vinegar is a natural fly repelant
This sounds silly but different areas have different flies. Check the ingredients. I boarded in 2 different barns and had to change fly spray brands. Currently I use Farnams Tri Tec 14. I need waterproof and sweatproof. I use SWAT cream around ears and for trail rides. I use Pyranha for horse shows cause it puts a mirror shine on my black horse and protects him too.