Flies in horse stables are a nuisance, just a few flies can ruin the ambiance of the nicest facility. You are never going to have a horse stable 100% fly free but there are steps to be taken that can be done to cut the fly problem to a very few.
The first step is the most logical one, Clean the stalls daily, if not twice a day.
Female flies seek out areas of fresh manure or decomposing organic material to lay their eggs as they feed on a variety of moist, fermenting organic matter, manure, rotting hay, feed silage, or garbage. The cycle from egg to adult takes from 7 to 14 days, depending on environmental conditions. Roughly 130 eggs are laid at a time in what is called the pupae. Limit the breeding areas and you limit the breeding. Removed manure and fly breeding materials should be composted or spread thinly in a field or pasture.
Beneficial insects, tiny fly parasites are a long term solution as they kill the fly larvae before the flies hatch and start to lay eggs. The female fly parasite instinctively knows how to find the fly pupae, bore a hole in the side and deposits her eggs inside. There the parasites feed on the fly eggs and larvae, breaking the life cycle of the fly.
Fly traps and sticky paper will capture adult flies. They may be most useful as a means of documenting fly numbers over time. A significant increase in catch from one week to the next can be a warning to check on sanitation and to increase fly control measures.
Insecticides can be used to kill adult flies after a problem has developed. While they can help to reduce fly numbers, they do not address their source – moist breeding areas. There are many alternatives for fly control but they should be viewed as a temporary solution until the root cause of the problem can be corrected. Large numbers of flies mean lots of flies breeding and a situation that cannot be corrected by insecticides alone. Flies can develop insecticide resistance, do not apply the same insecticide repeatedly throughout an entire season.
Foggers and misters can provide a quick knockdown of flies, especially in enclosed areas. Systems vary from foggers to timed release aerosols. Usually, these are pyrethrins with very short residues so treatments have to be repeated. Always read and follow directions when using a toxic pesticide.