We have discussed this several times on our TV show. We use Victor fly traps. They are large jars that you put some fly bait in and when the flies crawl in they can't get out. You can get them at your local farm supply store or through Jeffers supply. Through Jeffers they show the smaller jars, but when you select it you can then choose the gallon jars, which is what we use. You would not believe the number of flies you will catch in those things! Thousands! Then you can empty them and refill. We hang them on the outside of the pens. We put out about one per pen, but if they fill up in 10 days or so you should put out more. They won't eliminate flies, but they really cut down on them. Nothing makes you feel better than seeing a jar full of flies you've just eliminated!