Parasite control is an important and profitable part of a cattle producer’s yearly management program. The most common flies afflicting cattle include horn flies, face flies, heal flies, horse flies, deer flies, and stable flies. Of these flies, the horn fly is considered the most economically significant fly of pastured cattle. USDA estimated yearly losses in cattle production due to horn flies approach 730 million dollars.

Life Cycle
Horn flies typically appear in the early spring, and the population increases until it peaks during mid- to late-summer and early fall. The average life span of an adult horn fly is 10-21 days. Females lay eggs in fresh manure, usually within five minutes of manure deposition. The hatching larvae move into the manure where they feed and develop through three stages before pupating. The mature larvae generally move to the bottom of the pat or beneath the pat to the soil surface to pupate. Larvae development usually takes about one week and transformation from pupae to adult another week during summer months. As temperatures decline in the fall, a percentage of the pupae diapause (overwinter phase). The percentage of horn fly pupae in diapause increases until frost at which time reproduction ceases. In the spring, as temperatures warm the soil, pupation is completed and adults emerge.

Adult horn flies will spend most of their lives on cattle, but also have been observed on horses, sheep, goats, deer, and elk. Both male and female horn flies feed on cattle by piercing the animal’s skin with their bayonet-like mouthparts and ingesting a small amount of blood. Horn flies feed intermittently approximately 20-30 times daily for 10-25 minutes at a time. A single female can ingest 11-21 mg of blood daily.

Economic Significance The USDA estimates horn flies cost the cattle industry over $730 million due to lost production. Large horn fly populations can reduce a cow’s milk production by up to 20%, which can result in lighter calves weaned, typically 13 to 14 lb lighter. When significant numbers of horn flies are present on growing cattle, average daily gain can be reduced by 0.25 to 0.5 lb daily or 37.5 to 75 lb over a 150-day grazing period. From a dollars and cents standpoint, based on current market conditions, this can result in losses approaching $17.78 for weaned calves and $83.59 for cattle coming off grass (Table 1).

Table1  Economic Impact of Horn Flies on Weaned Calves and Grass Cattle
	Performance Loss	Average Sale Price	Economic Loss
Weaned Calf	13 to 14 lb weaning wt.	$127.00	$16.50 to $17.78
Grass Cattle	0.25 to 0.5 lb ADG	$114.50	$41.79 to $83.59

 

Economic Threshold
An important concept for producers to considering when evaluating horn fly control is the economic or action threshold.  This is usually defined as the number of flies on one side of a cow, which create production losses sufficient to justify treatment.  This number is determined by studies that attempt to control all factors affecting weight gain except for varying the number of horn flies per animal.

The action threshold producers should use will vary.  For average production situations, approximately 50 to 75 horn flies per side will be an appropriate threshold.  When forage is poor, the overall condition of the animals is poor, or several factors are stressing the cattle, reduce the action level toward 25 flies per side.  On the other hand, if forage is excellent, the weather is mild, and the animals are in excellent condition, increase the action threshold toward 100 flies per side.

Taking Counts
It is less disturbing to cattle and easier for producers to count flies on one side of an animal.  When counting horn flies, remember to adjust the counts on windy, sunny, and cool days.  On very windy days most of the flies will be on the downwind side. On sunny days, the flies will be on the shady side, and on cool days they will be on the sunny side, withers, or back.

To arrive at a practical estimate, count the number of flies on at least 10 animals, 1/2 from the upwind or sunny side and 1/2 from the downwind or cool side.  Then, use an average of the counts.  Bulls tend to have heavier fly burdens, so only count the number of flies on cows to get a more realistic picture of the herd’s average burden.

Count the Cost
Remember, the other element of the action threshold is cost of control.  These costs partially offset the benefit of improved production.  For more expensive control measures, increase the action threshold toward 100 flies per side and reduce it for less expensive measures.  Costs to consider are the actual cost of the control materials, the cost of labor to apply it, and the cost in lost energy due to any stress the animal might experience from the application.
 

Horn Fly Control Options
Many options exist for horn fly control (Table 2).  When deciding on a method and pesticide, several factors must be considered, such as cost, current management practices, labor, potential resistance, and ease of use.
 

Table 2  Common Horn Fly Control Options
Method	Use
Backrubbers	Use in loafing areas. Insecticide is applied when cattle use the backrubber.
Dust Bags	Place in location where cattle are forced to pass underneath the bag. Ensure dust bag is installed at correct height.
High Pressure Sprays	Use in confined area to apply insecticide. Retreatment may be necessary.
Insecticide Bolus	Large pill-like formulation that is given to the animal with a standard balling gun.  Active ingredient is usually an insect growth regulator.
Insecticide Ear Tags	Releases small amounts of insecticide, which is distributed over the animal during grooming.
Pour-on Insecticides	Ready-to-use formulations that are applied in measured doses to animals based on body weight.  Most function as contact insecticides.
Feed-through Products	Utilize methoprene (IGR) or an organophosphate in a nutritional supplement.  Cattle must consume product in appropriate amounts and in a timely manner to achieve satisfactory results.
Self-applicator Sprays	These are set-up on mineral feeders or gates between fields.  A switch, tripped by the animal, releases a small amount of spray.