Horse flies are rather large flies that attack mammals and are feared because of their painful bite. Horse flies are not a single species, but a whole family (called Tabanidae) with over 4000 species described so far.

A common feature is the large size and the painful bite.

They can be found worldwide, although the prevalent species vary in the different regions. Important genera are Chrysops, Tabanus, Hematopota, etc.

Chrysops spp, are the most common genera. Other regional common names for horse flies are: deer flies (mainly used for Chrysops spp), breeze flies, gad flies, March flies, clags, zimps, etc.

Many species attack wild and domestic mammals including livestock, cattle, dogs, cats and humans. Some species also attack birds and reptiles.

Biology and life cycle of horse flies

Adult horse flies are bulky, up to 2.5 cm in length. Only the females bite and suck blood, which they need for egg production. They lay eggs on stones or vegetation close to water. The hatching larvae get into the water where they are predators of smaller invertebrates such as other insects, crustaceans and snails. Males feed on nectar and pollen, do not have biting mouthparts and are not parasitic at all. 

Usually there is only one generation a year. Some species need even more than 2 years to complete development. However, since in most regions there are several species with slightly different life cycles, various waves of horse flies can appear during a season. In any case, the numbers in most places never become too high.

Fortunately horse flies do not build swarms, but are rather "individualistic", very much like mosquitoes and not like stable or horn flies that can attack livestock by the hundreds or thousands.  

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

Damage, harm and economic impact of horse flies

Horse flies are usually not an economic problem in livestock operations. Nevertheless, milk production on dairy cows on pasture may be reduced by up to 25%.

Blood loss after massive attacks can be significant and cause anemia, but it is rather unusual. The horse fly bite is very painful end stressing for the animals and it is reasonable to try to protect that most valuable animals (stud horses and bulls, race horses, pets, etc.).

Horse flies can transmit a number of diseases such as anaplasmosis, trypanosomiasis, anthrax, tularaemia, Loa Loa, etc. However, horse flies are usually not the most important and effective vectors of such diseases.

For dogs and cats (and their owners...) horse flies are especially irritating due to the painful bite.

Prevention and control of horse flies

Until now there is no really effective method to protect livestock, horses or pets (or humans) against horse flies. This has several reasons:

• the large size of the horse flies and the short time they remain on the host require a very high insecticide dose to be applied to the animals to kill the flies before they bite;
• the ability of horse flies to fly over very long distances from their breeding habitats in search of a pray makes it useless to try to control them in one property, since horse flies from other places can easily replace the killed ones;
• the high number of potential habitats where they can breed makes it virtually impossible to achieve population control by treating or modifying the environment.

Products containing pyrethroids (e.g. sprays, pour-ons, etc.) can provide a few days protection against horse flies, but certainly not weeks. Other pour-ons (e.g. with endectocides like ivermectin) or whatever injectables will have no effect on horse flies.

A lot of research has been carried out on repellents and traps against horse flies, but without practical result so far. Most repellents may keep flies away only for a few hours, if at all. Studies with traps found out that each species has a different profile regarding attractants (e.g. octenol, various animal urines, acetone, ammoniac, etc.). Since in most regions there are various different species, it is practically impossible to significantly reduce their population with traps.

For dogs and cats collars impregnated with pyrethroids can provide some relief. Other typical pet products (spot-ons, shampoos, soaps, sprays, etc.) are totally useless against horse flies.

There are no true vaccines that will protect livestock or pets from horse and deer flies.

There are so far no effective biological control methods against horse and deer flies. Learn more about biological control of flies and other insects.

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

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.

Resistance of horse or deer flies to insecticides

So far there are no reports on confirmed horse fly resistance to insecticides.

This means that if a particular product has not achieved the expected control, it is most likely because the product is not adequate or it was not used correctly, not because horse flies have become resistant. Incorrect use is the most frequent cause of product failure.

Learn more about parasite resistance and how it develops.