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Horn flies (Haematobia irritans or Lyperosia irritans) are tiny flies, one of the insects most damaging to cattle in Europe, North and South America, and Asia. They are obligate bloodsucking parasites. The buffalo fly (Haematobia irritans exigua), a close relative, occurs throughout Australia and shows a very similar behavior.

Adult horn fly (Haematobia irritans), chead and thorax. Note the protruding mouthparts. Picture of M. Campos Pereira.

The horn fly is native to Europe and was first introduced into North America from where it slowly spread southwards. It reached Argentina and Uruguay in the 1990's.

The horn fly is a parasite of cattle although it can also bite horses, sheep and goats, and occasionally humans as well. It affects mainly range and pasture herds and is less abundant in feedlots and dairy cattle or, generally, in indoor operations.

Horn flies are not parasitic of pig or poultry and they don't bite dogs or cats.

Biology and life cycle of horn flies

Adult horn flies are rather small (3 to 5 mm) and spend most of their time on the host, usually looking downwards. Preferred sites are the back, the flanks, the belly and the base of the horns. They may change the site on the same animal according to temperature and to avoid the wind. Both male and female flies suck blood intermittently the whole day through. Females leave the host only for egg laying.

Massive horn fly infestation of a cebu cattle.Populations peak during the summer in countries with temperate climate. Seasonality in tropical and subtropical regions may be more dependent on rain.

Female flies lay eggs on fresh cattle manure, even before the animal has finished defecating. A single female lays up to 360 eggs during her life, which lasts for 6 to 8 weeks. Larvae feed on manure and the adults may hatch 9 to 12 days after egg laying. Under suitable conditions 10 and more generations can follow during one year. The overwintering stage in cold regions is usually the pupa.

Horn flies can fly up to 15 km to find a host, although most flies will not leave the herd where they were born.

Click here to learn more about the general biology of insects.

Harm and economic loss due to horn flies

In endemic regions, it is not unusual for cattle in a herd to carry thousands of horn flies during the high season. This is why economic damage for the livestock industry can be substantial. Horn flies won't kill cattle but blood loss and stress due to biting, itching and scratching can reduce weight gains by up to 30%.  An infestation with 200 flies can reduce the daily milk production by 0.5 liters.

Severe infestations can also cause skin irritation and wounds that may attract other parasites such as screwworm flies.

Horn flies are vectors of Stephanofilaria stilesi, a helminth parasite of cattle.

Non-chemical prevention and control of horn flies

Intact cowpat: Ideal breeding place for hor fly larvae.Horn flies need an intact cowpat to complete development. Consequently, any measure that dries, destroys, eliminates or at least disturbs the cowpats will contribute to reducing the horn fly population in a property. Mechanical methods such as using a drag harrow or a chain in pastures may achieve this, but they may be detrimental for dung beetles and reduce pasture attractivieness for cattle.

Under intensive farming conditions as in most feedlots and many dairy farms, cowpats are spontaneously destroyed due to the high density of animals per surface unit. The cowpats are simply trampled and squashed, which accelerates drying and makes them unsuited for larval development of the horn flies. This usually results in lower hornfly populations than on range cattele, but not in suppression.

In the past, traps were used to catch and kill horn flies, but they were not efficient enough and have been vastly abandoned in many places.

Abundant research has been conducted on biological control using natural enemies of the horn flies, particularly on predators of the immature stages developing in the cowpats (e.g. Spalangia spp., a Hymenopteran wasp). So far, the levels of parasitism obtained in the field are insufficient to achieve a significant reduction of the adult horn fly population.

In Australia certain dung beetles that destroy the cowpats have been released with a reasonable success. However, this approach has not been effective enough in other countries. And releasing non-native dung beetle species for this purpose always bears the risk that such foreign species become invasive, get out of control and displace the native ones.

So far there are no commercial products based on natural pathogens of horn flies (bacteria, fungi, nematodes, etc.) suitable for significantly reducing horn fly populations in the field. Learn more about biological control of flies and other insects.

There are no true vaccines against horn flies either.

Click here if you are interested in medicinal plants for controlling flies and other external parasites of livestock and pets.

Chemical prevention and control of horn flies

Horn flies on cattle. Note their typical position looking downwardsHorn flies that are susceptible to insecticides (i.e. not yet resistant) are rather easy to control with insecticides applied to cattle. One reason is that since they spend most of their time on the host, if the host is treated, the flies will remain in contact with the insecticide during a very long period. And they are rather small, i.e., they need a relatively lower dose of the insecticide to be killed. This is not the case for many other flies such as stable flies or horse flies that are much larger in size and remain only for a few minutes on the animals.

Dusts bags (e.g. with coumaphos, an organophosphate) and back rubbers impregnated with various insecticides were vastly used in the past. However, these self-treating devices have the disadvantage that the exact dosing of each animal is not possible, which can create problems of overdosing and subsequent higher residues than those authorized. For this reason they are no more allowed in many countries.

Where there is still no resistance, dipping or spraying the animals with conventional insecticides (e.g. synthetic pyrethroids or organophosphates) may provide efficient control of existing infestations and protection against re-infestations for a few weeks. Pour-ons with similar insecticides and with macrocyclic lactones (e.g. doramectin, eprinomectinivermectin) should have the same efficacy and achieve a longer protection. The longest protection of 3 months and beyond can be achieved with ear-tags impregnated with insecticides, mainly organophosphates and synthetic pyrethroids. Where resistance is not yet a problem, it is highly recommended to periodically change the chemical class of the product used: this should contribute to delay the appearance of resistance.

Injectables or drenches with macrocyclic lactones (e.g. doramectinivermectin, moxidectin) are not efficient to control horn flies. Neither the effect on the adult flies through the blood, nor the indirect effect on the fly larvae in the cowpats are enough to control the populations.

Unfortunately, resistance (especially to synthetic pyrethroids) has already spread in many countries making it rather difficult to control horn flies. However, ear-tags impregnated with organophosphates will still work properly in most places giving up to 3 and 4 months protection. Pour-ons containing endectocides may last for 3 to 4 weeks.

In several Latin American countries (e.g. Brazil, Argentina, Uruguay, Colombia) there are pour-ons with fipronil allowed for use on cattle against horn flies, but they have withholding periods of up to 4 months. However, such products are not authorized in most other countries (e.g. in the US, Canada, the EU and Australia).

An alternative or a complement to attacking the adult flies by treating the hosts is attacking the larvae in the cowpats. This is best done with insect development inhibitors such as methoprene and diflubenzuron. These compounds are available in some countries in the form of slow-release boluses, or included in mineral blocks. There are also a few feed-thru products (e.g. with tetrachlorvinphos, an organophosphate) that can be mixed to cattle food. These compounds pass almost unchanged through the intestine of cattle directly into the fecal mass, where they interrupt the development of horn fly larvae. All these compounds have a large spectrum of activity and may be detrimental not only to horn flies, but also to other beneficial dung living fauna. In addition, this control method needs a few months for the fly populations to be significantly reduced, i.e. it is unsuited for quickly knocking down severe infestations. Since horn flies can fly long distances, this control method will not avoid that flies from neighboring properties continue infesting the own cattle. Best would be that all properties within a given area at risk coordinate their control measures to minimize such fly migrations.

So far there are no effective repellents against horn flies on cattle.

There is also additional information in this site on the general features of parasiticides and ectoparasiticides, as well as on parasiticidal chemical classes and active ingredients.

If available, follow more specific national or regional recommendations or regulations for horn fly control.


Insecticide resistance of horn flies

Horn flies are capable of developing resistance to insecticides very quickly, i.e., in a few years, regardless of the application method (dip, spray, pour-on, ear-tag, etc.). In fact, resistance to synthetic yrethroids is already very frequent in both North and South America, Australia and Southern Africa, and resistance factors can reach such high levels that many products have completely lost their efficacy against horn flies.

Resistance of hornflies to organophosphates has also been reported but is weaker and less widespread than resistance to synthetic pyrethroids.

So far there are no reports on field resistance to insect development inhibitorsmacrocyclic lactones or fipronil, a phenylpyrazole compound.

To avoid or at least delay the development of resistance it is highly recommended to periodically change the chemical class of the product used, before a resistance problem is suspected, e.g. every one or two years. This is usually called rotation.

A practical consequence of this is that if a particular product does not achieve the expected control of hornflies, it is reasonable to consider a resistance problem, especially if that product has been used for several consecutive years without rotation. However, experience shows that many cases of product failure are due to incorrect use and not to resistance.

Learn more about parasite resistance and how it develops.

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