Horse Flies

Facts, Identification & Control

Scientific Name

Tabanus spp.

Appearance

What Does a Horse Fly Look Like?

Horse Fly Illustration
Horse Fly Illustration

Horse flies varying in length from about ½ to 1 ¼ inches long, are black or gray in color and often have large, brilliant green eyes. The blood feeding female horse fly is equipped with blade-like mouthparts, which slash tissues and blood vessels and cause blood to flow to wounds. Females then use their sponge-like mouthparts for sucking up blood. Males only feed on pollen and nectar and have similar, but much weaker mouthparts. All horse flies have antennae that are shorter than the head.

Horse flies are similar to deer flies, and both are in the family Tabanidae. The two ways to tell them apart is to look at their overall size and their wings. Horse flies tend to be much larger with a stouter body and a very large head with very large eyes. Their wings are usually clear or cloudy whereas deer flies have dark bands or spots across their wings.

Behavior, Diet & Habit

Horse fly females are aggressive blood feeders, while males do not consume blood but feed on pollen and plant nectars. Female horse flies usually bite large, nonmoving mammals on the legs or body. Deer flies, in contrast, attack moving hosts and typically target high on the body, like the head or neck.
They rarely bite near the head. Horse flies have a range of hosts that include mammals of almost all sizes, livestock, humans, pets and birds. Should a female horse fly be interrupted when attempting to feed, they will fly off but quickly return to bite again, or go to another host to consume a complete blood meal. Horse fly larvae studied by field researchers feed on midges, crane flies and even other horse fly larvae. Because of their cannibalistic behaviors, horse fly larvae are usually found living alone. Deer fly larvae, on the other hand, usually live in groups. Pupae do not feed.

Similar to other blood-sucking insects such as mosquitoes for example, female horse flies use both chemical and visual cues to locate hosts. Carbon dioxide expelled by warm-blooded animals provides a long-range cue to attract flies from a distance, while visual cues such as motion, size, shape and dark color function to attract horse flies from shorter distances.

Horse fly development sites are freshwater and saltwater marshes and streams, moist forest soils and even moist decomposing wood. Females usually deposit egg masses on wet soil or vegetation that overhangs water. Larvae are active in moist or wet organic matter and look similar to house fly maggots.

Horse flies have from 6-13 larval stages, depending on the species. The final larval stage overwinters and then enters the pupal stage in the spring. The pupal stage only lasts about 1-3 weeks after which the adults emerge in the late spring or early summer. Fertile females will lay their eggs on the underside of leaves, and in about 2-3 days the larvae hatch out and drop from the leaf. Most horse fly species have only one generation per year, but others may take up to 2-3 years to complete their development.

Reproduction

Female horse flies must consume a blood meal in order to yield fertile fly eggs. One female can lay from 100-800 eggs per year.

Signs Of A Horse Fly Infestation

The most obvious sign of a horse fly infestation is the bothersome and painful biting caused by the adult female flies and the symptoms and reactions to their bites.

Distribution

Horse flies are found in most areas of the United States with more than 160 various species.

More Information

Chemical and source reduction control of horse flies is very difficult since they develop in natural habitats where insecticides, if legal to apply, offer little more than a minor, short-term degree of effectiveness. In addition, the extent of their developmental habitat is extremely wide-spread. Therefore, if a property owner needs to control horse flies, it is best to contact your pest management professional for advice on methods that are effective and to find out what can legally and practically be done for control. One method that is moderately helpful is the use of traps, although their effect is limited to a somewhat small scale.

Insect repellents are helpful, but even the best repellents are not overly effective. A better prevention option is to clothe and protect exposed parts of the body to reduce the likelihood of horse fly bites. When bitten, the symptoms and bite reactions can include:

  • Localized swelling and an itchy red area around the bite.
  • Persistent itching and scratching of bite wounds that can cause secondary bacterial infections if the bite is not kept clean and disinfected.
  • Since horse flies inject anticoagulant-containing saliva during blood feeding, some serious reactions may occur in people that are highly allergic to the anticoagulant compounds. Symptoms may include a rash on the body, wheezing, swelling around the eyes, swelling of the lips and dizziness or weakness.

Investigators have isolated many viruses, bacteria and protozoa from the sponge-like female mouthparts and their digestive system, but no studies to date show conclusive evidence that horse flies are capable of trans¬mitting diseases to humans.

Disease transmission to livestock is another matter. Adult flies may pass a number of disease agents and nematode parasites to animals. Equine infectious anemia (EIA), sometimes referred to as swamp fever, occurs in the southeastern United States and is mechanically transmitted to horses and other equines by horse fly bites. Symptoms in animals include lethargy, weight loss and sometimes death.
Anaplasmosis, found in the southeastern United States among cattle is a disease that causes symptoms of anemia, fever, weight loss and mortality.

Horse flies are an annoyance to livestock, as well as humans. Heavy biting rates can lead to weight loss in beef cattle, reduced milk yield in dairy cattle and hide damage from the fly’s puncture wounds.