When the H-trap is used to control horse flies nearby horses we recommend to place one H-trap with a maximum of 5 horses. Horse flies cannot see the difference between the H-trap and a horse. With a maximum of 5 horses we still have multiple chances a week to get the horse flies in the trap, and therefore built up a good control curve over time. Within a week one will then be able to see a noticeable reduction of the horse flies in the area.