There are numerous different horse fly repellents on the market from completely natural plant based repellents through to chemical based ones. How well a fly spray works is determined by the concentration of the active ingredient contained within the spray. These active ingredients include DEET, PMD (p-menthane-diol), citriodiol, saltidin and various flower and/or herbal extracts.
When choosing a fly spray the strength of the repellent is given on the bottle as a percentage of active ingredient, e.g. NAF DEET contains 19.6% Deet. The higher the percentage, the more of a particular active ingredient the fly spray contains. It has become common to change the water based spray for a 'stickier' coat conditioner base which can increase the amount of time a fly repellent remains on the coat. For example the Net tex Advanced fly repellent can last up to 72 hours.
Natural fly sprays include Barrier Super plus, Red Zone products, Citronella summer sprays and Eqyss Marigold spray, all of which produce an insect repellent effect with DEET and PMD.