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Why Do Horseflies Bite, Will They Chase You? 7 Facts

Last updated: August 2, 2023

By: Miles HenryFact Checked

Recently my neighbor’s son showed us his arm covered with horsefly bites. He swears horseflies chase him anytime he gets within a mile of one. His proclamation made me wonder why horseflies bite and if they really chase people.

Horseflies bite to ingest blood which is rich in protein. The protein is needed to develop their fertilized eggs. Only females need to bite since the males don’t produce eggs. And yes, horseflies will chase you down to get their meal. So be wary of them.

If horseflies have chased you, you understand how unpleasant an experience is, but do they bite and chase some people more than others? This is one of many questions surrounding horse fly behavior I answer.

picture of a horseflies bite on a humans arm.

Fact 1: The Biology Behind the Bite

Horseflies have a rather notorious reputation, and for good reason. Their bites can be painful, and if you’ve ever been bitten by one, you probably wondered why they bite and how they do it. Let’s delve into the biology behind the bite to understand this behavior.

Why Horseflies Bite

Unlike mosquitoes that feed on blood for reproductive purposes, female horseflies need the protein and iron found in blood to develop and lay eggs. It’s an essential part of their lifecycle. Male horseflies, on the other hand, feed primarily on nectar and don’t have the same need to bite.

The Mouthparts of Horseflies

Horseflies have a rather unique and efficient method of feeding. Their mouthparts are shaped like scissors, which they use to cut into the skin. Once they make that incision, they use a sponge-like part of their mouth to lap up the blood. This scissor-like action is what makes the bite so painful and different from other insect bites.

The female horsefly’s mouth is equipped with tiny, blade-like structures called mandibles. These are adapted to cut through the skin and access the blood vessels underneath. The feeding process is more like a slice and lap method, which might explain why their bites are more painful than those of other blood-sucking insects.

Male vs. Female Horseflies

Here’s a curious fact: If a horsefly bites you, you can be pretty certain it’s a female. That’s because male horseflies don’t bite humans at all. As mentioned earlier, males feed on nectar and other sweet plant juices, while female horse flies feed on blood.

This difference in feeding habits between males and females is tied to their biological roles. Females need the nutrients found in blood to nourish their eggs, while males don’t have the same reproductive requirement.

Understanding the biology behind a horsefly’s bite can transform it from a mysterious, painful nuisance into a fascinating aspect of nature. It’s not about aggression or malice; it’s about survival and reproduction.

The next time you encounter a horsefly, you’ll know that if it’s a biter, it’s a female fulfilling her biological role in the circle of life. Though that may not make the bite any less painful, it adds a layer of understanding to an otherwise unpleasant experience.

Horseflies are dangerous to horses because they release anticoagulants from their saliva when they bite to keep the blood flowing. The saliva can carry and transmit equine infectious anemia, which is fatal in some horses.

picture of a boy running from horseflies,

Fact 2: Will Horseflies Chase You?

The thought of being chased by an insect, especially one with a painful bite like a horsefly, is enough to send shivers down anyone’s spine. But is this fear grounded in reality? Do horseflies actually chase humans? Let’s explore the behavior of these intriguing insects to uncover the truth.

Horsefly Behavior

Horseflies are drawn to movement, warmth, and carbon dioxide, all things that humans naturally give off. When you move, especially if you’re sweaty after a jog or outdoor activity, you may attract a horsefly’s attention. If you’ve ever felt like a horsefly was following you, it’s likely due to these factors.

Do They Actually Chase Humans?

While it might feel like a horsefly is chasing you, what’s really happening is that the fly is drawn to the cues your body emits. If you start running, the increased movement, warmth, and carbon dioxide can make you even more attractive to the horsefly, making it seem like you’re being chased.

It’s important to note that the horsefly isn’t pursuing you with the intent to harm. It’s merely responding to sensory cues that signal a potential meal. However, this doesn’t lessen the fact that a horse fly bite hurts.

Why This Behavior Occurs

The “chasing” behavior stems from the horsefly’s need to feed. Female horseflies, in particular, are on the lookout for a blood meal to aid in reproduction. The signals you give off, especially when moving quickly or exerting yourself, resonate with the horsefly’s natural instincts to find food.

This behavior is not personal or targeted. It’s a biological response to environmental stimuli that signal the presence of a potential food source.

The idea of being chased by a horsefly may be unnerving, but understanding the reasoning behind this behavior can demystify the experience. Horseflies aren’t chasing you with intent or malice; they’re simply following signals that might lead to a meal.

If you find yourself the target of a persistent horsefly, remember, it’s not a chase; it’s a biological response, one that can usually be deterred by moving indoors or away from the area where the horsefly is active. It’s nature in action, fascinating and complex, even in something as simple as a fly following a jogger.

Picture of a two year old thoroughbred horse hot and sweaty, a perfect specimen for a horsefly.

Fact 3. Why Do Horseflies Bite When You’re Wet?

Our horse barn is over 100 yards from our swimming pool, yet we constantly have horse flies around the pool, and they seem to follow and bite the wet swimmers. I know they are attracted to movement and carbon dioxide, but why do horseflies bite when you are wet?

One theory is that horseflies are able to detect the lactic acid in sweat, which is more concentrated when you are wet. However, this doesn’t apply to wet swimmers. Another possibility is that horseflies are simply attracted to the moisture on your skin.

And they’re around swimming pools because there is some standing water nearby that is providing a breeding ground for them. In some cases, horse flies may also be attracted to the chlorine in pool water.

If you have horse flies around your swimming pool, you may want to take steps to eliminate any standing water sources that they could be breeding in.

Why are horseflies so aggressive?

Horseflies are known for their aggressive nature, which is due to their blood diet. The more time they spend around humans and other animals, the hungrier they get and the more aggressive they become when looking for food.

Female horseflies are aggressive; they fly around looking for blood meals that will provide the protein needed to grow their fertilized eggs; males just hang out on plants. Once a horsefly chooses a victim, they use its powerful jaws to rip loose skin and extract blood from animals and humans.

Most horsefly victims are livestock with thick skin, so their bite’s force must be powerful. And when horseflies bite humans, they don’t take it easy; they use the same biting power on us as they do a thick-skinned animal.

The force and ripping action of horsefly jaws are the reason their bites are so painful. Male horseflies survive on plant pollen and don’t bite humans.

Fact 4: What Attracts Horseflies to Humans

Horseflies don’t randomly select their targets. Specific factors attract them to humans, leading to those often-painful bites. In this section, we’ll explore what exactly draws horseflies to people and discuss how certain colors or clothing types might influence their attraction.

Factors That Attract Horseflies to Humans

  1. Body Heat: Horseflies are sensitive to temperature and are drawn to the warmth of our bodies. They can detect the heat emanating from our skin and are attracted to it.
  2. Movement: Quick or erratic movements can catch the attention of horseflies. If you’re jogging, dancing, or even waving your arms, you might attract their notice.
  3. Carbon Dioxide: We exhale carbon dioxide when we breathe, and horseflies are attracted to this gas. It signals to them that a potential food source is near.
  4. Sweat and Odor: The smell of sweat and other bodily odors can also be appealing to horseflies. These scents might signify a nutritious meal to them.

Colors and Clothing Types That Influence Horseflies

  1. Dark Colors: Some studies suggest that horseflies are more attracted to dark colors. Wearing black, navy, or dark brown might make you more appealing to them.
  2. Shiny Materials: Reflective or shiny fabrics can catch the light in a way that attracts horseflies. This might explain why wet skin, with its reflective surface, can also be alluring.
  3. Loose vs. Tight Clothing: While it may not prevent horseflies from being attracted to you, wearing tightly woven fabrics can make it harder for them to bite through. Loose clothing might provide easier access.
  4. Repellent Clothing: Some clothing items are treated with insect repellents, which can deter horseflies and other biting insects. Investing in these might be wise if you spend a lot of time outdoors.

Why Do Horseflies Bite Me?

Horse flies never seemed to bite my father, but they loved me. I figured it was because he was a smoker and coffee drinker, and the insects didn’t like his scent. But, after doing some research, I learned something different.

So why do horse flies bite me and not others? There are a few reasons. First, horseflies are attracted to movement. Well, I was always running and playing, so if you’re running or playing sports, you’re more likely to catch their attention than someone who is sitting still.

Second, horseflies are attracted to dark colors. So if you’re wearing dark clothes, you’re more likely to be targeted than someone who is wearing light-colored clothes. Finally, horseflies are attracted to areas of the skin that are exposed.

Understanding what attracts horseflies to humans provides insights into how to minimize or avoid their bites. It’s a blend of biology and behavior, with factors like body heat, movement, carbon dioxide, and even clothing playing roles in how these insects interact with us.

By recognizing what draws them in, you can take proactive measures, such as wearing repellent or choosing certain colors, to lessen their attraction. It’s an empowering knowledge that gives you some control over an aspect of nature that can often feel random and frustrating.

picture of a horsefly,

Fact 5: How to Prevent Horsefly Bites

Horseflies are more than just a painful nuisance; they can also be carriers of diseases and pose a real challenge to humans and animals alike. In this section, we will explore the broader methods of pest control and look into the prevention of diseases that horseflies might transmit.

Pest Control and Disease Prevention: Tackling the Challenge of Horseflies

Preventing horsefly bites is an essential part of managing their impact on humans and animals. Here are practical ways to mitigate the risk:

  1. Use Repellents: Applying insect repellents specifically designed to ward off horseflies can provide protection.
  2. Wear Appropriate Clothing: Light-colored clothing that covers the skin can deter horseflies, as they are attracted to dark and moving objects.
  3. Avoid Peak Times and Locations: Horseflies are most active during hot, humid days and near water bodies where they breed. Being aware of when and where they are most likely to be can help in avoiding them.
  4. Implement Environmental Controls: Removing standing water and keeping grass and vegetation trimmed can reduce breeding sites for horseflies.
  5. Consider Professional Pest Control: If horseflies are a consistent problem in a particular area, professional pest control services may be needed to address the issue comprehensively.

Prevention of Diseases Transmitted by Horseflies

Though rare, horseflies can transmit diseases in some parts of the world. Here’s how to reduce that risk:

  1. Understanding the Risk: Awareness of the diseases that horseflies can carry in your region helps in taking appropriate precautions.
  2. Regular Health Check-ups for Animals: If you keep animals that might be bitten by horseflies, regular veterinary check-ups can help in early detection and treatment of any diseases.
  3. Community Efforts: Collaboration with neighbors and local authorities to control horsefly populations can have a broader positive effect on the community’s health.

Horsefly bites can be painful and, in rare cases, might even transmit diseases. Understanding the biology and behavior of these insects, coupled with implementing practical preventative measures, can minimize their impact.

From personal protection strategies to broader pest control methods, a proactive approach to managing horseflies contributes to overall well-being and health. The challenge of horseflies is one that can be met with knowledge, preparation, and action.

How I keep these pest away?

We recently had some guests over and knew beforehand they would want to check out some of the horses we had in our barn. Knowing this, I took steps ahead of time to reduce the risk of horseflies aggravating us.

To keep horseflies from biting, it’s critical to use horsefly repellant, remove standing water, keep your barn clean of manure and debris, use horsefly traps, and burn citronella candles in the surrounding areas. But don’t burn candles in your barn.

Horseflies are a pain for both you and your horses. There are many products that will help to prevent them, including commercial horsefly sprays, masks, fans, or fly boots. I’ve written an article about these items- you may find helpful. Check it out here: Do Horse Fly Masks and Fly Boots Work? Which Ones Are Best?

Can Horse flies Bite Through Clothing?

Some people believe wearing long sleeves, shirts, and pants covering most of their skin will keep horseflies from biting them. Clothing may deter some insects, but not horseflies; they can bite through clothing, fur, and animal hide.

What Do Horse Flies Hate?

Fighting horseflies effectively starts by knowing your enemy. In other words, what do horseflies hate and will keep them at bay? Are there certain smells or colors that they avoid?

Horseflies hate some herbs such as eucalyptus, rosemary, basil, lemongrass, and clove. They also seem to avoid Avon’s Skin-So-Soft, apple cider vinegar, white vinegar, and citronella. Some of these scents are used in commercial horsefly sprays and effectively ward off horseflies.

Picture of flypaper in a horse barn.

Fact 6: The Impact of a Horsefly Bite

A horsefly bite is more than a fleeting annoyance; it can have physical effects that range from mild discomfort to serious health concerns. In this section, we’ll delve into what happens when a horsefly bites, how to treat it, and when it’s wise to seek professional medical help.

Physical Effects of a Horsefly Bite

  1. Immediate Pain: Horseflies have specialized mouthparts that slice into the skin, causing a sharp and often intense pain.
  2. Swelling and Redness: The area around the bite may become swollen and red as your body reacts to the horsefly’s saliva.
  3. Itching: As the bite heals, it might become itchy. Scratching can exacerbate the issue, leading to potential infection.
  4. Potential for Infection: If not treated properly, a horsefly bite can become infected, leading to more serious complications.

Treating a Horsefly Bite: Medical Advice

  1. Cleanse the Wound: Use soap and water to thoroughly cleanse the bite area, reducing the risk of infection.
  2. Apply a Cold Compress: To minimize swelling and alleviate pain, apply a cold compress or ice wrapped in a cloth.
  3. Use Over-the-Counter Medications: Creams containing antihistamines or hydrocortisone can be helpful in reducing itchiness and inflammation.The goal is to disinfect the wound and apply antiseptic cream.
  4. Avoid Scratching: As difficult as it may be, avoid scratching the affected area, as this can lead to infection.

When to Seek Professional Medical Help

  1. Signs of Infection: If you notice increased redness, swelling, warmth, or oozing at the bite site, it may be infected, and you should seek medical care.
  2. Severe Reactions: Some individuals may have a more severe reaction to a horsefly bite. If you experience symptoms such as widespread hives, shortness of breath, or swelling of the face or throat, seek medical help immediately.
  3. Persistent Symptoms: If the bite doesn’t improve or worsens over a few days, it’s advisable to consult with a healthcare provider for proper evaluation and treatment.

The impact of a horsefly bite on humans can be surprisingly significant. Recognizing the potential effects and knowing how to respond is crucial for effective treatment and prevention of complications.

Whether it’s immediate pain or a more prolonged issue like infection, understanding the proper care and knowing when to seek medical help ensures that a horsefly bite remains a manageable nuisance rather than a serious threat to your health.

Picture of horses in a pasture

Fact 7: The Role of Horseflies in the Ecosystem

While horseflies might be best known for their biting behavior, they also play a vital role in the ecosystem. Far from being mere pests, these insects contribute to biodiversity and have a significant influence on the environment. In this section, we’ll explore the ecological importance of horseflies, focusing on their role as pollinators and their position in the food chain.

Horseflies as Pollinators

  1. Unexpected Pollinators: Though not as famous as bees, horseflies also contribute to pollination. When feeding on nectar, they inadvertently transfer pollen between flowers, helping plants reproduce.
  2. Preference for Certain Plants: Some plants are specifically adapted to attract horseflies. The way these plants’ flowers are shaped, their color, and the scent they emit can draw horseflies, facilitating pollination.
  3. Impact on Agricultural Crops: Horseflies might even contribute to the pollination of certain crops, indirectly supporting agriculture and our food supply.

Horseflies in the Food Chain

  1. A Food Source for Predators: Horseflies are part of the diet for various animals, such as birds, frogs, and spiders. Their larvae also provide nourishment for fish and other aquatic creatures.
  2. Impact on Prey Populations: By preying on smaller insects and their larvae feeding on organic matter in water, horseflies can influence the population dynamics of other species, creating a balance within their habitats.
  3. Indicators of Environmental Health: The presence or absence of horseflies can be an indicator of environmental conditions. For example, a sudden decline in horseflies may signal an issue with water quality or other ecological factors.

Balancing Nuisance and Necessity

It’s easy to view horseflies solely as irritating pests, especially when considering their biting behavior. However, their role in pollination and their position in the food chain illustrate a more complex picture. Like many insects, they form a link in the intricate web of life, where even a small creature can have a large impact.

Horseflies, though often maligned for their painful bites, are essential components of many ecosystems. Their dual role as pollinators and a food source for various predators places them in a unique and vital position. Understanding their ecological importance encourages a more balanced perspective, one that recognizes their contribution to biodiversity and the interconnectedness of nature. It’s a view that elevates the horsefly from a mere nuisance to a valuable participant in the delicate balance of life.

My opinion: Horseflies Risk of Disease and Agravation outweigh their usefulness.

We know horseflies transmit fatal diseases, aggravate animals and humans, and pack a painful bite. But there is typically some utility in even the most mundane things in our life. So I wondered what benefits horseflies provide.

Horseflies are not good for anything. You can stretch your imagination and claim they are part of the ecosystem food chain. However, as a food source, its impact is negligible. Yes, birds eat horseflies, but not enough to make a meaningful impact on their diet.

Where Do Horseflies Live?

It seems horseflies are everywhere during the summer, and a couple of them frequently disrupt our guests at the swimming pool. Their presence made me wonder if they live nearby because our horse barn is over 100 yards away from the pool.

Horseflies are most active in the summer between May and September; they congregate around horse barns and pastures. They thrive in warm, moist areas. Stagnate water and manure or two favorite locations for immature horseflies to live.

Our swimming pool is behind our house but surrounded by a pasture on three sides. The pool and the surrounding area have a lot of standing water that attracts insects, and our guests’ movement also plays a role in the presence of horseflies around our house and swimming pool.

Below is a YouTube video that explains why horseflies are so aggressive.

YouTube video


Do homemade horsefly sprays work?


Yes, and some homemade horse fly sprays are as effective as commercial sprays. Plus, most are safer and cheaper to make than retail fly sprays.
You can learn more about homemade horsefly sprays in this article.

Do you know why horses attract horseflies?

Flies are attracted to large, warm, and dark moving objects, horses, and cows primarily, but humans in dark clothing will suffice.
If you want to read more about what attracts horses, check out this article: Why Do Horses Attract Flies? Number 2 May Surprise You