There are many ways, often used in various combinations, to control horn flies in a cattle herd. Please consult your veterinarian to design a horn fly control program that fits your operation. Briefly, horn flies are killed by exposing them to insecticides in ear tags, pour-on preparations, sprays, dust bags, or back rubs. Development of resistance by the flies has been a problem with sustainable control. There have been two classes of insecticides used in control of horn flies: 1) organic phosphates and 2) pyrethroids. Recently, spinosins, a new third class has been introduced and is available only as a pour-on. Horn flies are controlled by adding an insect growth regulator (IGR) to salt/trace mineral mixes. Our Extension entomologist suggests inserting an ear tag or ear tags in spring when fly levels approach 200 per cow and treat with a spray or pour-on at the same time to knock down the number of flies. The tag will last three to four months. To prevent development of resistance, the tags should be removed when they lose their effectiveness. Then you can put dust bags out and service the dust bags every three weeks by filling them with an insecticide of the opposite class (organic phosphates alternated with pyrethroid products). One bag should be used per 45 cows. Another program would be to replace them with ear tags of the opposite class instead of using dust bags when the ear tags lose their effectiveness.