Manna Pro Opti-Force Fly Spray is a sweat- and weather-resistant fly repellent for horses. Opti-Force Provides repellency and quick knockdown from horse flies, house flies, stable flies, face flies, horn flies, deer flies, gnats and mosquitoes (Culex spp.) that may transmit West Nile Virus. It lasts for 14 days, even when your horse sweats or after rainfall. This ready-to-use formula requires no mixing and can be used on horses and ponies.