I had a huge issue with horse flies with both my horses and dogs. I started looking at solutions because the horses were running themselves ragged. I learned that repelants don't work because they bite as they land (so no time to repel). I bought a trap, http://www.bitingflies.com/ and it works great. It truly made a dramatic difference in the number of flies in about 10 days. It is expensive, but there is no reason for it not to last. Best part is there are no poisions to hurt you furbabies!