Deer flies and horse flies belong to the Tabanidae family and are both known for their annoying and painful bites. These bloodsucking insects can be quite bothersome, especially to cattle, horses, and humans. Understanding the differences between them can help you better identify and manage these pests.
Deer flies are typically smaller, measuring between 10 to 13 mm long, with greenish-yellow thoraxes featuring dark stripes. Their wings appear tinted smokey gray-brown or showcase dark patterns. On the other hand, horse flies are larger, ranging from 14 to 19 mm in length, and possess clear wings and grayish-brown thoraxes.
Aside from their physical distinctions, each species’ behavior and habitat can also vary.
Deer Fly and Horse Fly: A Comparison
Deer flies and horse flies are bloodsucking insects with a few differences in appearance. Let’s examine their distinct characteristics:
Both deer flies and horse flies can be found in environments with nearby water sources due to their aquatic larval stages3. However, deer flies are more often encountered in wooded areas, while horse flies prefer open areas such as pastures and meadows4.
Deer flies (Chrysops) and horse flies (Tabanus) can be found across most regions of the United States, although they’re more prevalent in warmer climates5. Notable differences in distribution include:
- Deer flies: Higher concentrations in the southeastern US5
- Horse flies: Widespread throughout the US but less common in arid regions5
|Feature||Deer Fly||Horse Fly|
|Size||6-10 mm||20-25 mm|
|Wing Appearance||Dark bands||Clear or solid|
|Eye Color||Brightly colored||Vibrant|
|Preferred Habitat||Wooded areas||Open areas|
|Distribution||Southeastern US||Widespread in US|
Life Cycle and Reproduction
- Both deer flies and horse flies lay their eggs on vegetation near wetlands or aquatic habitats.
- Females typically lay eggs in specific locations, for example on vegetation overhanging water3.
- The larvae of both flies are generally white, brownish, or greenish in color1.
- Deer fly larvae are usually smaller than horse fly larvae1.
- Both types of larvae are spindle-shaped and taper to a point at both ends1.
- Larval stage feeding habits:
- Winter is spent in the larval stage, with pupation occurring in spring3.
- The pupal stage of both flies is spent in the same aquatic or wetland habitat as the larvae3.
- Adult emergence happens during late spring and summer3.
- Deer fly adults are 6-10 mm long and yellow to brown in color with patterned wings5.
- Horse flies range in size from 3/4 to 1-1/4 inches long, with clear or solidly colored wings and brightly colored eyes2.
|Aspect||Deer Fly||Horse Fly|
|Size||6-10 mm long5||3/4 to 1-1/4 inches long2|
|Wing Patterns||Patterned5||Clear or solidly colored2|
|Larval Habitat||Wet or semi-aquatic environments4||Wet areas or terrestrial environments5|
|Adult Habitat||Wetlands, aquatic1||Wetlands, aquatic1|
Feeding and Biting Habits
Adult Feeding Habits
Both horse flies and deer flies exhibit different feeding habits among their male and female counterparts. As adults, male horse flies and deer flies primarily feed on nectar, while their female counterparts need blood meals for egg production.
Female horse flies and deer flies are attracted to several factors from potential hosts, including carbon dioxide, body heat, and movement. For example, female deer flies tend to bite humans while they are outdoors near wooded areas.
The bites from both horse flies and deer flies can be quite painful, as their mouthparts cut the skin rather than pierce it. This blood-sucking habit can lead to the spread of various diseases and infections.
The bite differences can be seen below:
|Feature||Horse Fly Bite||Deer Fly Bite|
|Pain Level||More painful||Less painful|
|Bite location||Exposed skin||Exposed skin|
- Larger size (14 to 19 mm long)
- Clear wings
- Grayish-brown thorax
- More aggressive bites
- Found near livestock, such as in barns
- Smaller size (10 to 13 mm long)
- Tinted or patterned wings
- Greenish-yellow thorax with dark stripes
- Less aggressive bites
- Found in wooded and marshy areas
Some ways to protect yourself and animals from these bites include using essential oils, wearing protective clothing, and avoiding areas where these flies are more prevalent. Horse and deer flies are daytime biters, so turning horses out at night instead of the day can help reduce biting. Providing shelter or canopy trees far from wooded edges can also protect horses from bites.
Health Impacts and Control Methods
Deer flies and horse flies are blood-sucking insects that can be serious pests for livestock and humans. They can transmit various diseases in the process, some of which include:
- Anaplasmosis: A bacterial infection that affects mostly cattle, but can also impact sheep and goats.
- Tularemia: A bacterial disease affecting humans, known as “rabbit fever.”
- Equine infectious anemia: A viral disease that affects horses.
- Hog cholera: A highly contagious viral disease in pigs.
- Anthrax: A bacterial infection that can infect livestock and humans.
- Filariasis: A parasitic infection transmitted by deer flies that can cause health problems in humans.
Prevention and Repellents
Preventing deer fly and horse fly bites is essential in controlling the spread of diseases. Some effective measures include:
Regularly inspect and maintain fences and screens to minimize access for flies.
Use approved pesticides, like permethrin-based sprays, to control flies in livestock areas.
Apply repellents to livestock and humans when in infested areas.
Examples of deer fly and horse fly repellents:
- DEET-based repellents
- Picaridin-based repellents
- Natural oil-based repellents (e.g. eucalyptus or lemon oil)
|Feature||Deer Fly||Horse Fly|
|Size||6-10 mm long||14-19 mm long|
|Color||Yellow to brown||Grayish-brown|
As members of the family Tabanidae, deer flies and horse flies share some common characteristics. Tabanid flies are robust, capable of biting, and can transmit diseases to both livestock and humans. The health impacts of these pests can be severe if not controlled properly through prevention and the use of repellents.
Interesting Facts and Behaviors
Predators and Prey
Deer flies and horse flies are known as bloodsucking pests that can cause discomfort to humans and animals. They can be active in various environments, such as forests and wetland environments. Adult female deer flies and horse flies feed on the blood of mammals, while adult males feed mainly on pollen and nectar.
Some common predators of these flies include birds and other insects. Interestingly, some people utilize fly traps outdoors to control their numbers around their property.
Mating and Attraction
The mating habits of deer flies and horse flies are quite fascinating. Males and females come together in a specific location, usually near vegetation, to mate.
|Deer Flies||Horse Flies|
|Smaller in size (10 to 13 mm long)||Larger in size (14 to 19 mm long)|
|Greenish-yellow thorax||Grayish-brown thorax|
|Smokey gray-brown wings||Clear wings|
In order to attract mates, male flies often rely on visual cues. Once the female is attracted, the pair will engage in a flight dance before mating. After mating, female flies will search for a suitable host to feed on in order to obtain the nutrients necessary for egg development.
- Both flies undergo metamorphosis, passing through egg, larva, pupa, and adult stages.
- Can be found in outdoor and wetland environments.
While there are many similarities between deer flies and horse flies, they do have some different features that set them apart. By observing their distinct sizes, thorax colors, and wing patterns, it’s easier to identify which is which.
It’s important to protect yourself and your animals from these bloodsucking pests by using protective clothing, repellents, and fly traps when necessary.
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 – Horse Fly
August 16, 2009
This bug was filmed in keystone heights florida. What is it? It looks like a fly, but not. I did save it after it died and still have it.
I couldn’t get a decent photo of it, but here is a movie
This is Chlorotabanus crepuscularis, a species of Horse Fly. According to BugGuide: “Identification Body pale green, eyes and thorax yellowish green. The only green tabanid in NA. Range An eastern species occurring south of a line from Delaware to southern Texas. Habitat Larvae predaceous, usually in soil at edge of water and in floating vegetation, occasionally in forest soil. Adults in vicinity of larval habitats Season In Florida, flying from mid-March to mid-September with peak activity from May to mid-July. Food Females feed on mammalian blood Remarks As with all the blood-feeding tabanids, the females are responsive to Carbon Dioxide. I caught over 500 females in one night with a trap baited with dry ice in coastal South Carolina. Will also come to lights at night.
Regarded as a pest species in Florida“
Letter 2 – Horse Fly
July 5, 2010
This really, really big fly was sitting on a leaf and sat still while I got a close shot (I was taking pics of a butterfly when I saw him). We live in Delaware and I have seen these large flies around and they scare the bjesus outta me. I have searched with no success….any clue?
Kent County, Delaware
This is a Horse Fly, and though Horse Flies are known to bite animals and humans, you have nothing to fear from this individual. Only female Horse Flies are blood suckers. The male Horse Flies do not bite. The males can be identified by the structure of the eyes. The two compound eyes of the male Horse Flies have no space between them while the eyes of the females have a separation. We believe, based on photos posted to BugGuide, that this might be Tabanus abdominalis, though an expert in Horse Flies might be required to give a conclusive species identification.
Letter 3 – Horse Fly
Large fly with horn-like antennae(?)
Location: New Boston, MI
August 15, 2010 10:19 pm
Found this guy in the first half of August in southeast Michigan. It landed on my car in a rural area, with fields, woodlands, marshlands, and a small river all nearby. It was fairly large, about an inch long or more. I spent years of my childhood exploring the area and have never seen anything like it.
We believe we have properly identified your Horse Fly as Tabanus sackeni based on images posted to BugGuide. It appears that your specimen is a female because of the space between her eyes. Like Mosquitoes, Female Horse Flies bite while males do not.
Letter 4 – Horse Fly
Hitch-hiking Horse Fly
Location: North Middle Tennessee
August 16, 2010 8:22 pm
While driving down a country road looking for insects to photograph this one found me. It landed by the passenger side window. My wife saw it first and said, ”I don’t know what the heck that is.” I knew it was a horse fly but that was about all. I just now looked at the website to try and ID it. The very first horse fly (from yesterday) looks like a match. It appears this is a ”Tabanus sackeni” I wonder if they have a habit of hitching rides. Randall from Mi said his landed on the car as well. Thanks and have a great day.
Our first thought upon looking at your images was that Randall sent more photos because your images are so similar to his. We agree that this is another female Tabanus sackeni, though we always allow for expert corrections to our amateur attempts. You pose an interesting question. We doubt that Horse Flies have evolved to the point that they are using Phoresy to get around. Phoresy is the act of one creature hitching a ride on another creature. We suspect it is more a matter of being attracted to the color of the car, or the reflectance, or something that we just don’t understand, but whatever the reason, it might make a nice study for a research paper.
Here is one more photo for laughs.
Letter 5 – Horse Fly
Please identify each bug
Location: Southeastern Iowa, Van Buren County, United States.
September 12, 2010 6:45 pm
I would like you to identify each of these bugs for me please. I am 14 and need them identified for a school project.
Signature: Carson Schuck
We do not do people’s homework since we have enough of our own preparation to do for school. Your fly is a Horse Fly, but we have not been able to identify the species on BugGuide, though we believe we found an unidentified match in the genus Tabanus on BugGuide, though because of the angle on your photograph, we are unable to determine if it has the same number of abdominal stripes as the BugGuide image.
Letter 6 – Horse Fly
Picture I took – I swear on a stack of bug books
Location: Scott Lane, Sandown, NH
August 17, 2011 1:43 pm
I know this is a 3 spot horse fly but thought other people might enjoy seeing it as well.
Signature: Bev Manning
We looked on BugGuide for photos of a Three Spot Horse Fly, and we could not find anything with that common name, however, it is logical that Tabanus trimaculatus could have that common name, as indicated in this BugGuide posting. It is difficult to tell from your photo if your fly matches the pinned specimen on World Field Guide. In a more practical vein, most people who encounter Horse Flies might be more concerned if the fly was a male or female since only the females are blood sucking biters. Males feed on nectar, though we have read that females will also feed on nectar. The eyes are the best way to quickly distinguish males from females, but alas, your lateral view does not illustrate the spacing between the eyes. Your Horse Flies presence on a blossom would make us speculate that it is more likely a male than a female, but we cannot be certain.
Closest I got to the top of this fly. Thanks very much for the reply
Hi again Beverly,
We can’t really disagree about this being Tabanus trimaculatus, and this new view shows clearly she is a female because the space between the eyes is evident.
Yay – thank you. Your time is very much appreciated.
Letter 7 – Horse Fly
Location: Big Pine Key, FL
November 13, 2011 9:53 pm
Hello, I saw this fly this morning on my porch’s railing. I live in the Florida Keys. Didn’t seem scared by us. Feeds on nectar? Thanks
You have the family correct, but not the sex. This is a Horse Fly, but the space between the eyes indicates she is a blood sucking female. We are unable to identify the species at this time.
Letter 8 – Horse Fly
Subject: Loud & Fast; Scares the Dog
Location: Missouri, 60 miles from St Louis
September 30, 2013 2:31 am
Several of these show up around our house each spring-early fall. They will hover in place for a while, then take off really fast, zooming around for a while before suddenly stopping to hover in place. The hum/buzz of their wings is loud, my dog even refuses to go outside when he hears them. They fly very fast when they aren’t hovering (my grandpa started calling them ’bullet bugs’). I finally managed to catch one with a grocery bag so I could photograph it. If you can identify it, I’d love to know what this is that scares my dog (and if he has good reason to be scared!)
This is a Horse Fly in the family Tabanidae, and according to BugGuide: “Females (but not males) suck vertebrate blood which they need to produce eggs.” We are not certain of the species, but your individual resembles Tabanus calens which is pictured on BugGuide. Your individual appears to be a male, which will not bite. If you dog has had a bad experience with one or more females buzzing prior to biting, you dog might be remembering the experience. Horse Flies generally feed on livestock, but when livestock is not available, other warm blooded prey, including humans, will suffice.
Thanks so much for that! I searched around for a while trying to find out what they are, but didn’t have luck. I’m usually better at identification when it comes to spiders (Love them so much).
Next time I get really good spider pics, I’ll be sharing them like I did my fishing spider before.
Letter 9 – Horse Fly
Subject: cicada killer wasp??
Location: Mason, MI
September 6, 2014 8:35 pm
This is the 2nd time I’ve seen this insect–both times I was inside my car. The first time I was driving when my son noticed it on the windshield and it stayed put until my speed reached about 50-55 mph. This one landed on my car when I reached my destination and I hadn’t gotten out yet. So I took a picture so I could try to identify it later. It’s at least 1 inch or bigger and I thought the first one looked like it had striped-like markings of a wasp but this one doesn’t appear to have those as much but it could be the angle of the picture. A friend thought it might be a cicada killer wasp but I wasn’t as convinced.
This is a Horse Fly, not a Cicada Killer.
Letter 10 – Horse Fly
Subject: Is this a Bot Fly?
Location: South Windsor ct
July 24, 2015 4:08 pm
Hello I am in south Windsor ct and the almost 2 inch fly wondered into our shop.
I looked up bot fly and saw there was different types and did find one that looked like this.
Signature: Is this a bot fly?
This is most certainly NOT a Bot Fly, but a Horse Fly in the family Tabanidae, and based on the spacing between the eyes, it is a blood sucking female. We believe it is Tabanus stygius based on this BugGuide image.
Thank you Daniel, I only asked because there was a Bot fly found in Westport Ct.
Letter 11 – Horse Fly
Subject: What type of fly is this?
Location: Chatham, ON, Canada
July 26, 2016 6:31 am
Hi. Recently, I took a picture of this fly on my car and I’m not sure what kind it is. Most people say it’s a horse fly but I’m not sure. Any idea?
This certainly is a Horse Fly, and upon scrolling through BugGuide pages, we believe we have correctly identified it as Tabanus stygius. According to BugGuide‘s data, this wide ranging species is found in Ontario. We cannot tell from your image if this is a female or a male as the space between the eyes is not visible, but only female Horse Flies are blood-sucking biters.
Letter 12 – Horse Fly
Subject: Large Fly
Geographic location of the bug: Michigan
Time: 09:23 AM EDT
Your letter to the bugman: I saw this fly sitting on my trash can. Hot weather. The ledge he is sitting on is about 1 1/4″ wide. I can’t find any Google images that match. Can you identify this bug?
How you want your letter signed: Tom
This is an impressive female Horse Fly and we believe we have correctly identified her as Tabanus stygius thanks to this BugGuide image.
Thank you for your response and identifying the fly. I’m impressed you replied so quickly. Thank you.
You happened to send your request during the window of time in the morning we try to devote to responding.