Horse Fly

What it does

Only female horse flies bite; they need blood to develop eggs, while males feed on nectar. Females are persistent, strong daytime fliers that circle a host and cut the skin with scissor-like mouthparts, causing a painful bite that often bleeds. They are attracted to movement, warmth, dark colors, and carbon dioxide.

Where it lives

Horse flies breed in wet soil and shallow water at the edges of ponds, marshes, streams, and irrigated pasture. Adults range widely over fields, trails, beaches, and around livestock. They are found throughout the US, with heavy populations near wetlands and coastal marshes such as the Southeast and Gulf Coast.

How it breeds

Females lay egg masses on vegetation over water or wet soil. Larvae drop into moist mud, where they develop over months, often overwintering, before pupating. Most species produce a single generation per year, so populations build slowly and peak during the warm summer months.

What it eats

Adult females feed on the blood of horses, cattle, deer, other mammals, and people. Males and non-biting females feed on nectar and plant juices. The predatory larvae feed on small organisms in wet soil and mud.

Identification

Horse flies are large, 10-28 mm, robust flies with very large, often iridescent green or purple eyes that nearly meet on top of the head. They hold their wings out to the sides at rest. They are much larger than house flies and fly with a loud buzz.

Signs of an infestation

• Large flies persistently circling people or animals
• Sudden sharp, painful bites during the day outdoors
• Bites that bleed and swell on exposed skin
• Restless, tail-swishing livestock during summer
• Many big flies near ponds, marshes, or pasture

Prevention tips

• Wear light-colored, long-sleeved clothing outdoors
• Apply an EPA-registered repellent to exposed skin
• Avoid marshy areas during peak midday activity
• Use fans and stabling to protect livestock
• Reduce standing water and wet muck where possible

Professional solutions

Horse flies are hard to control because they breed over large wetland areas and fly in from a distance, so consumer sprays rarely reduce numbers. Management focuses on personal protection, physical traps, and protecting animals. On farms, an extension agent or livestock specialist can advise on trapping and herd protection.