Flies are the bane of every horse owner and rider. There are many options in the battle against flies including fly sprays, fly masks, sheets and leg wraps. I tend to do a multi-pronged approach that includes fly spraying before riding and using ear bonnets. For trail riding, I spray the horse very well and coat the ear bonnets with fly spray. Some of the more aggressive species of flies breed around the borders of marshes and streams, which you are more likely to encounter during a trail ride.
I let the horses have stall access during the day with fans blowing, which really helps keep them comfortable and reduce the presence of flies. For safety reasons, try to purchase barn fans with enclosed motors so the dust present in the barn doesn't cause the fan to short out or catch on fire. I also keep the barn as dark as possible, because some fly species prefer sunlight. Late afternoon/evening turnout reduces the amount of time the horses are being bothered by flies. Unfortunately, mosquitoes feed at dusk and dawn, so the horses aren't totally free from insect pests.
I have used fly predators with success on my farm. I have purchased mine from Arbico Organics. It is suggested to release the predators at dusk or dawn when the bird population isn't as active. I never remember to release at those times, and I think I ended up inadvertantly feeding the pupae to the birds. I have found that releasing the pupae under the edges of the tarps of my compost bins seems to allow hatching with minimal loss due to bird predation.
For turnout, I use a combination of fly masks, sheets and fly leg wraps. I use fly masks for the horses that tend to get "weepy eyes" that are further bothered by flies. I like the fly masks with the built in ears for additional protection. For light skinned horses or those with white noses, the extended nose fly masks will help protect from sunburn.
My favorite fly sheets are the ones made by Horseware Ireland. The Amigo Bug Buster has an insecticide built into the fly sheet to help repel insects. It is also a soft, comfortable material. I have also used the Amigo Mio, an inexpensive fly sheet also made from a soft fabric. If you have horses that play hard, these sheets will rip more easily than the textilene style sheets but they seem to be cooler for the horses to wear. When we have excessive heat warnings, I don't use fly sheets because I think it makes the horses hotter wearing them.
One of my favorite items for flies is the fly leg wraps. They are effective in reducing the stomping associated with flies landing on the horses legs. I can always tell if I've forgotten to put on leg wraps because I hear a lot more stomping! There are quite a few brands on the market. My favorite is the Cashel Crusader fly leg guards. These guards follow the contours of the horses legs, so you get a more custom fit with them than some of the other products. They don't tend to be as tall as some other fly guards, so that is one thing to keep in mind.
Indie wearing the Cashel Crusaders:
Kensington also makes fly wraps. The Kensingtons are taller than the Cashels, but I haven't had as much luck with them lasting as long. It seems that the material stretches more easily and then starts to separate. I know that many people like the Kensingtons and have had good success with them.
Indie wearing the Kensington Fly Boots:
Roma makes an inexpensive set of fly boots that are a good value. These boots are as tall as the Kensingtons and they have the tightest weave fabric of the three sets of boots. They are not form fitting, so they have a tendency to slip if not applied carefully. They have fleece at the top and bottom of the boots. I'd prefer the fleece only be at the top of the boot, because it attracts burrs otherwise. They will fit the widest range of leg sizes, from regular to draft. A petite legged horse would do better in the Cashels or Kensingtons. The Roma boots last a long time and are half the price of the others.
Indie wearing the Roma Fly Boots:
Even though it takes extra time and effort, I've found the best way to avoid problems such as rubs and sagging with all brands of fly boots is to take them off daily and rinse them thoroughly. I keep two sets of boots per horse and alternate them. In the mornings, I take off the boots, hang them on a pipe gate (this allows you to clean both sides of the boots simultaneously) and rinse thoroughly. I let them dry on the gate while I check the legs and put the alternate set of boots on the horse. I have found I can get several seasons out of one set of boots by doing it this way.
Fly sprays are a mainstay in the summer. I only use sprays before riding and use mechanical barriers (masks, sheets, fans) for the rest of the time to attempt to minimize chemical exposure. If you are spraying multiple horses, a garden sprayer works very well and is quiet to use for the more skittish horse.
I have also found that ear nets (also called ear bonnets) really help keep the horse from being distracted by flies while being ridden. They stay on best if placed under the crownpiece of the bridle. I rinse mine out after each use to keep them soft and comfortable for the horse.