Flea Repellent for your Pets! 5-6 lemons, sliced. 2 1/2 cups of water. Combine in saucepan. Cook on medium for 20-30 minutes. Let cool. Move to spray bottle. Spray your pet weekly, avoiding eyes and any irritated skin. The lemon mixture will make your pets skin more acidic; not so tasty for fleas and other biting insects.