Horse flies and deer flies (family Tabanidae) are large biting flies that bother people, horses, cattle, dogs, and wildlife across much of the United States. They’re most active in warm weather, especially on sunny days, and their bites can be painful because they cut the skin rather than “sting.”
This article explains how to identify horse flies and deer flies, where they’re most common in the U.S., what attracts them, how to reduce bites, and what you can realistically do for control.
Important note on terminology
Your source text also describes horse bot flies (Gasterophilus intestinalis), which are a different group of flies with larvae that develop inside horses. In the U.S., both tabanids (horse/deer flies) and bot flies are common seasonal problems for horses, but they require different control approaches. The sections below cover both, clearly separated.
What Are Horse Flies and Deer Flies?
Tabanids are stout flies with large eyes and strong flight.
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Horse flies are usually larger (often 1/2–1 inch long), robust, and can be strong fliers.
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Deer flies are typically smaller and often have patterned wings; they can be very persistent around the head and shoulders.
Feeding behavior
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Both sexes feed on nectar and plant fluids.
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Females require a blood meal to develop eggs, so only females bite.
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They most often bite mammals, but they may also feed on birds or reptiles.
When and Where Are They Most Active in the U.S.?
Horse flies and deer flies are mainly a late spring through early fall problem, with peak pressure varying by region.
They’re most common:
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Near wetlands, ponds, streams, marshy pastures, and irrigated areas (larval development needs moisture)
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In wooded edges next to open fields
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On hot, sunny, low-wind days, usually during daylight hours
What Do Their Bites Feel Like?
Tabanids don’t “sting” like bees. They bite, creating a small wound.
Typical bite reactions:
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Immediate sharp pain
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Redness and swelling
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Itching as it heals
Some people develop larger localized swelling or hives. Seek medical care if you have spreading redness, drainage, fever, severe swelling, or signs of an allergic reaction.
How to Avoid Horse Fly and Deer Fly Bites
Because broad “kill” control is difficult, prevention is usually the best strategy.
Personal protection (people)
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Use EPA-registered repellents with DEET, picaridin, IR3535, or oil of lemon eucalyptus
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Wear long sleeves/pants and light-colored clothing when possible
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Avoid peak fly areas (wet edges) during the hottest parts of sunny days
Protecting horses and livestock
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Use fly sheets, fly masks, and leg wraps (tabanids often target legs and lower body)
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Apply equine-labeled fly repellents consistently, especially before turnout
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Provide shelter or shaded areas; pressure is often lower in breezy shade than in still, sunny spots
Can You Control Horse Flies and Deer Flies?
Realistically, you can reduce pressure on a property, but you rarely eliminate them completely.
Effective approaches:
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Reduce breeding habitat where practical: improve drainage around high-traffic animal areas, manage standing water that’s under your control (not natural wetlands)
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Manure management helps overall fly loads, but tabanids usually breed in wet soils rather than manure, so this is supportive—not a complete solution
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Traps can help locally: commercially available traps (often dark, moving/heat-reflective designs) can reduce biting pressure in specific areas if placed correctly (sunny, open, away from where animals and people spend time)
What usually doesn’t work well:
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Yard fogging or general insecticide spraying: tabanids are strong fliers, often arrive from off-property, and are not easily controlled this way
First Aid for Bites
For most people:
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Wash with soap and water
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Cold compress for swelling
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Anti-itch products (e.g., hydrocortisone, oral antihistamines if appropriate)
Avoid scratching to reduce infection risk.
Horse Bot Flies in Horses (Bots) – Gasterophilus intestinalis and Related Species
Horse bot flies are not horse flies or deer flies. Adult bot flies resemble bee-like flies and are mainly a problem because they lay eggs on the horse, and larvae later develop inside the horse’s digestive tract.
How to identify bot fly activity
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Yellow eggs glued to hairs, commonly on the forelegs, shoulders, and along the body
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Horses may react strongly to the adult flies hovering around them
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Adults do not bite or sting
Bot fly life cycle (U.S. overview)
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Eggs are laid on the coat
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Eggs hatch with warmth/moisture and larvae enter the mouth when the horse licks or rubs
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Larvae migrate briefly in oral tissues, then attach in the stomach/upper intestine
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Mature larvae pass out in manure, pupate in soil, and emerge as adults
Signs and problems in horses
Most horses tolerate small numbers, but heavier burdens can be associated with:
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Mouth irritation (early stage)
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Gastric irritation; in some cases, colic-like signs or poor condition
Preventing bot flies
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Inspect and remove eggs frequently (daily during peak season if possible)
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Use a bot knife/bot block, grooming tools, or careful clipping
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Use fly sheets/masks and equine repellents to reduce egg-laying attempts
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Good manure cleanup helps overall fly pressure, though bots pupate in soil
Controlling bots inside the horse
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Dewormers containing ivermectin or moxidectin are commonly used for bots in the U.S.
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Timing is often planned for late fall/winter (many owners treat after the first hard frost in their area), but the best schedule depends on local climate and veterinary guidance
If you want, I can rewrite this as two separate USA-facing articles—one strictly for Tabanidae (people + livestock bite prevention) and one strictly for horse bot flies (equine parasite management)—so the headings and messaging match each topic perfectly.