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Deer Fly vs Horse Fly: What’s the Difference? (With Pictures)

If you own livestock or horses, you likely already know about the nuisances that are Deer Flies and Horse Flies. These two fly breeds are alarmingly large, and the females feed on blood. Not only are their bites painful, but they can also spread infectious diseases and parasites.

Deer Flies and Horse Flies are similar in a lot of ways. They are different, however, because of their body build. To the naked eye, you should be able to tell the difference between these two species just by looking at their size. Horse Flies can be nearly an inch bigger than Deer Flies. This drastic size difference makes it easy to distinguish between the two.

To find out more about these flies, keep reading. In this guide, we give you an overview of both fly types and provide tips for protecting your livestock from them. Let’s get the buzz about these flies.

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Visual Differences

Deer Fly vv Horse Fly
Image Credit: Left – Deer Fly (Paul Reeves Photography, Shutterstock); Right – Horse Fly (RealityImages, Shutterstock)

At a Glance

Deer Fly
  • Average length (adult): ¼ – ½ inch
  • Lifespan: 30 – 60 days
  • Blood Sucking: Yes (female only)
  • Transmits Diseases and Parasites: Often
  • Location: Global, with the exception of polar lands and certain islands
Horse Fly
  • Average length (adult): 1-½ – 1-¼ inches
  • Lifespan: 30 – 60 days
  • Blood Sucking: Yes (female only)
  • Transmits Diseases and Parasites: Often
  • Location: Global, with the exception of polar lands and certain islands

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Deer Fly Overview

Deer Flies (sometimes known as Sheep Flies in the US) are a bloodsucking insect that prey on humans, cattle, and other livestock. Their bites can be very painful, and they can spread a number of diseases. These insects can be found across the entire globe, with the exception of Greenland, Iceland, and Hawaii.

Appearance

The Deer Fly is a large insect that comes in 250 varieties. A single Deer Fly will be bigger than a house fly but smaller than a Horse Fly. It has brightly colored compound eyes and large clear wings to match. These wings will come with dark bands.

Breeding

A single Deer Fly can lay between 100 and 800 eggs per batch. Typically, the female Deer Fly will lay the batch on vegetation around water or damp areas. Whenever the eggs enter the larval stage, they often feed on small creatures and rotting organic matter that can be found around the water. This larval stage can last between one and three years.

They then go through a pupal stage, allowing them to become adults sometime between late spring and summer. As adults, the males will feed on pollen, while the females feed on blood, which is required to produce the eggs.

Attraction to Blood

Since blood is required to produce eggs, it is only the females that feed on blood. Though they can eat a variety of blood types, they prefer mammals. They typically choose prey by smell, site, or detected carbon dioxide.

More so, females may determine their prey by body heat, dark colors, and movement. Light in the night can also attract Deer Flies since they typically hunt during the day. They are most often active during direct sunshine when the temperature is around 71.6 degrees Fahrenheit.

Whenever it is time to feed, they use mandibles and maxillae in a scissor like motion. This creates an incision so that they can suck up the blood. This process is really painful. To ensure that the fly can suck the blood, it has anticoagulants within its saliva, which can cause allergic reactions.

Unfortunately, diseases and parasites can be transmitted through the Deer Fly bite. This includes anaplasmosis, anthrax, equine infectious anemia, filariasis, hog cholera, and tularemia.

Location

Deer Flies are an issue around the globe. With the exception of Iceland, Greenland, and Hawaii, they are located everywhere. Experts suspect that Deer Flies aren’t found in these locations because of their isolation from main continents.

Where You’ll Find Them

If you live in an area with damp forest or wetland environments, you are likely to have Deer Flies in your area. This is especially true if the environment is rural. Expect their activity to be heightened between June and July.

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Horse Fly Overview

Horse Flies (sometimes called gadflies) are a rather frightening breed of fly that are incredibly large and prey on blood. They are most often seen during the day and tend to stay inactive during the night time. Much like Deer Flies, they can be found all around the world, except for polar regions, Hawaii, Greenland, and Iceland.

Appearance

Horse Flies are rather noticeable flies. They can be as large as 1-¼ inch, making them more than twice the size of many Deer Flies. They also have compound eyes and widebodies. The bodies will typically be patterned and brightly covered.

Breeding

Horse Fly mating happens in swarms. Whenever it’s time to lay eggs, the females will lay them on stones or vegetation around water. There can be clusters of up to 1000 eggs in certain water regions. The eggs are white at first but darken in a few days. The eggs typically hatch six days after being laid.

The larvae will fall onto the moist ground or water below. There, they will consume different organic matter, like worms or other larvae. The older they get, the more they will move into drier land. The pupae period lasts for around 2 weeks, which is when the metamorphosis will be complete.

Males typically appear first, followed by the females. After both of the sexes have emerged, they will begin mating. Courtship starts in the air, but it finishes on the ground.

Attraction to Blood

Before the females can lay their eggs, they have to eat blood. The blood is required for the production of eggs. As a result, females have stronger mouths than the males so that they can eat blood from mammals and other animals. Typically, however, females only bite out of necessity.

How females extract blood is through specific mouthparts that are formed as a stabbing organ. This includes two cutting blades and a sponge like section, allowing for the fly to lap up the blood. As you would expect, Horse Fly bites are incredibly painful.

Female Horse flies often transfer blood borne diseases through their bites. This can include equine infectious anemia virus, trypanosomes, filarial worm Loa loa, anthrax, and tularemia.

Location

Horse Flies can be found all around the globe, but they are not found in polar regions and certain islands, like Iceland, Greenland, or Hawaii.

Where You’ll Find Them

Like Deer Flies, you will most likely find these in damp forest or wetland environments. If you have a marsh, pond, or stream on your land, you might have Horse Flies.

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How to Get Rid Of Deer and Horse Flies

Unfortunately, getting rid of Deer and Horse Flies is nearly impossible, unless they are found in your home. Most often, these flies are found in natural environments, making it impossible to use insecticides as you would on other pests.

Not to mention, most insecticides are designed for smaller insects. Because of how large Deer and Horse Flies are, insecticides often do not kill the flies, meaning that you waste your money on insecticides that don’t even work. For these two reasons, insecticides pose no real threat to Deer or Horse Flies.

You can try adding trapping devices around any areas that you want to mitigate there exposure. For example, you might want to add traps inside your barn or shed, especially if you have livestock. However, these flies typically don’t go into shaded areas, making this a very ineffective method.

How to Protect Your Animals

Because these flies can spread infectious diseases and parasites, it is important to protect your animals from them. There are Permethrin-based sprays that you can use for livestock and horses. The point of this insecticide is to irritate the fly, forcing them to leave after landing.

If you do not spray the livestock completely, including the underbelly and legs, the flies will continue to swarm around the animal’s body. Additionally, you will need to repeat applications because the spray will wear off eventually.

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What’s the Difference?

Deer Flies and Horse Flies are very similar, but they are not identical. If you are unsure which fly you have, the easiest way to determine the species is to simply look at their bodies. Horse Flies are notably bigger than Deer Flies. If the fly is alarmingly large, it is most likely a Horse Fly.

If you are still unsure, it may be a good idea to contact a pest control specialist. They will determine definitively which breed is on your land, and they may be able to give you tips for mitigating their exposure. Still, it is next to impossible to eradicate these pesky flies.

Since eradication is next to impossible, get the needed supplies to protect your livestock and yourself from their bites. Since these flies can cause parasites and diseases, the last thing you want is for your livestock to get sick after being bit by one of these creatures.

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