Natural fly spray is a repellent, not an insecticide. If you think about it, this makes more sense, whether you want to be all natural, or not. For an insecticidal spray to work, the fly has to land on your horse, and in some cases, bite him! Exactly what we want to prevent. So rather than the harsh poisons, which cost more and can pollute our ground water, go with a natural repellent that creates a scent barrier around your horse and keeps (most) flies from even lighting on his sensitive, ticklish skin!
I have two that I rotate between: Liquid Net and Aloe Herbal Fly Repellent. I get both from Horseloverz.com.
I like to buy the concentrate, and mix it up myself- it costs a bit less that way. I have also bought the fly predators in the past. Can't tell you for certain if they make a difference, but I think they did...
Yay fly repellent! Now everyone can get back to enjoying themselves!