Black fly bites in Pennsylvania
Pennsylvania has 278 verified black fly records in the GBIF biodiversity database — one of the highest counts in the country. Sightings concentrate in May to August. The bite itself appears often painless at first, itch builds over hours and lasts up to a week, sometimes with lasting swelling.
| Verified records | 278 |
|---|---|
| Peak sightings | May to August |
| State bite season | April to October |
| Active | daylight, worst in late spring near running water |
| Habitat | fast-flowing streams and rivers — larvae need moving water, so they are absent from still ponds |
| Bite appears | often painless at first, itch builds over hours |
Identifying it in Pennsylvania
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.
Locally, the species most often confused with it are honey bee sting, bald-faced hornet sting, carpenter ant bite — all of which are also recorded in Pennsylvania.
Seek medical care if
- marked swelling around the eyes
- fever or spreading redness
- breathing difficulty (rare allergic reaction)
What else it could be in Pennsylvania
These are recorded in Pennsylvania too and are the ones most often confused with it here. The differences below are the ones that actually separate them.
| Black fly bite | Mosquito bite | |
|---|---|---|
| Pattern | several, along the hairline, ears and ankles | single bumps scattered over exposed skin |
| Where on body | hairline, behind the ears, ankles | ankles, lower legs, arms |
| Appears | often painless at first, itch builds over hours | within minutes |
| Records here | 278 | 14 |
Season in Pennsylvania specifically
Records in Pennsylvania concentrate in May to August, which is what local sighting data shows rather than a national average — seasonality shifts substantially with latitude. The general bite season in Pennsylvania runs April to October.
Because fast-flowing streams and rivers — larvae need moving water, so they are absent from still ponds, exposure in Pennsylvania tends to be concentrated in specific places rather than spread evenly — which is why a bite history ("where were you in the 24 hours before?") narrows the answer faster than the appearance of the mark.
Care
- wash, cold pack, antihistamine
- avoid scratching — these bleed and crust easily
Avoiding them in Pennsylvania
- fine-mesh head net in black fly season
- cover wrists and ankles
- avoid riverside trails in late spring at peak emergence
Questions
When are black flys active in Pennsylvania?
Pennsylvania records peak in May to August. The general bite season in Pennsylvania runs April to October.
Are they common in Pennsylvania?
Pennsylvania accounts for 278 verified records in GBIF, ranking it 1st among states for this species. Records reflect reporting effort as well as abundance.
What does the bite look like?
a small bleeding point that swells into an itchy lump, red, often with a central blood spot. Several, along the hairline, ears and ankles.
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.