Are Horse Flies Dangerous? Uncovering the Truth

Horse flies and deer flies are known for their bothersome presence around humans, livestock, and wildlife.

These insects are bloodsuckers, often causing painful bites in their quest to feed.

While their annoying nature is undeniable, the question arises: are horse flies dangerous?

The concern over their potential to transmit diseases is valid, as their bloodsucking habits may introduce pathogens to their hosts.

Despite this risk, it’s important to understand the biology of these insects to properly assess their impact on public health and learn how to minimize the chances of being bitten.

Differentiating between horse flies and deer flies can help in understanding their potential hazards.

Horse flies are typically larger, with clear wings and grayish-brown thoraxes, while deer flies are smaller, sporting tinted wings and greenish-yellow thoraxes with dark stripes.

Their varying appearances can clue individuals into which type of fly they may be dealing with and what precautions to take.

Basic Information about Horse Flies

Physical Characteristics of Horse Flies

Horse flies belong to the Tabanidae family and are known for their large size and distinct appearance. Some common features of horseflies include:

• Size range: 3/4 to 1-1/4 inches long
• Large, brightly colored eyes
• Clear or solidly colored wings
• Grayish-brown or blackish body color

Gender Differences: Male Vs. Female Horse Flies

There are notable differences between male and female horse flies. Key distinctions can be found in the table below:  

Feature	Male Horse Flies	Female Horse Flies
Mouthparts	Non-biting	Biting
Feeding	Nectar and pollen	Blood meal

Male horse flies have large eyes, which almost touch, while female horse flies have smaller and more separated eyes. The primary difference lies in their feeding habits.

Horse Fly Bites

Why Horse Flies Bite

Horse flies bite humans and animals for one primary reason: to obtain blood. Female horse flies need the nutrients in their blood to produce eggs. These flies have specialized mouthparts to effectively access blood from their victims.

They use a scissor-like mandible to cut through the skin, which can cause significant pain for the one being bitten.

Symptoms of Horse Fly Bites

Horse fly bites can lead to various symptoms:

• Pain: The bite is usually painful due to their cutting mouthparts.
• Redness: The bitten area often becomes red and inflamed.
• Swelling: Bites can lead to puffiness and swelling around the bite site.
• Itching: As with many insect bites, itching is a common symptom.
• Rash or hives: Some individuals may develop a rash or hives near the bite.

In some cases, horsefly bites can lead to more severe symptoms, including:

• Fever: A high temperature may indicate an infection or an allergic reaction.
• Vomiting: Severe reactions to horsefly bites may cause vomiting.
• Bacterial infection: Bacteria from the fly’s mouthparts can enter the bite wound, leading to infection.

Symptoms	Common	Severe
Pain	Yes	Yes
Redness	Yes	Yes
Swelling	Yes	Yes
Itching	Yes	Yes
Rash or hives	Yes	Yes
Fever	No	Yes
Vomiting	No	Yes
Infection	Possible	More likely

While horse flies don’t directly transmit diseases, the risk of bacterial infection and potential allergic reactions makes their bites concerning.

In horses, horse fly bites have been associated with transmitting equine infectious anemia.

Prevention and Control

Protecting Livestock and Pets

In summer, horse flies can be quite bothersome to livestock and pets. To protect them from these pests, consider the following:

• Use fly sheets, masks, and boots to physically exclude flies from contacting your animals.
• Place fans in stable areas to interrupt flight and make it difficult for flies to land.
• Provide your animals with deep shade or housing, which also helps manage water sources and reduce exposure to aquatic biting flies¹.

Effective Traps and Repellents

Various methods can help control horse populations, including some traps and repellents. For example:

• Horse Pal and Epps Biting Fly Trap can be effective in reducing horse fly populations.
• Insect repellents containing DEET or picaridin can offer some protection against deer flies.
• Using light-colored clothing may help reduce the attraction to and bites from these flies.

Here’s a comparison table of these repellent and trap options:

Method	Pros	Cons
Fly sheets, masks, and boots	A physical barrier, reusable	Limited area of protection, animals may outgrow or damage
Fans	Non-chemical, energy-efficient	Requires electricity
Housing, shade	Provides shelter	Takes up space, requires maintenance
Traps	Targets specific fly species	May require maintenance, costly replacement parts
Repellents	Applied directly to the skin	Needs reapplication, may cause skin irritation

By using these preventative measures and techniques, you can help reduce the risk of your animals and yourself being affected by horse flies.

Horse Fly Life Cycle

The Breeding Environment

Horse flies are known for their blood-sucking behavior during daylight hours. They belong to the family Tabanidae and thrive in moist environments, such as marshes, swamps, and damp areas.

Some characteristics of their breeding environment include:

• Wet areas with standing water
• Presence of plants and decaying organic matter
• Close proximity to animals, as they provide a source of protein for the female horse flies

Development from Eggs to Adult Horse Flies

Horse flies undergo a complete life cycle, including the stages of eggs, larvae, pupae, and adults. Here is an outline of their development:

1. Eggs: Female horse flies lay their eggs on plants near water bodies or in marshy areas, providing the larvae with a suitable environment to develop.
2. Larvae: Once hatched, the larvae consume organic matter, including decaying plants and small organisms like mosquito larvae and ticks. In some cases, they also feed on frogs.
3. Pupae: After reaching a certain size, the larvae form pupae and undergo metamorphosis. At this stage, they are relatively inactive and can remain in the pupal case for several days to months.
4. Adult Horse Flies: They emerge from the pupae and feed on flower nectar and plant sap for energy. Females require blood meals for reproduction, and their bites can be painful for animals and humans alike.

Potential Health Risks

Allergic Reactions

Horse flies are known to bite both humans and animals, such as pets and equine. While their bites can be painful and cause itching, some individuals may experience more severe reactions. Allergic reactions to horsefly bites can include:

• Wheezing: Difficulty breathing, potentially requiring medical attention.
• Dizziness: Feeling light-headed or unsteady on your feet.
• Swelling: At the site of the bite or, in severe cases, in the face or throat, which could lead to difficulty swallowing.

Infections and Complications

The risk of infection from horse fly bites is relatively low compared to other insect bites such as mosquitoes or house flies. However, if proper care is not taken, complications can arise.

Bacteria can enter the bite wound and lead to infection. To minimize these risks, follow these simple steps:

• Clean the area: Use soap and water to disinfect the bite.
• Ice pack: Apply an ice pack to reduce swelling and numb pain.
• Avoid itching: Scratching the bite can introduce bacteria and increase the risk of infection.

In some cases, horsefly bites may transmit diseases such as swamp fever in North America. This disease mainly affects equine populations and is rare in humans.

To protect yourself from horse flies when outdoors, especially in hot and swampy environments, consider using insect repellents containing DEET.

Method	Pros	Cons
Insect repellents (e.g., DEET)	Effective in repelling horse flies	May be harmful to certain environments or some individuals

It’s essential to remain vigilant and take necessary precautions when spending time outdoors, particularly in areas with a known presence of horse flies.

Conclusion

In conclusion, horseflies are considered nuisances due to their painful bites and intimidating appearance. These insects raise valid concerns about their potential dangers.

They don’t directly transmit diseases like mosquitoes, but since the females consume blood, they pose various health risks. The bites are painful and can cause discomfort, itching, and localized swelling.

These bites can also trigger an allergic reaction and can lead to more severe symptoms, such as wheezing, dizziness, and extensive swelling.

The bites also add the risk of infection because the bacteria from the flies’ mouthparts can enter the wound.

Use the tips and tricks mentioned in the article to avoid these bites and to be safe from these insects.

Footnotes

1. Horse Flies and Deer Flies | Entomology – University of Kentucky ↩ ↩²

2. Horse and Deer Flies | Missouri Department of Conservation ↩ ↩²

3. University of Kentucky Entomology ↩

4. Purdue University Public Health and Medical Entomology ↩

Reader Emails

Over the years, our website, whatsthatbug.com has received hundreds of letters and some interesting images asking us about these insects. Scroll down to have a look at some of them.

Letter 1 – #9996: Greenhead Horse Fly

2 1/2″ Fly with Large Blue Eyes
May 13, 2010
This bug was on our driveway and I thought it was dying. It was huge (2 1/2″ long) and I went to get my camera so I could look it up in my bug book. It was still there and I was able to get one picture.

The eyes were very blue. When I moved it to get a picture straight out of its eyes it flew away! I was so disappointed. Obviously, it wasn’t dying, but possibly just hatched. Please tell me what it is if you can. I’ve never seen anything like it and I take a lot of pictures of bugs, birds, etc. in our yard.
Sandi
Boca Raton, FL

Hi Sandi,
This is one impressive Horse Fly, but we do not know the species.  Perhaps one of our readers will have better luck researching than we have had.

Thanks for your quick response once again!  All I can say is that boy must be on steroids!!!  He was really large for a fly.
Sandi

Eric Eaton identifies Greenhead Horse Fly
Hi, Daniel!  Hope you had a great trip to see Mom for Mother’s Day 🙂
The horsefly is no doubt the “Greenhead,” Tabanus americanus.  Would not want to be bitten by that one!
Eric

Thanks, Eric,
According to BugGuide, this is the Earth’s largest Tabanid.

Letter 2 – Orange Banded Horse Fly

Subject:  What’s this big black fly with yellow middle?
Geographic location of the bug:  Tweed River, Pittsfield VT
Date: 07/17/2018
Time: 02:07 PM EDT
Your letter to the Bugman:  Please help us identify this enormous fly or whatever the heck it is!! We took the kids and dog for a swim in the River at the end of a hot day and these flies were relentless!

We’ve never seen them before and I can’t find anything similar on the internet. If anyone knows what this is, it’s you.
How you want your letter signed:  Thank you!

It’s an orange-banded horse fly! I posted to Facebook and a friend helped us identify. So no need to waste your time on us. Horse fly?? I feel kinda silly I even asked!! LOL.
Thank you!
Michelle D.

Dear Michelle,
After verifying the identity of this Orange Banded Horse Fly, Hybomitra cincta, on BugGuide, our first thought is that it is a stunningly beautiful Horse Fly and BugGuide does note:  “Females have first three segments of abdomen orange, rest of abdomen black (sharply delimited), and wings dark.

Males are harder to identify.”  Don’t underestimate the amazing diversity of Horse Flies.  Some especially striking examples from our site are the only green North American Horse Fly Chlorotabanus crepuscularis, the American Horse Fly, and the Western Horse Fly.

Letter 3 – Dark Giant Horse Fly from Scotland

Subject: I was wondering what this was
Location: Central Scotland
July 1, 2017, 6:09 am
Hello
We were trying to find out what this was. We had thought it was some kind of hoverfly but we have never seen one so big.
Signature: Best wishes, Dawn

Dear Dawn,
This is a female Horse Fly, Tabanus sudeticus, and we just posted images of a male.  Only female Horse Flies are blood-sucking biters, and they can be distinguished quickly from males by the spacing between their eyes.  Males do not have a space between their eyes. 

According to Influential Points, the common name for this species is Dark Giant Horse Fly and the site states: 

“The dark giant horsefly flies in July and August and commonly feeds on the blood of cattle and ponies. In Europe … Tabanus sudeticus flies from the end of June and through July and August. Krčmar (2005)  reports that it reaches its maximum abundance in the third week of July.

In Britain, it mainly lives in boggy areas in the north and west, although it is also quite common in the New Forest. Tabanus sudeticus is distributed widely in northern Europe into Russia.” 

According to iNaturalist:  “a species of biting horse-fly. It is the heaviest fly in Europe.”  According to the Nature Net Ranger’s Blog: 

“This is Tabanus sudeticus, sometimes called the dark giant horsefly. It seems, oddly enough, that this impressive insect has not really got a commonly-accepted English name. It’s referred to in one place as the “dark behemothic horsefly”: a charmingly descriptive name, albeit a little cumbersome.

Yes, it’s sitting on my finger and no it didn’t bite me. They can be up to 25mm long (that’s one inch) and 50mm across the wings – a massive fly and the largest dipteran in Europe (I think it was bigger!

Having measured the Ranger’s finger, the fly could’ve been at least 30mm – The Cat). Horseflies are big, fast-flying creatures, and they will bite any big mammal, including humans.

The bite is very painful, and as horseflies cut the skin when they bite (rather than pierce it), horsefly bites can take a long time to heal and can cause infection.

Unlike insects which surreptitiously puncture the skin with needle-like organs, horse flies have mandibles like tiny serrated scimitars, which they use to rip and slice flesh apart.”

Letter 4 – American Horse Fly

Green Eyed Bug
Two of these showed up this morning. They are at least an inch long, (although it looked a lot bigger when it landed on my leg!)
Thanks, Rich Armstrong
North Stonington, CT

Hi Rich,
Luckily you didn’t get bitten by the American Horsefly, Tabanus americanus. It is usually found near swamps, marshes, and ponds. The male eats pollen and nectar but the female takes blood from large mammals, including man.

According to the Audubon Guide: “When the female bites, the wound inflicted often continues to bleed for several minutes because the fly’s saliva contains an anticoagulant that prevents clotting.

A single animal may suffer a debilitating loss of blood if many of these insects attack it.”

Letter 5 – Biting Fly from Jordan is Horse Fly

Subject: Beautiful biter
Location: Jordan
May 30, 2012, 9:55 am
Ok, here’s a fly that found me instead of the other way around!
It is attractive, with those eyes and patterned wings, but its bite was painful.
I’d love to know what it is, so any help identifying it would be great!
Signature: Ben from Israel

Hi Ben,
We do not recognize this Biting Fly.  We will post your photo in the hope that we can get to this at a later date or that one of our readers will have luck with an identification.

Karl identifies the Horse Fly
Hi Daniel and Ben from Israel:
This is a Horsefly in the family Tabanidae. It looks very similar to the Cleg Fly from Europe, Haematopota pluvialis, but the wing patterns are not quite right and I don’t think that species makes it as far south as Jordan.

I believe it is probably another fly in the same genus; this kind of wing pattern is apparently diagnostic for the genus. There is an excellent paper titled “Systematics and distribution of horse flies (Diptera: Tabanidae) of Jordan” (H. Al-Talafha, et al. 2004), accessible online at:

http://www.sove.org/Society_for_Vector_Ecology/Journal/Entries/2005/6/1_Volume_30,_Number_1_files/8Al-Tafaha%20et%20al.pdf.

According to this paper, there are 3 species of Haematopota in Jordan. Although not all are described in detail, I believe the posted image is probably an H. coronata.

The above paper includes an identification key with the following description for H. coronata: “Wing tip is always clear (Plate I-e), a brown band extends between antennal base to the lateral sides of the eyes”. I hope this helps. Regards.  Karl

Letter 6 – American Horse Fly

Subject: Unknown Fly
Location: Houston
May 11, 2016, 9:22 am
Can you id this fly? About the size of the blowfly. Found outside the warehouse, near the septic tank.
Signature: Richard

Dear Richard,
We quickly identified your female Horse Fly on BugGuide as Tabanus americanus, but we cannot fathom why it was not given the common name American Horse Fly based on its scientific species name. 

According to BugGuide:  “Planet Earth’s largest tabanid.”  That would make it a pretty large Horse Fly.  Only female Horse Flies bite and feed on blood, and when there is no livestock available, they will bite humans. 

The Encyclopedia of Life does refer to this as the American Horse Fly.

Letter 7 – Bug of the Month July 2018: Giant Horse Fly Creates Media Buzz in England

Subject:  What the hell is it??
The geographic location of the bug:  Bassenthwaite Cumbria England
Date: 07/01/2018
Time: 12:20 PM EDT
Your letter to the Bugman:  Please help me with what the hell this is!!
How you want your letter signed:  Gail.

Dear Gail,
Congratulations on being chosen Bug of the Month for July 2018 with your query of this Giant Horse Fly, in the genus Tabanus.    You are the third identification request we have received this week, and we quickly linked to a Huffington Post posting. 

We cannot tell due to the camera angle if this is a male or female Giant Horse Fly.  Males in the genus have compound eyes that nearly touch one another while the eyes of the female have a space between them. 

Only the female Giant Horse Fly will bite as the male does not feed on blood which is necessary for the female to lay viable eggs. 

That blood generally comes from livestock including horses and cattle, but when livestock or other large mammals are not available, the opportunistic Horse Flies might bite humans try to remember after viewing the images in that Huffington Post article that most encounters between humans and Horse Flies do not end with bites. 

The Gadfly that tormented Io in Greek mythology was most likely a Giant Horse Fly as Wikipedia confirms.  Long ago, the mythological Io was also the inspiration for the name of the lovely North American Io Moth as was consistent with the pattern set with 18th Century taxonomists like Linnaeus and Fabricius.

Letter 8 – Dark Giant Horse Fly from the UK

Subject:  Giant  Horsefly???
The geographic location of the bug:  Bournemouth UK
Date: 07/17/2021
Time: 10:33 AM EDT
Your letter to the Bugman:  Is this a Giant Dark Horsefly? It landed in the garden yesterday and we were curious.
How you want your letter signed:  Jane B

Dear Jane,
We agree with you that this is a Dark Giant Horse Fly and it appears to be a female.

Letter 9 – Beautiful White Male Horse Fly with no common name: Leucotabanus annulatus

Subject:  Is it a Fly?
Geographic location of the bug:  Madison, Florida
Date: 08/13/2021
Time: 06:56 AM EDT
Your letter to the bugman:  Can’t find anything like it in my bug books.
Have not found it on any web searches. Is it a fly or a moth?
How you want your letter signed:  Farmer Bob

Dear Farmer Bob,
This is definitely a fly and the large, close-set eyes indicate a male.  We were nearly certain this is a Horse Fly but we cannot ever remember seeing a white Horse Fly.  Our web search quickly brought up a FlickR posting of Leucotabanus annulatus.

We then confirmed its identity on BugGuide and learned the preferred habitat is “places where there are trees and rotting wood.” 

We are very excited as this is a new species for our site and it is a magnificent species.  As an aside, female Horse Flies suck the blood of warm-blooded animals and they will bite humans if there is no other source of blood. 

Males do not bite, do not feed on blood, and pose no threat to humans.