Black Flies: Bites, Season and How to Avoid Them

Black flies have a clear calendar and a clear cause: a nearby stream or river. Most bad black fly advice treats them like a garden pest that can be sprayed away, which extension entomologists agree it cannot.

What black flies look like, and what they are not

NC State Extension gives adults as usually 1.2 to 3 mm long, dark and humpbacked, hence the name buffalo gnat, with broad wings. Penn State and Iowa State say slightly less than a quarter inch, and Minnesota’s Metropolitan Mosquito Control District (MMCD) gives females as an eighth to a quarter inch (about 3 to 6 mm). Mississippi State notes they can be grey, tan or greenish. Purdue lists adults at 5 to 15 mm, which matches NC State’s larval range, so we have not used it. The hump is the best field mark.

Compiled by The Pest Control Brief from NC State Extension, Minnesota MMCD, Purdue, Colorado State University Extension and the University of Kentucky.

Are black flies the same as horse flies? No. Both slice skin rather than pierce it, but horse flies belong to a different family (Tabanidae), are roughly ten times longer, and their larvae live in wet soil and shallow water rather than attached to rocks in a current. See our guides to horse flies and deer flies and to no-see-ums, the midges that slip through screens at dusk.

Where do black flies come from?

Every black fly starts in moving water. UNH Extension says they breed exclusively in running water, from fast rivers to sluggish rivulets, and that big populations indicate clean streams because most species will not tolerate organic pollution. Purdue adds that dams and concrete channels make excellent larval habitat, and that cleaning up New England streams created habitat for some of the worst pest species. Adults travel: South Dakota State estimates 5 to 8 miles in search of hosts, so a stream you cannot see from the house can still be the source.

The life cycle

Purdue says a female lays one batch of 200 to 500 eggs after a blood meal, mostly on or in flowing water. Eggs of most species hatch in 4 to 30 days, larvae cling to rocks for 1 to 6 months, and adults emerge from the pupa in 4 to 7 days, riding a bubble of air to the surface.

What is a black fly’s lifespan?

Short, as adults: about 3 weeks according to Iowa State, 4 to 5 weeks according to Mississippi State, and up to two months according to California’s San Gabriel Valley district. The full cycle can take most of a year, because many species spend winter as eggs or larvae in the stream. Compare other flies in how long flies live.

When are black flies worst? Season by region

Timing matters most, because avoidance is the main defence. Purdue says most species are active from mid-May to July, worst in the upper Midwest and Northeast. The detail varies by region:

Compiled by The Pest Control Brief from the extension services and agencies named.

Within a day, UNH and Penn State both put peak biting at about 9 to 11 a.m. and 4 to 7 p.m., worst on humid, cloudy days and just before storms. Black flies do not bite at night.

Our own data shows how rarely they are photographed. The Pest Control Brief analysis of iNaturalist research-grade records, pulled October 2026, found only 42 black fly records tagged to a US state, 18 of them in California. That is too few to map, and it does not mean black flies are rare: they are tiny, fast and swatted rather than photographed. The extension records above are the better guide.

Do black flies bite, and what happens if you get bit by a black fly?

Yes, but only the females, which need blood for their eggs; males feed on nectar. Penn State explains that rather than piercing like a mosquito, the mouthparts slice the skin and the fly laps the pooled blood. MMCD says she injects anticoagulants, a pain killer and toxins, and the swelling is an allergic reaction to them. They favour thin skin at the neck, wrists, eyes and ears, and Iowa State says just beneath the hat rim.

MMCD describes the bite as a reddened weal with a wound in the centre. Purdue describes reactions from a small puncture to a golf-ball-sized swelling, plus “black fly fever”: headache, nausea, fever and swollen lymph nodes in the neck. Iowa State notes rare deaths from toxaemia or anaphylactic shock. See a doctor for spreading swelling, breathing trouble or fever after a heavy attack. To tell bites apart, see our bite identification guide.

Not every black fly bites. UNH says only two of New Hampshire’s roughly 40 species consistently and abundantly bite people; others just swarm the face and crawl into ears and eyes.

Are black flies dangerous? Are they bad?

For people in the US, mostly no. Purdue states that none are known to transmit disease agents to humans in the US; elsewhere they carry the worm behind river blindness (onchocerciasis) in equatorial Africa and parts of Central and South America.

For animals it is different. NC State notes they transmit Leucocytozoon simondi, a poultry parasite, in the US. Mississippi State reports chickens suffocating after gnats clogged their airways, livestock stampeding, and a toxic shock-like syndrome from the saliva. A USDA memorandum by entomologist G. H. Bradley tallied approximately 527 mules killed by the southern buffalo gnat in five Arkansas counties in April 1934, after 500 in a 1931 outbreak.

Bradley’s 1931 report holds the clearest lesson for animal keepers. In Coahoma County, Mississippi, a farm with no stables lost 35 of 65 mules, the Levee Board lost two of 170 stabled mules, and two farms that boarded or sacked over barn openings lost none. NC State and Iowa State advise the same today: shelter livestock and poultry during heavy flights and run fans. Ask a vet about on-animal products.

Why are there black flies in my house?

Usually, they are not black flies. Purdue says black flies do not enter human structures to seek blood, and Penn State notes they are not attracted to lights. Small dark flies indoors are more likely:

• Fungus gnats, about 1/8 inch, delicate, near houseplants. Colorado State says they do not bite and peak in late fall and winter.
• Drain flies or fruit flies, around sinks and fruit bowls; see fruit flies, drain flies and gnats.
• House flies or cluster flies, which are much larger than a black fly; see flies in the house all of a sudden.

Small flies biting indoors at dusk are probably no-see-ums, which Purdue notes readily enter dwellings. Our small flying bugs in the house guide covers the rest.

How do you get rid of black flies?

Not at the scale of one property, and every extension source we read says so. NC State says spraying shrubs or fogging a yard is not likely to be of much help; UNH says fogging gives relief measured in hours, and that treating surface water in New Hampshire needs a special permit and a commercial licence. Penn State warns that yard foggers harm pollinators while the flies return immediately.

What does work: community larval programs

Real control treats larvae in streams with Bti (Bacillus thuringiensis israelensis), a bacterium with little effect on other aquatic insects, run by public agencies:

• Pennsylvania DEP, in the program’s 41st year in 2026, treats 48 rivers and streams across more than 1,800 miles by helicopter and ground crews.
• Minnesota’s MMCD monitors about 170 small stream and 27 large river sites, treating when larvae pass a threshold; its stream program began in 1984.
• San Gabriel Valley MVCD treats foothill rivers and sites such as Morris Dam.

Ask your county extension office or mosquito control district whether a program covers your area.

What you can do: avoid the bite

1. Time your outdoor work for early morning, midday or late evening in peak weeks.
2. Wear light colours. UNH says orange, yellow and light blue attract fewer than dark green, brown and red.
3. Close the gaps. Tuck trousers into socks, prefer zips to buttons, and fasten cuffs, because black flies crawl under clothing.
4. Use a head net in heavy areas; MMCD notes they swarm the face.
5. Use an EPA-registered repellent on exposed skin, wrists, ankles and belt line. UNH names DEET as most effective, with picaridin and IR3535 nearly as effective; MMCD adds oil of lemon eucalyptus. UNH says repellents do not last as long against black flies as against mosquitoes, so reapply, and follow the label, especially for children.
6. Run a fan on the patio. Penn State says the breeze defeats weak fliers and disperses your carbon dioxide.
7. Turn off running water features for a day once a week, as the San Gabriel Valley district advises, since backyard waterfalls can breed them.

Penn State says living plants do not repel black flies; our review of mosquito repellent plants finds the same for mosquitoes. Nor is this a job for a pest control service: no residential treatment changes what comes off a river.

Frequently asked questions

Are black flies bad?

For people in the US they are a nuisance rather than a health threat: Purdue says none are known to transmit disease to humans here, though sensitive people can develop large swellings or “black fly fever”. For animals they are worse. Heavy swarms have killed poultry and livestock through toxic reactions and suffocation, and USDA records show about 527 mules killed in Arkansas in April 1934.

What causes black flies in a house?

True black flies rarely come indoors. Purdue says they do not enter buildings to feed, and NC State says they usually do not enter in large numbers. Small dark flies in a house are usually fungus gnats from houseplant soil, drain flies or fruit flies, or cluster flies. Small biting flies indoors at dusk are more likely no-see-ums, which can pass through ordinary window screens.

What happens if you get bit by a black fly?

The female slices the skin and feeds on the pooled blood, leaving a red weal with a small wound in the centre. Swelling ranges from a pinprick to golf-ball size and usually itches. Some people get “black fly fever”, with headache, nausea, fever and swollen neck glands. Iowa State notes rare severe allergic reactions. Seek medical advice for spreading swelling, fever or breathing trouble.

How do you get rid of black flies?

Not by spraying a yard. Extension entomologists agree that fogging gives a few hours of relief at most, because adults fly in from streams miles away. Effective control means treating larvae in the water with Bti, which state and district programs such as Pennsylvania DEP and Minnesota’s MMCD do. At home, rely on timing, light clothing, head nets, fans and an EPA-registered repellent.

How big are black flies?

Small. NC State gives adults as usually 1.2 to 3 mm long; Penn State and Iowa State describe them as slightly less than a quarter inch, and Minnesota’s MMCD gives females as an eighth to a quarter inch. The humped back and broad wings set them apart from no-see-ums, which are 1 to 3 mm, and from deer flies, which are 10 to 13 mm.

Sources

• Purdue University Public Health and Medical Entomology, Black Flies: Biology and Public Health Risk; and Biting Midges.
• University of New Hampshire Extension, Black Flies fact sheet.
• NC State Extension, Black Flies and Their Control; and Horse Flies and Deer Flies.
• Penn State Extension, Black Flies in Our Gardens.
• Iowa State University Extension and Outreach, Yard and Garden Encyclopedia: Black Fly, 2016.
• South Dakota State University Extension, What Are Those Little Flies That Keep Biting Me?, 2019.
• Mississippi State University Extension, Publication 3755, What to Do about Black Flies (Buffalo Gnats), J. Goddard, 2025.
• Colorado State University Extension, Fungus Gnats as Houseplant and Indoor Pests.
• University of Kentucky Entomology, ENTFACT-511, Horse Flies and Deer Flies.
• Metropolitan Mosquito Control District (Minnesota), Biting Gnat/Black Fly Control.
• San Gabriel Valley Mosquito and Vector Control District, Black Flies.
• Pennsylvania Department of Environmental Protection, DEP Launches 2026 Black Fly Suppression Program, April 2026.
• US Environmental Protection Agency, Find the Repellent that is Right for You.
• G. H. Bradley, USDA Bureau of Entomology and Plant Quarantine, Report on the Outbreak of Buffalo Gnats in Mississippi and Arkansas, 1931; and Memorandum concerning Losses Due to the Southern Buffalo Gnat in Arkansas, 1934 (Mississippi Entomological Museum archive).
• The Pest Control Brief analysis of iNaturalist research-grade records, pulled October 2026.