Tabanus spp. Linnaeus, 1758

Horse fly

Big, fast, painful biters the size of a thumbnail, the females need blood to make eggs.

Order: Diptera
Family: Tabanidae
Group: Flies
Ontario status: Established, seasonal (near water)

Why it matters

Painful biters that can ruin outdoor work and recreation. They can mechanically carry germs between animals, including the agents of tularemia and equine infectious anemia in livestock.

Origin

Native worldwide; Ontario has many native species, most active in early to mid-summer.

Behavior

Females deliver a painful slash-and-sponge bite, they cut the skin and lap the blood. Persistent: swatting rarely deters them. Males never bite (they drink nectar).

Living conditions

Larvae grow in wet mud and shallow water at pond and stream edges. Adults hunt on hot, sunny days, especially near water, livestock, and woodland trails.

Signs of presence

Large flies circling persistently on hot sunny days
Painful bites that bleed slightly, near water or livestock
A loud, deep buzzing unlike smaller flies

Weaknesses

Only females bite, and only by day, evening activities avoid them
They need wet mud for larvae; drained, managed shorelines reduce breeding
Dark moving targets attract them; light clothing and stillness help
They cannot bite through tightly woven fabric

Myths and facts

Myth They're attracted to you because you're dirty or sweet.
Reality They hunt by sight, movement, CO2 and heat, any large mammal, clean or not, is a target. Only egg-making females bite.

Research notes

Morphology

Adult 10-25+ mm, among the largest flies. Huge compound eyes, often iridescent green/purple, meeting at the top of the head in males, separated in females. Third antennal segment stout with annuli (rings). Wings clear (vs. deer flies' patterned wings). Females: blade-like mouthparts for slashing.

Diagnostics

Confirm by: very large size, huge eyes, clear wings, painful daytime bite. Larvae: tapered, segmented, predatory, in wet mud.

Easily confused with

Deer fly, Chrysops spp. Deer flies are smaller (8-12 mm) with dark-patterned wings; horse flies are larger with clear wings.

Life cycle

Egg: Laid in masses on vegetation overhanging water or wet mud
Nymph or larva: Aquatic predators of worms and small invertebrates in mud; the long stage, months to over a year
Adult: Males feed on nectar/pollen; females take blood meals for egg development; live a few weeks
Egg to adult: Typically about one year in Ontario; the larval stage dominates the timeline
Generations per year: One in Ontario; adults peak June, August

Development

Complete metamorphosis: egg → larva (aquatic/semi-aquatic predator, many instars) → pupa → adult.

Reproduction

Females need one or more blood meals per egg batch; lay several hundred eggs per mass over water.

Distribution

Native range: Worldwide family; Holarctic species in Ontario
Ontario: Common near water, wetlands and livestock country; strictly seasonal
Global: Worldwide except extreme polar regions

Ecology

Larval habitat: wet mud, pond and stream margins. Adults are strong fliers (several km). Only females blood-feed. Seasonal: adults in summer, larvae year-round in mud. Mechanical vectors of tularemia, anaplasmosis and equine infectious anemia among animals.

Resistance

Unknown, not subject to insecticide programs; managed by avoidance and barriers.

IPM implications

No indoor or structural relevance, an outdoor avoidance pest. The biology says: manage the interface (timing of outdoor work, protective clothing, head nets, repellents on exposed skin) because the larval habitat (wetlands) is not treatable and should not be. Visual canopy traps exploit host-seeking for monitoring and local relief around high-use areas.

Chemical ecology

Host location is primarily visual (large dark moving objects) supplemented by CO2 and heat; oviposition follows wet-mud cues at water margins.

Genetics and DNA

No reference genome published as of 2026.

Taxonomy

Diptera: Tabanidae. Genus Tabanus Linnaeus, 1758. Ontario species include T. similis and T. sulcifrons.

Open questions

Species-level adult activity calendars for Ontario; larval ecology of the common species in the province's wetlands.

References

Teskey (1990), The Horse Flies and Deer Flies of Canada, Agriculture Canada monograph
Foil (1989), Annual Review of Entomology, tabanid biology and disease links
UF/IFAS Extension, horse fly and deer fly biology