Black fly bite
A black fly bite usually shows up as a small bleeding point that swells into an itchy lump, red, often with a central blood spot, appearing often painless at first, itch builds over hours. It peaks around day 1 to 2 and generally lasts up to a week, sometimes with lasting swelling. Several, along the hairline, ears and ankles is the usual arrangement.
| Looks like | a small bleeding point that swells into an itchy lump |
|---|---|
| Pattern | several, along the hairline, ears and ankles |
| Size | 3–25 mm |
| Appears | often painless at first, itch builds over hours |
| Worst at | day 1 to 2 |
| Lasts | up to a week, sometimes with lasting swelling |
| Itch | ●●●●● 5/5 |
| Pain | ●●○○○ 2/5 |
| Usually on | hairline, behind the ears, ankles, wrists |
| Active | April to July, daylight, worst in late spring near running water |
How to tell it apart
Black flies cut a small hole and pool blood, so the classic finding is a pinpoint blood spot that later swells and itches intensely. They crawl into hair and under collars, which is why bites cluster at the hairline and behind the ears rather than on open skin.
Where and when you meet it
Habitat: fast-flowing streams and rivers — larvae need moving water, so they are absent from still ponds.
Across 927 verified US records in the GBIF biodiversity database, sightings of Simulium venustum concentrate in May to August. That is the window when bites are most likely — it is observation data rather than a guess, and it shifts with latitude.
States with the most recorded sightings
- Pennsylvania 278 records
- Michigan 218 records
- Wisconsin 99 records
- Alaska 63 records
- New York 53 records
- Illinois 44 records
- New Hampshire 38 records
- Vermont 26 records
- Colorado 24 records
- Connecticut 11 records
- Maine 8 records
- Minnesota 7 records
Record counts reflect where people report and identify specimens, not exact abundance. They are useful for ruling a species in, not for ruling it out.
Where it is recorded
| Country | Records | Peak months there |
|---|---|---|
| United States | 927 | May to August |
| Canada | 406 | June to August |
Verified records from the GBIF biodiversity database across the seven countries we cover. A country missing from this table means we have no records for it, not that the species is absent.
What it means for you
The season is short and intense — a few weeks after spring emergence in the north, near running water. Reactions can be disproportionate, with swelling around the eyes that looks alarming and settles over several days.
Seek medical care if
- marked swelling around the eyes
- fever or spreading redness
- breathing difficulty (rare allergic reaction)
Care at home
- wash, cold pack, antihistamine
- avoid scratching — these bleed and crust easily
Preventing the next one
- fine-mesh head net in black fly season
- cover wrists and ankles
- avoid riverside trails in late spring at peak emergence
Common questions
How do I know it was a black fly?
Black flies cut a small hole and pool blood, so the classic finding is a pinpoint blood spot that later swells and itches intensely. They crawl into hair and under collars, which is why bites cluster at the hairline and behind the ears rather than on open skin.
How long does a black fly bite last?
Typically up to a week, sometimes with lasting swelling, peaking around day 1 to 2.
When is it a problem?
Seek care for: marked swelling around the eyes; fever or spreading redness; breathing difficulty (rare allergic reaction).
Can it transmit disease?
No human disease is known to be transmitted by this species in the United States. The practical risk is infection from scratching.
Get a photo assessment in the app
Everything above is free to read. The app adds what a page cannot: point your camera at the bite and get an assessment of the most likely cause, a severity read, and a day-by-day care plan — including the specific signs that mean you should be seen.
iPhone and iPad. Free to download.