Horse Flies
Horse flies are large, stout-bodied flies with powerful slicing mouthparts that deliver painful, bleeding bites. They are among the largest biting flies in North America, with some species reaching over an inch in length. Females require blood meals for egg production and are aggressive daytime biters that target livestock, horses, deer, and humans. They breed in wet, marshy areas, and populations are highest near bodies of water, swamps, and irrigated agricultural land. Their large size, persistent attack behavior, and painful bites make them a significant outdoor nuisance.
Species in This Group (2)
Key Behaviors
Female horse flies locate hosts by visual cues including movement, dark colors, and carbon dioxide. They are strong, persistent fliers that will follow targets for considerable distances. Their slicing mouthparts create a wound that bleeds freely, and they lap up the blood. They are most active on warm, sunny, calm days and are especially problematic near water. Males do not bite and feed on nectar and pollen.
Risks & Concerns
Horse fly bites are immediately painful and produce bleeding wounds that can become infected. They can transmit equine infectious anemia to horses and tularemia to humans, though transmission is uncommon. Large populations make outdoor work and recreation near waterways intolerable during peak season. Livestock experience stress, reduced weight gain, and decreased milk production from persistent horse fly attack.
Quick Comparison
| Species | Size | Color | Key Trait |
|---|---|---|---|
| Horse Fly | 10-25 mm | Dark brown to black or gray with large iridescent compound eyes | Large size; powerful, painful bite that draws blood; large iridescent eyes. |
| Deer Fly | 8-12 mm | Yellow to brown with patterned wings and bright green or gold banded eyes | Smaller than horse flies; patterned wings; bright banded eyes; circles the head persistently before biting. |
Need Help With Horse flies?
If you're seeing signs of horse flies in or around your home, professional treatment is the fastest way to stop the problem. Lotus Pest Control offers free inspections to identify the issue and recommend the right solution.