Are you ready to protect your horses from flies this season? Economical in this gallon refill size and effective, Pro-Force Fly Spray can be used on horses, ponies, dogs and their environments. Try Pro-Force Fly Repellent from Manna Pro. Effective for up to 14 days, this powerful fly spray has a rapid knockdown and kill for almost 70 species of nasty insects, including those that spread West Nile Virus.