Horse Flies

What Are Horse Flies?

The horse fly is a large, bloodsucking fly known for its painful bite. A general term, the common name of the horse fly applies to a variety of species of biting flies.

These flies can cause a lot of the misery, as the female flies, just like mosquitoes, feed on blood. People call them “horse” or “deer” flies.

They have a pair of mandibles that act much like scissors, slashing the skin to cause the blood to flow, at which point they lap it up with their mouths. The male flies feed on nectar and other sweet liquids, but the females are biters. The larvae of the Tabanids are another group of predators, feeding on whatever insect larvae they come across in damp soils, in rotting logs, under debris on the soil, or even along the edges of ponds. So, we can thank them for their work as larvae, but wish they’d go away as adults.

What Do They Look Like?

horsefly large eyes
deer fly
deer fly

Large and robust, the horse fly can measure up to an inch in length. A large pair of eyes dominates the head, with females exhibiting separated eyes and males sporting contiguous eyes. Horse flies are black or dark brown in color and have translucent, body-sized wings. Larvae are spindle-shaped and creamy white or tan in color.

Appearance / Identification

Size: 3/8-inch to more than 1 inch depending on species
Color: Color varies — most are brown or black
Top of Prothorax: May have stripes or markings

Other Identifying Characters:
Sharp proboscis (mouthparts); eyes large and often iridescent head; sharply arched (humped) thorax

Horse flies are generally large flies which are brown or black in color. Many different species might be encountered some of which will be up to one inch or more in length. The eyes of horse flies are very large and are often colorful or iridescent. The mouthparts of these flies are piercing/sucking in design and extend sword-like from the bottom of the head.

Bites

Horse flies are biting flies which can and will bite people. Bites can occur on any part of the body and are very painful.

What Do The Bites Look Like?

The bite of large horse flies can even result in visible, bleeding wounds.  Bite sites typically develop into red lumps and may itch for one to two days.

horsefly bite on skin
horsefly bite

Symptoms

Common bite areas include the necks and legs of victims, which are rich with large, healthy arteries.

The bites of horse flies are generally painful and result in lesions and swelling, depending on the part of the body affected. Females often leave noticeable, swollen wounds, sometimes visibly bleeding, once finished with a blood meal or shooed away.

The insects often swarm to feed, with as many as 100 horse flies converging on a single animal to draw blood meals, at times. Daybreak and the hours just before sunset represent the most common times of day female horse flies feed.

Potential for Transfer of Disease

Due to the deep penetrations of horse fly wounds and the aspect of feeding associated with the bites, the transmission of disease remains possible if sustained contact occurs. Bites should receive immediate attention. The possibility of disease transmission mainly comes from the multiple feedings that take place across several species of mammals in a given day or within relatively short periods of time which leave trace particles of flesh and blood from other animals on horse fly mandibles, which are then potentially transferred to subsequent hosts upon biting.

Treatment

If bitten by the pest insect, victims should take appropriate actions to limit the possible transmission of disease and chances of infection.

Soap and water solutions serve as acceptable means of cleaning a horse fly bite in addition to using saliva, which contains a protein that can often act as a temporary disinfectant until the proper materials are accessible to clean and dress the bite mark. If an allergic reaction occurs, Benadryl or other anti-allergen ointments and medicines may reduce the amount of swelling, itching, or general discomfort associated with such a reaction.

Victims should avoid scratching the abrasion and visit a licensed doctor if pain, swelling, or significant reactions occur or persist. On average, horse fly bites may take a week or so to fully heal. Bandages or other medical sutures may serve as barriers to limit the potential for infection and the ability to scratch at the wound while it heals.

Where Do They Live?

Horse flies are common in wet sites like wooded, swamp, and beach environments. The pests are active during the warm summer months, and female flies are usually seen during the daytime searching for blood meals. Attracted to warmth and carbon dioxide from the breath of living creatures, female horse flies will leave breeding sites to seek out hosts.

Barns or other ill-lit structures providing shade found in rural areas serve as ideal hiding spots for horse flies.

What Does a Horse Fly Eat?

Female horse flies feed on blood in order to gain protein that aids in egg production. Horses, cattle, and humans are common hosts to these blood-feeding insects. Male horse flies primarily feed on pollen and nectar. During development stages, larvae feed on decaying organic matter and soft-bodied animals such as earthworms and grubs.

Life Cycle & Reproduction

Female horse flies deposit eggs in the soft mud of aquatic and semiaquatic environments like river banks, marshes, and the edges of ponds.

Gestation Period

Females lay between 25 and 1,000 eggs at a time, and eggs usually hatch within a week. Horse flies remain larvae for one or two years and pupate in the spring and early summer months. A summertime pest, the adult horse fly has a lifespan of one to two months.

The larvae of these flies develop primarily in aquatic areas and are predaceous on other insects. The adults, therefore, are most common during hot summer months in areas where ponds, lakes, marshes and other wetlands are located.

What Problems Do They Cause?

Certain species in the Tabanidae family are known to carry and transmit diseases, such as tularemia and anthrax. Fortunately, transmission to humans is rare.

Horse flies are rarely a significant problem inside structures. These flies are most commonly found inside large, commercial buildings, especially warehouses, where overhead doors are left open to allow ventilation.

These flies will be attracted by the cooler air currents emitting from these doorways and will fly to and enter the building. Once inside, a horse fly or deer fly may land on or bite a person working in the building. Bites inside buildings, however, are not very common.

Where Do They Live?

Homes near water are most susceptible to infestations of horse flies. Seeing multiple flies darting around the home may indicate an infestation. The pests generally do not breed inside of structures, and prolonged infestations are an uncommon occurrence with the horse fly.

Prevention

How Do You Keep Them Away?

Homeowners want to keep horse flies out of the house by keeping windows and doors closed and sealing up any cracks and openings into the home. Traps may also be used to combat the pests, but the practice is not an efficient method of control. Pesticide control methods are even less effective because breeding grounds are usually away from structures the flies inhabit. Individuals expecting to encounter horse flies outdoors may wish to cover the skin with long clothing to prevent bites.

Because infestations of horse flies usually involve only a few individuals, extensive inspections are not necessary. Breeding sites will generally be located a good distance from the building. Should numerous horse flies be found in a particular building, it would be a good idea to survey the area surrounding the structure for possible breeding sites. Ponds, marshes, and lakes within a mile of the building are likely breeding sources.

How Do I Get Rid of Flies in My House?