Ours is caused by our mulberry and pear trees :/ This is our first summer with poultry and I'm hoping they like mulberries. If not, hopefully they like the fly feast under the tree lol
I use store bought traps now but a setup like this works well.
http://www.thehealthyhomeeconomist.com/quick-and-easy-to-make-highly-effective-fly-trap/
I didn't read to see what they're using for bait but an inch or two of soda certainly gets them coming around.