By gaining some basic understanding of fly biology, particularly what encourages and discourages their reproduction, you can reduce fly numbers to tolerable levels, with no or only a minimal use of insecticides.
Often the simple release of Fly Predators, plus modest improvements in manure management and perhaps a few fly traps is all that will be needed.
More comprehensive approaches, including initial careful and selective use of insecticides, may be necessary for the fastest control if the pest fly problem has been allowed to become severe. This is due to the fact that most of the “natural” controls prevent future flies but do not affect existing adults. Since adult House Flies live approximately 21 days, you will either need to put up with them for that long or use traps and baits or careful pesticide application to reduce their population rapidly.
The first step is to identify the insect(s) that are plaguing you and your animals. With horses, the majority of your bothersome flies will likely be the House Fly (80%) and the biting Stable Fly (20%). When the fly is identified, then use the methods listed that focus on the prevention and control of that species. This is important as different flies can come from dramatically different places. Preventing flies fixes the problem. Just going after the adult flies you see only affects 15% of the population, the rest will be there bothering you in a week or so.
Using a preventive solution like Fly Predators or making the area less “fly friendly” requires that you treat the locations where the pest flies are reproducing. In almost all cases this is NOT where you see the adult flies that are bothering you or your animals. Often you will see the most flies in the barn but it’s highly likely that only a small percentage of those flies came from the barn. Instead they most likely came from the pasture, corrals, the manure pile, the neighbors animals, etc. If you’ve only been spraying, using traps, or baits you have to think differently when using prevention.
It’s been said that every creature has its purpose. For flies their purpose is to decompose rotting organic matter such as manure, dead plants and animals. If you have an abundance of material needing decomposition, you get an abundance of flies.
Flies have four stages in their life cycle (shown in the diagram). Adult House Flies lay eggs on or close to a larval food source of rotting organic matter (i.e., manure but also compost piles, rotting hay, etc.) the eggs hatch within hours into larvae (maggots). The larvae feed and then in no less than 5 days they pupate forming a cocoon. Inside this cocoon they metamorphosis into an adult fly which takes at least 3 days. This is the same process a caterpillar goes through as it turns into a butterfly. You can use these minimum transformation days to your advantage.
The pupa is light to dark brown and looks very much like a dark grain of rice or a rodent dropping. This is the stage the Fly Predator takes over killing that immature fly. The pupa is also the stage that flies “hibernate” when the temperature drops below 47℉ allowing them to make it through the winter. Note there are no “baby” flies, when they emerge from the pupa, they are full size. The smaller flies you see are just different species.