Horseflies, deer flies and the yellow fly are in the same family and can be effectivly trapped. Visit http://www.bitingflies.com for more information.
Or just to improve this answer a little, you could go and buy some fly spray? Its quite cheap and you can get it at all local tack shops near you :) if your horse has a problem with flys in his/hers ears i'd recommend you get the fly gel as you can rub that in there ears or you can just spray a small amout on your hand and rub AROUND the ear, not in. Hope this helped :)