The Guaranteed Horse Products Fly Bye! Plus Horse Insect Repellent Spray works to protect your horse from pesky insects without using any pesticides. This spray works on a molecular level, creating a molecular change in a fly’s exoskeleton that encourages the fly to move away from the area. In addition to protecting your horse from flies, the spray contains geranium oil for protection against mosquitoes and gnats, too. It’s non-toxic and safe for your horse and the environment.