For the past few years, I have been adding neem oil to my regular fly spray, and it does an amazing job at keeping the flies away. In addition, it is pretty viscous stuff (goes solid below 70 degrees) so mixing it with something that cuts it is great, and it stays on better than anything else I have tried. I also use a mitt to apply the spray- as opposed to spraying it on, and you seem to get a whole lot more milage out of a quart of spray that way.
Downsides? It smells nasty, and it will discolor white markings- not permanently- but not so you’d like to go into the show ring looking a bit yellow! The plusses totally out weigh this, however.
There is a someone (I think they are in FL- no, it isn’t me) on Ebay selling 2.5 gallon containers- which is a lot, I know, but you can get a great price, and share with friends. We also have kept it over the winter, and it was fine the next Spring.
Just thought I’d share- I know people are always looking for help with those pesky flies, this time of year. :winkgrin: