Chrysops spp. Meigen, 1803

Deer fly

Smaller cousins of horse flies with patterned wings, the classic early-summer woodland biter.

Order: Diptera
Family: Tabanidae
Group: Flies
Ontario status: Established, seasonal (woodlands/wetlands)

Why it matters

Among the most aggressive human-biting flies in Ontario's outdoors. They can carry the bacteria causing tularemia (rabbit fever) between rabbits and, rarely, to people.

Origin

Native across North America; Ontario's woodlands and wetlands host many species.

Behavior

Fast, persistent daytime biters, most intense in June and July. They circle the head and bite the neck and scalp. Only females bite.

Living conditions

Larvae in wet mud and shallow wetland edges, like horse flies. Adults hunt along trails, shorelines and forest edges on warm days.

Signs of presence

Small fast flies circling your head on woodland trails in early summer
Painful bites on the neck and scalp
A high whining buzz distinct from mosquitoes

Weaknesses

Daytime-only biters, dawn, dusk and night are safe
Attracted to movement and dark colours; stillness and light clothing reduce bites
Head nets are the single most effective personal protection
They stay near wet breeding areas; dry open ground has fewer

Myths and facts

Myth They're baby horse flies.
Reality A separate genus, full-grown at 8-12 mm. The patterned wings are the giveaway, horse flies have clear wings.

Research notes

Morphology

Adult 8-12 mm; dark body; wings with bold dark patterns/bands, the genus diagnostic, visible in flight. Eyes often iridescent. Smaller and more agile than Tabanus.

Diagnostics

Confirm by: 8-12 mm, dark-patterned wings, painful daytime bite around head and neck, woodland/wetland habitat.

Easily confused with

Horse fly, Tabanus spp. Horse flies are larger (10-25+ mm) with clear, unpatterned wings; deer flies are smaller with unmistakable wing patterns.

Life cycle

Egg: Masses on vegetation over wet mud
Nymph or larva: Predatory larvae in wet soil and mud; long development
Adult: Females blood-feed; males on nectar; short adult life of weeks
Egg to adult: About one year in Ontario
Generations per year: One; adults June, August with a sharp early-summer peak

Development

Complete metamorphosis: egg → larva (aquatic predator in mud) → pupa → adult.

Reproduction

Blood meals fuel egg batches; females lay masses over larval habitat.

Distribution

Native range: Holarctic
Ontario: Abundant in woodlands, wetlands and cottage country each early summer
Global: Northern hemisphere temperate regions

Ecology

Wetland and woodland-edge species. Strong fliers but stay near breeding habitat. Vectors of Francisella tularensis (tularemia) among lagomorphs and rodents; occasional bridge to humans. Larvae are themselves predators in the mud community.

Resistance

Unknown, no insecticide programs target them.

IPM implications

Outdoor avoidance pest with no structural dimension: personal protection (head nets, repellents, clothing), timing of trail and shoreline use, and visual traps for local monitoring. Wetland larval habitat is ecologically off-limits for treatment, the strategy is entirely about the human, fly interface.

Chemical ecology

Host-seeking dominated by vision and movement, with CO2 as a close-range cue.

Genetics and DNA

No reference genome published as of 2026.

Taxonomy

Diptera: Tabanidae. Genus Chrysops Meigen, 1803. Common Ontario species include C. cincticornis and C. excitans.

Open questions

Fine-scale habitat partitioning among Ontario Chrysops species; quantitative tularemia bridge risk in the province.

References

Teskey (1990), The Horse Flies and Deer Flies of Canada, Agriculture Canada
Foil (1989), Annual Review of Entomology, tabanid disease links
Ontario public health resources, tularemia and biting flies