The horn fly Haematobia irritans (L.), a blood-feeding fly, spends most of its adult life on cattle, feeding numerous times daily. The females deposit 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 activity varies throughout the country depending on temperatures. In Nebraska overwintering adult flies emerge from late March until early June. In central Texas overwintering emergence starts in mid-February. In south Texas and Florida horn fly adults are present year-round. Horn fly numbers range from less than 100 per head in northern areas to 1,000 or more in the southern states.

Sprays and Self-Treatment Devices
Control of the horn fly has evolved through several systems. Laake's work in Kansas in the mid-forties with new chlorinated hydrocarbon insecticide sprays gave ranchers an appreciation of the impact of horn flies on weight gains of cattle. However, spraying requires labor and stresses range cattle as cattle must be gathered into corrals for treatment. The cost of the spray per animal per spraying at the time of Laake's study was under $0.50 per head, but labor and stress to the cattle probably accounted for another $0.25–$0.50 per animal. However, the weight gains indicated by the Laake study would have provided returns of $4–$8 per $1 invested for the spray.

   Sprays were followed by self- treatment devises called "rubbers" or "oilers." These were made from burlap-wrapped chain or wire suspended between two posts. The burlap was usually treated at weekly intervals with a hydrocarbon insecticide diluted with number two diesel fuel. The rubbers were placed at locations where cattle obtained water, mineral, or shade. When rubbers/oilers were first used, maintained properly, and placed correctly, they provided good control for an investment of approximately $0.25 per head monthly.

   When the chlorinated hydrocarbon insecticides were restricted by the Environmental Protection Agency (EPA) from use on livestock and replaced with organophosphate insecticides (OP), the efficiency declined, primarily because of the reduction in residual value of the insecticides. Meanwhile, cost of insecticides increased.

Dust Bags
Dust bags gradually replaced oilers in much of the range country. Dust bags contained OP dusts which filtered through the bottom of the bag when cattle contacted the bag as they passed under it. Dust bags were particularly effective when cattle were forced to pass under them to obtain water, feed, or mineral. This was generally accomplished by fencing the water tank and suspending the dust bags in the entrance-exit gate. This was termed "forced-use" of dust bags. This method was particularly popular in areas where both face flies and horn flies were pests of cattle, because the faces of cattle were dusted as they passed under the bag. However, many range cattle water from streams or ponds, thus preventing the use of dust bags.

   Dust bags were generally used on a "free-choice" basis when cattle didn't obtain water from tanks. Dust bags were placed in locations frequented by cattle. In time, cattle learned to use the bags, but often there were not enough bag stations present and older cattle and bulls dominated the use of the few available. Horn fly control from free-choice use of dust bags is usually 25%–50% less compared to cattle forced to use bags.

   Dust bags possessed some problems until they were modified and ranchers learned how to use them more efficiently. The first bags were made from burlap which allowed rain to soak the bags and cake the dust. Bags were modified with plastic covers for rain protection and now have a netting which traps the dust until an animal rubs against it.

   Generally, 50 pounds of dust will treat about 50 cattle for one month. The dust currently sells for approximately $1–$1.50 per lb, which translates to $1–$1.50 per head monthly for treatment. When used correctly, the oiler or dust bag treatments generally keep horn fly populations below economic threshold levels. Also, these treatments, although effective, are not persistent enough to cause resistance in horn fly populations.

Ear Tags
Dust bags were followed by the more efficient insecticide-impregnated ear tags. The insecticide was incorporated into the matrix of the ear tag. Tag contact with the animal's shoulder results in insecticide, which "blooms" to the surface of the tag, transfers onto the hair of the animal. Permethrin and fenvalerate formulations used in the first ear tags were somewhat oily and migrated through the animal's hair. Control was so effective when the ear tags were first used that most cattle producers used them. However, this success was short-lived. Within two years, resistance of horn flies to insecticides used in the tags was detected in Florida. Within four or five years resistance was present throughout the United States, Canada, and Mexico.

   The cost of ear tags varies depending on the number used. In areas that had both face flies and horn flies, ear tag companies recommended two tags per cow and two per calf, which currently means a cost of $4.00–$5.50 per cow/calf pair. But for the most part, the recommendation was one ear tag per cow. Given an ear tag cost of $1 per cow and effective horn fly control (when only one tag was used and depending on the price of cattle), the result was a $4–$8 gain for each dollar spent. When resistance to the pyrethroid insecticides developed, animal health companies and extension entomologists often recommended returning to dust bags and oilers or to use feed additives.

   After pyrethroid ear tags were introduced, animal health companies developed ear tags incorporating organophosphate insecticides. These ear tags can be used as an alternative where pyrethroid resistance is a problem. As soon as resistance to a chemical is apparent, the treatment using that chemical should be discontinued even if applied at higher concentrations.

   The newer OP ear tags generally cost over a dollar per tag (up to $1.50 per tag) and almost all companies recommend two tags per cow and often suggest tagging the calf too. These tags do not provide the degree of control compared to the first pyrethroid tags, nor does the control last as long (10 to 12 weeks). Consequently, many producers have stopped using ear tags or have chosen other control systems or possibly have stopped trying to control horn flies altogether.

Feed Additives/Boluses
Another type of insecticide is feed additives, which are typically incorporated into mineral blocks consumed by grazing cattle or added to mineral or feed. Rabon^®* oral larvicide was an early feed additive. Later, methoprene, an insect growth regulator (IGR), was approved for use as a feed additive. While both are feed additives, the mode of action between these two compounds differs. Rabon, an organophosphate, kills developing fly larvae in the manure of treated cattle, while methoprene  interrupts the horn fly's development by keeping it in the juvenile stage (prevents development into mature flies), thus breaking the horn fly life cycle. Cost for feed additives range from $0.02–$0.05 per animal daily.

   Insect growth regulators were later incorporated into bolus formulations. Currently, one bolus is available--VIGILANTE^ª*** (dimilan). Once the bolus enters the cow's reticulum, it gradually erodes, releasing the insect growth regulator into the digestive system. Once voided in the manure, the insect growth regulator contacts horn fly larvae. Cost of bolus treatment is approximately $0.04–$0.05 per animal daily.

   IGRs are very effective when cattle consume sufficient levels or receive the bolus. The reduction in horn fly numbers is also dependent upon the number and proximity of untreated cattle.

Resistance
The development of chemical insecticide resistance in horn flies has greatly increased the problem of fly control. If resistance exists to pyrethroids, the problem will only be increased by using another pyrethroid at an even higher concentration. The newer pyrethroid tags may provide control for a time, but will only increase resistance in the long term. Use of organophosphate tags is an alternative if ear tags are used. However, with continuous use of OP treatments, the same future can be experienced if not managed properly. Horn flies in Mexico have become pyrethroid-resistant because animals were dipped for tick control every two weeks. It does not matter how or what chemical insecticide is used, if used continuously, resistance will develop.

   On a positive note, to date there is no known resistance to insect growth regulators. The use of insect growth regulators provides an alternative biological control method and alternative tool in the management of horn fly populations.

*Rabon is a registered trademark of Fermenta Animal Health.
***VIGILANTE is a trademark of Solvay Duphar, distributed by Hoechst Roussel Vet.