Last Spring I built a Manitoba fly trap. They can be purchased for about $200 and last a few years. Mine cost maybe a hundred and it's built out of plywood and metal screening and a beach ball. Ugly but it trapped a lot of horse flies. This is a large trap standing close to 6' tall with a base of about 5'x5', triangular in shape. Horse flies circle the swinging black painted beach ball which hangs down the middle then fly upward where the screen allows plenty of light. As they continue their way upward they wind up in a gallon bottle up top with a mesh cone which prevents them coming back down. Most didn't make it to the bottle as sparrows were swooping in there and feasting on the flies as they were feeding their young. Stable flies, the biters, unfortunately can come from thousands of miles distant, flying as high as five miles and riding the prevailings. This is why they are so difficult to control. The best method during the day is to allow the horse to enter a dark barn, shelter with canvas tarps, old carpeting covering the openings and windows. The darker the better. It's best if the ones covering the doorways touch the ground and then some to reduce little streams of light coming in. Neither flies nor mosquitoes like the dark and will stay out.