The horn fly, Haematobia irritans (L.), is one of the most important blood-sucking pests of pastured cattle in North America. In the U.S. alone, annual losses in cattle production and control costs exceed $780 million. Horn flies are very common on beef cattle in Montana. When abundant, cattle experience pain and annoyance from fly bites, which interfere with normal activities of cattle including feeding and resting. The result may be decreased milk production, reduced weight gains and poor feeding efficiency. Consequently, it is important that livestock producers be aware of the economic importance of this pest and select an appropriate control method for its management.
Horn fly biology
Adult horn flies are 3/16 inch long and are about half the size of a housefly. They are dark gray in color, with two dark stripes on the thorax and a few faint spots on the top of the abdomen. While at rest on the animal, the wings are held partially open, forming a V shape. These flies are external blood-feeders and an individual fly will bite an animal and feed on blood 20 to 30 times per day. Depending on the infestation level, this can calculate into thousands of bites per day that an animal tolerates.
Both male and female flies spend the majority of their time in the summer on the animal, moving from the backs and sides to belly during the heat of the day. Females leave the animal to deposit eggs in fresh manure and then return to the animal to resume feeding. A female will deposit up to 500 eggs during her lifetime. Horn fly larvae, which are typical-looking fly maggots, hatch and develop in the manure. Ten to 14 days are required in the summer to complete larval development and transform to the pupal stage. There are several generations of flies during the summer. A rapid buildup of flies occurs in mid-summer with adult populations generally peaking in late summer. As daily temperatures decline in the fall, the horn fly pupae go into hibernation and spend the winter in the soil. Adult flies emerge from the hibernating pupae as early as April.
Horn flies are fairly specific for cattle, but in their absence they will feed on other animals such as horses. The flies show a preference for larger animals (bulls, cows, steers, heifers) and tend not to bother calves until the end of the summer when calves have grown. Cattle infested with 200 or more flies will bunch together, expend energy attempting to escape from the flies, reduce their food intake while combating flies and alter grazing behavior due to fly irritation. The pain and annoyance caused by these flies is often manifested in reduced milk production and lighter calves at weaning time. Nebraska studies demonstrated calf weaning weights were 10 to 20 pounds higher when horn flies were controlled on cows. Other studies around the U.S. and Canada have shown improved weight gains of stocker cattle and replacement heifers when comparing treated to untreated animals. For example, in a three year Louisiana study, yearling replacement heifers treated for horn flies had a 17 percent weight gain advantage over untreated heifers. Some of these studies have indicated that as few as 200 flies per animal will result in reduced feed efficiency and lowered weight gains. In addition, horn flies have been implicated in the spread of summer mastitis.
Because of the economic losses associated with horn flies they should be controlled on beef cattle. A Canadian study demonstrated that economic benefits (i.e., increased weight gains, increased milk production, better pasture utilization, and higher feed efficiency) are maximized if fly-free grazing is maintained for 115 days. Generally, insecticides are the primary method used for controlling horn flies because biological control and cultural tactics have proven ineffective. There are many effective insecticide application methods available to control horn flies on pastured cattle. These include dust bags, back rubbers (oilers), insecticidal ear tags, sprays, pour-ons, boluses, and oral larvicides (feed-additives). Because flies remain on the animal, self-treatment control devices are effective for horn fly control. Efficacy, cost, convenience, and herd management practices should be considered when designing a horn fly control program.