Not every fly buzzing around a Pennsylvania backyard is the same harmless nuisance swatted away at a picnic. Some species draw blood with scissor-like mouthparts, others breed by the thousands along rushing rivers, and a few simply want to sneak indoors before winter. Learning the different types of flies in Pennsylvania makes it easier to know which ones to worry about, which ones to ignore, and which ones actually help keep the ecosystem in balance.
From the imposing black horse fly to the tiny fruit fly hovering over a bowl of ripening bananas, each species has its own size, habitat, and level of nuisance. This guide breaks down six flies commonly found across the state, what makes each one identifiable, and why they show up where they do.
Pro Tip: Most biting flies in Pennsylvania are most active during daylight hours in late spring through early fall, so scheduling outdoor activities for early morning or evening can reduce encounters.
1. Black Horse Fly (Tabanus atratus)
The black horse fly (Tabanus atratus) is one of the largest and most intimidating flies a person can encounter in Pennsylvania. It is a large, dark-colored biting fly found across much of eastern and central North America, with a glossy black body, smoky wings, and unusually large eyes. Most adults measure roughly 0.8 to 1.1 inches long, making them noticeably bigger than a typical housefly.
The species name itself hints at its appearance: its scientific name, Tabanus atratus, refers to its dark appearance, since “atratus” means clothed or dressed in black. Black horse flies may be encountered in Florida, Texas, Illinois, Wisconsin, Maryland, Pennsylvania, and many other states with suitable wet breeding habitats, which explains why they turn up near ponds, streams, and wooded wetlands throughout the Keystone State.
Only females bite. Adult females feed on mammalian blood while males survive mainly on nectar and plant juices, a pattern shared across the broader horse fly and deer fly family. According to the University of Maryland Extension, females are blood-sucking and inflict a painful bite, and horse flies overall run about 3/8 to 1 1/8 inches long, usually gray or blackish. That bite has earned a reputation, often described as “a sharp, sudden rip” rather than a simple pinprick, since the mouthparts slash rather than pierce.
2. Deer Fly (Chrysops cincticornis)
Deer flies belong to the genus Chrysops, and one of the more recognizable Pennsylvania species is Chrysops cincticornis. An adult female of this species lays eggs on vegetation near damp ground, since the larvae need moisture to develop.
Compared with their horse fly relatives, deer flies run smaller. Deer flies range from 7 to 10 millimeters in length, are yellow to black, have stripes on the abdomen, and possess mottled wings with dark patches, while horse flies grow considerably larger. The larvae of Chrysops are aquatic, semi-aquatic, or terrestrial, thriving in the same wet meadows and stream edges favored by their larger cousins.
Anyone who has walked a wooded trail in July has likely experienced the species’ most annoying trait. Deer flies may irritate humans by circling around the head continuously before landing to bite. Tabanids, including deer flies, are most likely encountered during hot summer and early fall weather, which lines up with peak hiking and camping season across Pennsylvania’s state forests. For anyone dealing with biting flies around pets as well, the guide to types of flies that affect dogs covers additional species worth watching for.
| Common Name | Approximate Size | Bites Humans? | Typical Habitat |
|---|---|---|---|
| Black Horse Fly | 0.8–1.1 inches | Yes (females only) | Wetlands, ponds, wooded streams |
| Deer Fly | 0.25–0.4 inches | Yes (females only) | Wooded trails, damp meadows |
| House Fly | About 0.25 inches | No | Homes, farms, garbage areas |
| Fruit Fly | About 0.1 inches | No | Kitchens, ripening produce |
| Black Fly | Under 0.2 inches | Yes (females only) | Fast-moving rivers and streams |
| Cluster Fly | Slightly larger than house fly | No | Attics, wall voids, sunny outdoor walls |
3. House Fly
The house fly (Musca domestica) is the species most people picture when they hear the word “fly.” It is typically gray, about a quarter inch long, with four dark stripes on its thorax and large, reddish compound eyes. It is highly mobile and attracted to a wide range of organic materials, from decaying food to animal waste.
House flies don’t bite, but that doesn’t make them harmless. House flies do not bite or sting, but they are carriers of more than 100 diseases, picked up as they move between garbage, manure, and food surfaces. Alongside fruit flies, house flies are one of the two common types of flies that plague homeowners in Pennsylvania, and the house fly is by far the most common.
Pest control operators in the state generally recommend the same handful of sanitation steps to keep populations down:
- Remove household trash daily rather than letting it accumulate indoors
- Wipe down kitchen counters and other food-prep surfaces after every meal
- Repair torn or gapped window and door screens
- Store food in containers with tight-fitting lids
- Clean up pet or livestock waste promptly
These prevention tips mirror the guidance from Pennsylvania pest control professionals: remove trash daily, clean kitchen counters, repair holes in screens, seal food in containers with tight-fitting lids, and discard animal waste.
House flies share space with other common household invaders across the state, including several types of ants found in Pennsylvania that respond to similar sanitation-based control strategies.
4. Fruit Fly
The fruit fly, scientifically known as Drosophila melanogaster, is one of the smallest flies on this list but arguably the most familiar in kitchens. These flies have a short lifespan, measure around 3mm in length, have red eyes, and are commonly found near decaying fruits and vegetables where they feed on the yeast and bacteria that thrive in those environments.
Their life cycle moves fast, which is exactly why a few overripe bananas can turn into a swarm within days. With a lifecycle of 10 to 14 days, these flies can become a nuisance, especially in kitchens, dustbins, or anywhere decaying fruit sits out. Getting rid of an infestation rarely requires chemical treatment.
Instead, most households find success with a short sequence:
- Locate and discard any overripe or rotting fruit, including pieces hidden in fruit bowls or produce drawers
- Empty and rinse kitchen trash and compost bins that may be holding food residue
- Wipe down counters, drains, and sticky spots where fermenting liquid may have collected
- Set out a shallow dish of vinegar to trap remaining adults while the source is eliminated
If you want to get rid of these flies, removing fruit that has rotted or any rotten organic matter in the home is usually all that’s required. Anglers who tie imitation insects for trout streams sometimes model patterns after fruit flies and other small Diptera, a topic covered in more detail in the guide to different types of flies used for fishing.
5. Black Fly (Simulium jenningsi)
Black flies look nothing like the flies most people expect. Simulium jenningsi is the species most responsible for Pennsylvania’s black fly problems along waterways. Black flies don’t look like typical house flies; they more closely resemble fruit flies in appearance, and some people distinguish them from house and horse flies by calling them gnats.
Important Note: According to the Pennsylvania Department of Environmental Protection, Simulium jenningsi is particularly troublesome and is the most common black fly pest in Pennsylvania, and it loves to bite.
Black flies need flowing water to complete their life cycle, which is why certain rivers see far heavier swarms than others. Eastern Pennsylvania’s Lehigh, Schuylkill, Delaware, Lackawanna, Lackawaxen, and Susquehanna Rivers and their tributaries are ideal breeding grounds for some of the most problematic Simulium jenningsi subspecies. Black flies breed in moving water found in rivers and streams, which sets them apart from house flies and cluster flies that breed on land.
Their bites are more than an itchy annoyance for some residents. Bites may swell and cause worse reactions if the victim scratches the area, and a small number of people experience severe reactions to black fly bites. Because black flies are such a nuisance to residents and tourists alike, the Pennsylvania Department of Environmental Protection oversees a program to help eradicate the annoying gnats along the state’s most affected rivers.
Readers interested in other river-adjacent wildlife under state monitoring may also want to review the list of endangered animals in Pennsylvania, several of which share the same waterway habitats.
6. Cluster Fly
Cluster flies are easy to mistake for house flies at a glance, but a closer look reveals clear differences. The species Pollenia rudis is the one most homeowners in Pennsylvania encounter each fall. These flies are larger and darker than common house flies, often metallic blue-green or gray, and covered with fine, yellowish hairs.
Unlike house flies, which breed year-round wherever food waste accumulates, cluster flies follow a very different seasonal pattern. Cluster flies and similar insects often become pests in Pennsylvania homes while seeking shelter from changing weather conditions, typically slipping into wall voids and attic spaces as temperatures drop in autumn. On warm winter days, they may emerge inside the house, drawn toward sunny windows before returning to dormancy.
Pro Tip: Sealing gaps around windows, siding, and roof soffits before fall is far more effective against cluster flies than trying to remove them once they’ve moved into wall voids for the winter.
Because cluster flies overwinter in structures rather than breeding indoors, large numbers appearing in a single room usually point to an entry point somewhere nearby rather than an active infestation. Comparing their habits with other regional pests, such as the biting species covered in the guide to types of flies in Delaware, helps put Pennsylvania’s own fly diversity into a broader regional context.
Pennsylvania’s fly population ranges from harmless kitchen visitors to genuine biting nuisances, and correct identification is the first step toward deciding whether action is even necessary. Species like the fruit fly and cluster fly rarely warrant more than basic sanitation and sealing, while black horse flies, deer flies, and river-breeding black flies call for more deliberate avoidance during peak biting season.
Whether the goal is protecting an outdoor gathering or simply understanding the insects sharing the yard, recognizing these six species offers a solid starting point for anyone curious about the wider world of flies found throughout Pennsylvania, as well as the state’s many spider species that often feed on them.