Living way out of town with nearby properties running cattle and horses, flies can be a problem in the warmer months. Typically, November is the worst month- generally plenty of spring rain and the temperatures are warming up- ideal conditions in the weeks leading up to November for flies to reproduce. They appear in their thousands- it makes riding my bike at slower speeds, such as up hills, rather unpleasant as flies swarm around looking for a drink of sweat, and getting in my mouth.
There isn't much I can do about the flies on surrounding properties and roads where I ride, but around the house, there is a natural solution that can help in the garden too
Fortunately there is a good use for dead flies- they make excellent fertilser
I've used 2l plastic juice bottles, and even some 20l detergent containers from a cafe in town, to catch them in, they just need a few small flaps cut into it near the top on the handle (if it has one) side, so you can still pour the dead fly mix (DFM) out the usual opening without making a mess. Initially, you need to put a bit of water and something that attracts flies into the containers, but once a few flies die in there, greater and greater numbers of flies are attracted, they are just about queing up to get inside!
Chook poo, fish, meat, prawns etc usually do the trick in getting it started. Before too long, the flies are accumulating, and more water will need to be added so that new flies cant just walk around on top of the dead flies- they need to get caught in the liquid. Maggots start appearing, but they also eventually die in the liquid.
While the level is still at a reasonable level to pour out easily, and more liquid usually needs to be added at this stage, you can pour them out into a larger container for the "fermentation" phase. In summer, I generally find a few weeks in the sun is enough. Then it's just a matter of diluting the mix with some water and watering the plants and trees with it. Fruit trees thrive on it, and in my early experiments with it about 15 years ago, I picked over 1300 passionfruit from 2 vines in one season, they were fed solely on the DFM and kept well watered. The first year I collected the flies, I filled several 20litre containers with them! In recent years the numbers are greatly reduced, perhaps due to me catching a good number of them each summer.
Overall, I find it's a great way to make the outdoors more pleasant over the warmer months when the flies are thick, and to naturally fertilise the garden at the same time.