In most parts of the country, those pesky flies are beginning to hatch. Besides just being a pain in the neck, flies can transmit diseases, so an effective fly control plan is a necessity.
To develop a good fly program, think of the lifecycle of the fly. First and foremost is to prevent the flies from breeding. Then it's necessary to keep them from hatching. The final stage is catching and killing adult flies.
No single fly control method can control all those stages, however, there are easy and relatively cost-effective ways to fly-proof your horse's environment.
To prevent breeding:
Clean up manure daily. Eliminate areas of standing water.
To prevent hatching:
Oral feed-through wormers kill off the larvae in the stomach. Additionally, fly predators, which are nocturnal wasps, eat fly larvae.
To kill adults:
Pesticides, fly traps, strips and zappers can be used to reduce the adult fly population.