What to Know About Horseflies

Medically Reviewed by Mahammad Juber, MD on April 10, 2022
4 min read

Horseflies are a type of fly that belong to the Tabanidae family of insects. The female horsefly depends on blood for nutrition to make eggs. This is because animal blood has a certain protein they need to develop their eggs. Horseflies like feeding on large, dark-colored animals like horses. But this does not prevent them from biting humans when they get a chance.

The Tabanidae family of insects is made up of about 4,450 species, and about 400 of these species are present in the U.S. Horseflies breed and thrive in wet areas, like coastal areas.

Horseflies are generally larger than the houseflies you may be familiar with. They're short, sturdy flies with large eyes, segmented antennae, and strong, blade-like mouthparts. Horseflies are usually about six to twenty millimeters long.

Male horseflies only feed on flower nectar. While the females also mostly feed on nectar, they have to feed on blood when their mating time comes around. This is when horseflies become dangerous. Female horseflies have to drink blood to reproduce. They mate during the summer, which, coincidentally, is the same time that humans like to go outside in shorts and short-sleeved shirts. This makes humans the perfect horsefly target.

Horseflies have strong and sharp mouthparts that work like scissors when cutting through the skin to get to the blood. The horse fly bite hurts a lot, much unlike a mosquito or a tick bite. After mating and feeding on blood, the female horseflies find a damp area to lay their eggs. Common places for horseflies to lay their eggs include:

  • Creeks
  • Marshes
  • Beach dunes
  • Pond shores
  • Waterfalls
  • Termite mounds

The horsefly eggs then hatch into venomous, predatory larvae, or maggots. The larvae make a meal out of small animals like frogs and minnows. They develop into pupae during the winter and become adults at the end of spring. After they become adults, they go on to feed, reproduce, and then die.

Horseflies track their prey by size, color, and smell. As mentioned earlier, they like large, dark-colored animals. When they get close enough to the big, dark object they’ve spotted, they then follow the source of carbon dioxide exhaled by animals when they breathe. Once they've traced the source, they proceed to bite.

Unlike mosquitos and ticks, horseflies are not major disease vectors. This doesn't mean that they can’t transmit diseases — only that they're not as big of a problem as, say, mosquitos and dengue or ticks and Lyme disease.

Horseflies actually benefit the environment. Because they mostly feed on nectar, horseflies help a lot of plants when it comes to pollination. Many plants even rely on horseflies for pollination.

A horsefly bite mainly causes pain, redness, and minor swelling of the affected area. These bites should go away on their own over a few hours or days. You may also experience other symptoms, like:

  • A red, raised rash (hives)
  • Wheezing
  • Weakness
  • Dizziness
  • Puffiness and swelling of part of your body

If you’ve just been bit by a horsefly, the following steps may help ease your discomfort:

  • Clean the bite site with soap and water.
  • Do a cold press by placing an ice pack on the affected area for 10 minutes to ease the swelling.
  • Raise the affected area (if possible) to try and manage the swelling.
  • Try not to scratch the bite site, which could lead to infection or form a blister. If a child has been bit, consider cleaning and trimming their fingernails.
  • Avoid applying substances considered to be home remedies (such as vinegar or sodium bicarbonate) to the affected area. They may not help.
  • Use over-the-counter medications to manage pain and discomfort.
  • If you're itching a lot, talk to your doctor about the best treatment (creams or medications) to relieve the itch.

In some cases, you may get severe symptoms. If you notice yourself feeling any of the following, call your doctor immediately:

  • Dizziness
  • Wheezing or difficulty breathing
  • Problems with swallowing
  • Swelling in the throat, mouth, or face
  • Increased heart rate
  • Loss of consciousness
  • Vomiting
  • Any symptoms of infection, like pus at the bite site, fever, or swollen glands

To avoid horse fly bites in the first place, here are some strategies you could try:

Insect repellents. Insect repellents offer some degree of protection against horse flies. However, you may not get the same results as you do with mosquitos or ticks. 

White clothing. Because horseflies look for large, dark-colored objects when looking for prey, wearing a white shirt or clothing may make you unattractive to horseflies. However, if you are in an area known to have ticks, this may not be the best idea. Removing the target on your back for horseflies may make you easy prey for ticks, which are attracted to the color white.

Body paints. Having brightly colored body painting may help to throw off your local horseflies due to their preference for seeking out dark colors. Just note that although such body paints may be able to keep horseflies at bay, most are used for specific cultural reasons among indigenous communities on various continents.

Traps. Laying horsefly traps can be effective when it comes to smaller areas like yards, swimming pools, and camping sites. This method might also work if you're trying to keep horseflies away from cattle in a confined place. An example of a trap that works is hanging black, shiny, sticky balls that attract horseflies as they're moved by the wind. Another option is a malaise trap — a large piece of netting that catches flies simply by being in their flight path.

Manipulating the environment. This involves clearing bushes or woods in residential areas and draining damp areas to eliminate the available breeding places for horseflies. This can be quite effective because horseflies will only lay their eggs in wet areas.