There are a few all natural fly sprays that are made for horses and dogs. Farrier's Wife is one that I use for both my horse and my dog on long hikes. I believe they have a website if you search. Stop in your local horse supply shop and ask.. Just don't get one with lots of chemicals and make sure it says safe for doggies..
My favorite Repellent for Canines AND their Owners is an all natural herbal spray. The name is: "Herbal Defense" and is sold by "Only Natural Pet Store" in Boulder, CO. They have a wonderful online store. The spray consists of Neem oil, Lemongrass, Catnip, Citronella, & other essential oils. It smells Great! Can be used on Dogs, cats, puppies, kittens, and Humans...I use it on me, all the time....Good luck...Karla