Most people meet cluster flies without ever learning what they are. The flies turn up at an upstairs window, move too slowly to be house flies, die on the sill, and get swept up. Then more arrive. The usual conclusion is that something is rotting somewhere, and that conclusion is almost always wrong.
What They Are
Cluster flies are Pollenia species — Penn State Extension and University of Kentucky Extension both name Pollenia rudis specifically, and Kentucky refers to Pollenia rudis and relatives, which is the more accurate framing since several closely related species behave the same way.
They belong to the family Calliphoridae, the blow flies. That surprises people, because blow flies are the metallic green ones associated with carrion. The cluster fly is the odd relative that went in a completely different direction: its larvae are parasites of earthworms.
Telling Them Apart
Four features, in the order you will actually notice them:
- They are slow. University of Kentucky Extension states the cluster fly moves sluggishly. This is the first thing anyone notices, and on its own it nearly settles the identification. A house fly is difficult to swat. A cluster fly can be picked up.
- They are slightly larger than a house fly. Kentucky gives “a little larger than the common housefly”; Penn State gives “slightly larger than the common house fly, Musca domestica”.
- Golden hairs on the thorax. Kentucky describes short, golden colored hairs on the thorax. Penn State describes golden-yellow hairs on the thorax, more numerous on the underside between and near the legs. On a specimen on a windowsill, catch it in the light and the sheen is visible.
- Dull gray with black markings, and the wings overlap at rest. Penn State gives the coloring. A house fly at rest holds its wings apart in a shallow V; a cluster fly lays one over the other along its back.
Penn State also describes the larvae, if you ever see one: cream-colored maggots of an elongated wedge shape.
The Earthworm Connection
This is the fact that makes every other fact about cluster flies fall into place, and it is almost never mentioned in the material people find first.
University of Kentucky Extension states that the larvae develop as parasites in the bodies of earthworms. Penn State Extension describes the maggots burrowing into earthworms on which they feed, and explains the cycle: adults lay eggs on soil containing earthworms, the larvae develop, pupate, and emerge as adults. Penn State reports that in Pennsylvania there may be three to four, or more, generations of cluster flies per season.
Everything follows from this:
- They come from the lawn, not the house. The nursery is soil with earthworms in it. Houses with grass, pasture or open ground around them get cluster flies; there is nothing indoors that produces them.
- Nothing indoors will stop them. No amount of cleaning affects a population whose larvae are inside earthworms in the yard.
- They are not attracted to your food. They are looking for a crevice to overwinter in.
Not a Filth Fly
The term “filth fly” describes flies whose larvae develop in garbage, manure or carrion and which can therefore move pathogens onto surfaces. House flies and blow flies fit it. Cluster flies do not.
University of Kentucky Extension states that cluster flies constitute no health hazard. Penn State Extension states they are not known to carry any diseases of medical importance to humans.
This matters practically as well as reputationally. If the flies in your house are cluster flies, there is no contamination question to chase, no drain to investigate and no dead animal to find. The problem is a building-sealing problem and nothing more.
Why Your House
Penn State Extension puts it simply: in the fall, cluster flies frequent the sunny sides of homes in search of protected overwintering sites. University of Kentucky Extension describes them crawling into the home through small openings in the walls and seeking protected places to spend the winter within the walls, attics and basements.
Kentucky’s common name for them is the more descriptive one: attic flies. If your problem is in an upstairs room, that is exactly where the sources say to expect it.
The sunny-wall behavior is shared with every other insect in this cluster — boxelder bugs on the south side, Asian lady beetles on the south and southwest, stink bugs in the upper areas of buildings. They are all reading the same building the same way.
Why They Reappear in Winter
Because they never left. Penn State Extension notes that cluster flies emerge from wall voids on warm winter days, through cracks under or behind baseboards, around window and door trim, and around exhaust fans or lights in ceilings. University of Kentucky Extension describes them appearing in late fall or early winter and again on warm, sunny days in early spring.
So the ten flies at the window in February are not a new problem. They are part of the population that entered in September, warmed up by sunshine or the furnace, and heading for the light. It is worth knowing, because otherwise a mild winter looks like a recurring infestation.
If It Is Not a Cluster Fly
University of Kentucky Extension covers two others that overwinter or appear indoors, and the distinction is worth carrying:
- Face flies (Musca autumnalis) resemble house flies closely enough that Kentucky says only an expert can tell them apart. Their larvae develop in cattle manure and the adults feed on secretions from the eyes and noses of cattle and horses. They have similar overwintering habits to cluster flies, so on a farm both are plausible.
- Blow flies have shiny metallic bodies, are strong fliers attracted to lights, and develop in manure or carrion. Kentucky notes they are usually apparent in small numbers. A sudden run of shiny metallic flies indoors is the one that deserves investigation, because it can mean an animal has died in a wall, a chimney or a crawl space. A slow dull-gray fly does not carry that implication at all.
What to Do
Seal, in summer. Penn State Extension calls mechanical exclusion the best method and specifies sealing cracks around windows, doors, siding, utility pipes, behind chimneys, and underneath the fascia. University of Kentucky Extension emphasizes exclusion as time-consuming but necessary. The work has to be done before the flies arrive in the fall, which means summer.
Vacuum or swat the ones inside. University of Kentucky Extension recommends mechanical control — fly swatters and vacuums — for active flies indoors. Their sluggishness makes this genuinely practical in a way it would not be for house flies.
Be careful with insecticidal dust indoors. Penn State Extension advises avoiding insecticidal dust inside structures, to prevent carpet beetle problems. It is the same failure mode that applies to every insect in this cluster: dead insects in a void become food for a pest that damages fabric. University of Kentucky Extension does discuss insecticide sprays and dusts for attics, basements and wall voids; we make no claim here about how well any product performs, and the two sources differ in emphasis, which is worth knowing before you choose.
Questions
What are the slow flies at my window in the fall?
Almost certainly cluster flies, Pollenia species. University of Kentucky Extension describes the cluster fly as a little larger than the common housefly, moving sluggishly, with short golden colored hairs on its thorax. The sluggishness is the giveaway: a house fly is hard to swat and a cluster fly is not.
Are cluster flies dirty? Do they carry disease?
No, and this is the single most important thing to know about them. University of Kentucky Extension states they constitute no health hazard. Penn State Extension states they are not known to carry any diseases of medical importance to humans. They are not filth flies. They did not come from garbage or a drain, and their presence says nothing about how clean the house is.
Where do cluster flies come from? What do the larvae eat?
Earthworms. University of Kentucky Extension states the larvae develop as parasites in the bodies of earthworms. Penn State Extension describes the maggots as burrowing into earthworms on which they feed, and notes the adults lay eggs on soil containing earthworms. This is why they appear in the fall around houses with lawns and pasture, and why nothing indoors is producing them.
How do I tell a cluster fly from a house fly?
Size, speed, hairs and wings. University of Kentucky Extension gives a little larger than the common housefly, moving sluggishly, with short golden hairs on the thorax. Penn State Extension describes them as dull gray with black markings, with golden-yellow hairs on the thorax that are more numerous on the underside between and near the legs. Their wings overlap when at rest, whereas a house fly holds its wings apart.
Are they breeding in my house?
No. University of Kentucky Extension states they are not breeding in the home. Penn State Extension states they will not reproduce within the structure. The flies emerging in your bedroom in February entered the building the previous fall.
Why do they appear on warm winter days?
Because warmth wakes them. Penn State Extension notes they emerge from wall voids on warm winter days, coming out through cracks under or behind baseboards, around window and door trim, and around exhaust fans or lights in ceilings. University of Kentucky Extension describes them appearing in late fall or early winter and again on warm, sunny days in early spring. A February sunbeam on a south wall is enough.
Should I be worried if the flies are shiny and metallic instead?
That is a different fly and a different problem. University of Kentucky Extension describes blow flies as having shiny metallic bodies, being strong fliers attracted to lights, and developing in manure or carrion. A sudden run of shiny metallic flies indoors can mean something has died in a wall, a chimney or a crawl space, which is worth investigating. A slow dull-gray fly is a cluster fly and means nothing of the kind.
How do I get rid of cluster flies?
Exclusion, then mechanical removal. Penn State Extension calls mechanical exclusion the best method and specifies sealing cracks around windows, doors, siding, utility pipes, behind chimneys and underneath the fascia. University of Kentucky Extension recommends mechanical control — fly swatters and vacuums — for active flies indoors, and emphasizes exclusion as time-consuming but necessary. Penn State advises against insecticidal dust inside structures because of the carpet beetle problem it creates.
Sources
- Penn State Extension (Steve Jacobs, Sr. Extension Associate). Cluster Flies. Updated June 24, 2026. https://extension.psu.edu/cluster-flies
- University of Kentucky Entomology (Michael F. Potter). Cluster Flies, Face Flies, and Blow Flies in Homes (ENTFACT-624). Revised March 2026. https://entomology.ca.uky.edu/ef624