Cluster Flies

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.

If what you have is none of these, house flies are covered separately: our house flies page sets out what the sources say they carry and what they support doing about it. And a much smaller, red-eyed fly gathering around ripening fruit, a trash can or a slow drain is a different insect again, not a cluster fly at any stage of its life — see our fruit flies page for that one.

What to Do

Seal, in summer. Penn State Extension puts mechanical exclusion ahead of every other method it discusses 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.

Caulk for the summer sealing work

These are examples of one material, for the sealing Penn State Extension puts first. They are in no order of preference, none is called better than another, and nothing here is a claim about how well any of them performs — the limit stated just above applies to these cards too.

Where the caulk specification comes from, stated plainly. Penn State Extension names the places to seal on this page — cracks around windows, doors, siding, utility pipes, behind chimneys and underneath the fascia — but the material itself, good quality silicone or silicone-latex caulk, is a specification the same source gives for the other fall invaders rather than for cluster flies. It is attributed here rather than presented as a cluster fly finding. And the timing is the part that matters most: University of Kentucky Extension calls exclusion time-consuming but necessary, and the work has to be done before the flies arrive, which means summer.

The sealants below are named because every source on this page says the answer is closing the building rather than treating the insect.

Questions

Sources