Merril solves the problem with his custom-blended fly spray:
The hardest part is finding pure eucalyptus oil. Don't get the fake stuff. A small bottle will run about $4**.
The biggest advantage to Merril's spray is that it stays put, leaving a thin harmless film where the commercial stuff evaporates or sweats off. It smells good, too, and is economical. Unlike similar concoctions, it does not attract yellow jackets.
I made labels (Karen's Bug-Off) for my spray bottles at the barn, lest someone mistake it for something else, and I listed the ingredients on the bulletin board so others could make their own. Everyone at the barn now uses, and swears by, Merril's special blend.
Merril sprays this concoction on his mules as needed and says it "works better than any of the commercial brands I've used." I, too, have tried a lot of commercial preparations and this mixture works better than any of them. It's really oily, and a light misting over the animal is adequate and doesn't sweat off as quickly as the other stuff, so you use less.
**UPDATE - Although eucalyptus oil is more readily available today (2018) the price is certainly a bit more than $4!