# Black horse fly (Tabanus atratus) > A large, glossy black horse fly whose females bite mammals to feed on blood and can be an important pest of livestock. Source: https://insectscanner.com/species/black-horse-fly/ (updated 2026-10-10) ## Is it dangerous? **Painful bite.** Female black horse flies deliver a painful, cutting bite to obtain blood. Bites may bleed, itch, swell, and cause local irritation; allergic reactions are uncommon but possible. They are not known to be major vectors of human disease in North America. First aid: wash the bite with soap and water, apply cold compress to reduce swelling, and use over-the-counter topical antiseptic or antihistamine creams for itching; seek medical care for severe allergic reactions or signs of infection. **Pets:** Livestock and pets can receive multiple bites that cause stress, blood loss (in heavy infestations) and reduced grazing in animals. Protect animals with repellents, fly sheets, and barning when flies are active. Severe reactions are uncommon but monitor for secondary infections. ## Quick facts - Scientific name: Tabanus atratus - Order: Diptera (Flies) - Family: Tabanidae (Horse flies and deer flies) - Size: About 20-28 mm body length - Colours: Black, Very dark brown, Olive green (eye reflections) - Habitat: Wet or marshy ground, edges of streams, ponds, moist meadows and wood margins; adults patrol open areas near hosts. - Range: Eastern and central North America (United States and southern Canada). - Active: Warm months (spring to late summer); adults active during daylight, especially sunny and warm periods. - Lifespan: Adults live days to a few weeks; full life cycle typically 1 year but can vary with climate. - Effect on plants: Harmless - Pest: yes ## Overview Tabanus atratus is a large species of horse fly found across much of eastern and central North America. Adults are robust, fast-flying flies; females require a blood meal for egg development and are often persistent biters of mammals, including humans and livestock. Males do not blood-feed and visit flowers for nectar. Larvae develop in wet or semi-aquatic soils and organic-rich habitats. ## How to identify it Adults are very large and robust, with a black body that often has a faint bluish sheen and dark, smoky black wings. The head is broad with large, uniformly dark compound eyes that lack the bright colored bands seen in many other horse flies; in males the eyes meet on top of the head. Antennae are short and stout, and the mouthparts of females are blade-like, adapted for cutting skin to obtain blood. - All-black body and dark smoky wings, without the banded wing pattern of deer flies. - Very large, robust size (about 20-28 mm) and broad head with large, uniformly dark eyes. - Females have prominent blade-like mouthparts; look for females persistently landing on mammals. - Absence of bright patterned eye bands or strongly patterned thorax distinguishes it from many other Tabanus and Chrysops species. ## Lookalikes - **Greenhead horse fly** (Tabanus nigrovittatus): Greenhead has a greener head and different habitat (salt marshes) and often more patterned thorax; T. atratus is uniformly darker and more inland. - **Striped horse fly** (Tabanus lineola): T. lineola often shows thoracic striping and different eye patterns; T. atratus is more uniformly black without clear striping. ## Life cycle - **Egg:** Females lay clusters of eggs on vegetation overhanging wet soil or on stems near water or moist ground. - **Larva:** Larvae are aquatic or semi-aquatic predators/ detritivores that develop in wet soil, mud, or moist organic substrates; they feed on small invertebrates or organic matter and molt through several instars. - **Pupa:** Pupation occurs in drier soil; the pupal stage lasts days to weeks depending on temperature. - **Adult:** Adults emerge and are strong fliers. Males feed on nectar; females seek blood meals to mature eggs. ## Diet and predators - Young eat: Larvae feed on small invertebrates and organic matter in wet or semi-aquatic soils. - Adults eat: Males feed on nectar and plant juices; females feed on blood from mammals (including horses, cattle, deer, and humans) and also take nectar. - Predators of the young: Aquatic and semi-aquatic predators such as beetle larvae, amphibians and other invertebrates. - Predators of adults: Birds, bats, dragonflies, and predatory insects; spiders may catch some individuals. ## Effect on plants and gardens Adults do not damage plants; males visit flowers for nectar and can act as incidental pollinators. ### How to get rid of it Reduce exposure: use fine mesh screens on windows, keep animals in barns or under shelter during peak fly activity, and deploy horse-fly traps (black sphere/decoy traps or sticky/CO2-baited traps) around pastures. For livestock, use approved topical repellents/insecticides and physical barriers such as fly masks, sheets, and head nets. Avoid outdoor exercise for people and animals during hottest, sunniest parts of the day when females are most active. ## Where and when to find it Adults: flying in sunny open areas, along forest edges, near pastures and water. Females often found on or near mammals. Larvae: muddy margins, wet meadows and organic-rich wet soils. - Best time: Late spring through summer on warm, sunny days; daytime activity peaks in the hottest hours. - Tips: Use a telephoto or zoom lens to photograph them in flight or on vegetation. Wear long sleeves and repellents when photographing in areas with biting flies. ## Fun facts - Only female horse flies bite; males feed on nectar. - Horse flies have cutting mouthparts that slice skin rather than piercing like mosquitoes. - Their powerful flight lets them chase hosts over some distance — they can be persistent and hard to swat away. ## Frequently asked questions ### Do black horse flies bite? Yes — females bite to obtain blood for egg development; their bites are painful because they cut the skin rather than pierce it. ### Are black horse fly bites dangerous? Only mildly. Bites are painful and may swell or itch but are rarely dangerous to healthy people; livestock can suffer more from multiple bites and stress. Seek care for heavy bleeding, infection, or a severe allergic reaction. ### Why are black horse flies in my yard? They are attracted to open areas near water or moist soil where larvae develop and to mammals for blood meals. Warm, sunny weather increases their activity. ### How do I get rid of black horse flies? Reduce breeding habitat (drain or manage wet areas), use traps (black sphere or CO2-baited traps), apply repellents or protective coverings for livestock, and limit outdoor exposure during peak times. Complete elimination is difficult because adults fly in from surrounding areas. ### Do black horse flies transmit diseases? They can mechanically transmit some livestock pathogens under certain conditions, but they are not major vectors of human disease in North America. --- More about this species: [iNaturalist](https://www.inaturalist.org/taxa/116680), [Wikipedia](https://en.wikipedia.org/wiki/Tabanus). Identify insects and spiders from a photo with the Insect Scanner iPhone app: https://apps.apple.com/app/id1543946563?pt=126685406&ct=web-markdown&mt=8 Identification from photos is never certain: don't handle insects you can't identify, and get medical advice for bites, stings or allergic reactions.