Topical fly sprays are by far the most commonly used products to keep flies and other insects off of horses. There are so many options out there it can sometimes be confusing about which one to use, but with a little knowledge the selecting process can be made much easier. One of the best ways to choose a fly deterrent is to simply read the label. Most fly sprays use the active ingredient Permethrin. It is one of the best insecticides still labeled for use on horses. Other active ingredients are usually added to protect the Permethrin from sunlight in order that the it will work for a longer period of time. Other additives can include things like citronella scents to help repel insects.
Another great way to reduce the fly population in your area is to feed a supplemental Insect Growth Regulator (IGR) along with your horses daily feed. These products work by sterilizing your horses manure, making it impossible for flies to lay their eggs in it. It is a program that can help to control the population before it has a chance to grow.