Every autumn some houses get a particular kind of fly. It is grey, slow enough to catch in your hand, and it gathers at the upstairs windows on sunny afternoons and again on mild days in winter. Those are cluster flies: one of the least troublesome visitors a house can have, if one of the most persistent.
What cluster flies are
Cluster flies are a small group of flies in the genus Pollenia, relatives of the blow flies. The common one in houses on both sides of the Atlantic is Pollenia rudis. Penn State Extension notes it is thought to be native to Europe and may have reached North America in ship ballast containing soil and earthworms.
They are named for the habit that brings them to your attention: they gather in clusters to spend the winter. English Heritage, which deals with them in historic houses, says those clusters may be over 1,000 flies, usually in attics and wall voids.
They are not in our Field Guide yet, so there is no card for them. Of the 181 cards, 27 are tagged as found indoors, and among the flies on that list are the house fly and the bluebottle, the two they are most often mistaken for.
Why they are in your windows
The life cycle explains almost everything. In spring, Penn State says, adults leave their winter shelter and lay eggs on soil that has earthworms in it. The maggots burrow into earthworms and feed inside them until they are ready to pupate. UConn’s Home and Garden Education Center puts the whole cycle at 27 to 39 days depending on temperature, so there are several generations each summer. UConn says two or more, Penn State three to four or more in Pennsylvania.
In autumn the adults need somewhere dry to spend the winter. Outdoors that might be loose bark or a hollow tree. Next to a lawn or a field, it is more often a house. Cotswold District Council describes them gathering on sunny, light-coloured walls and crawling into crevices as the afternoon cools, mostly under the eaves and fascia boards into the roof space. UConn notes they start looking for shelter once outdoor temperatures fall to the mid-50s °F (about 13 °C).
Indoors they mostly sleep. When the sun warms the wall or the heating comes on, they wake and head for the light. The University of Minnesota Extension lists them in and around homes in autumn and spring, and on mild days in winter too. They are not hatching indoors; they are trying to get back outside.
Why they come back every year
Cotswold District Council is unusually frank about this. The flies tend to return to the same property year after year, a similar house next door may never get them, and nobody knows for sure why. Lighter colours, sunnier spots and scent left at old roosts have all been suggested. The council stresses that it has nothing to do with poor hygiene.
What we can say is that the ingredients do not change. The lawn and fields around the house still have earthworms. The University of Maine Cooperative Extension notes that earthworm numbers depend largely on soil moisture, so a damp summer can mean more flies the following autumn. The sunny wall is still sunny, and the gaps under the eaves are still there.
Cluster fly or house fly?
| Cluster fly | House fly | |
|---|---|---|
| Size | A little larger, about a centimetre | A little smaller |
| Colour | Dull grey, golden-yellow hairs on the thorax | Grey with dark stripes on the thorax |
| Wings at rest | Overlapping over the back | Held apart |
| Movement | Slow, clumsy, easy to catch | Quick, hard to swat |
| When | Autumn to spring, in crowds | Mostly spring to autumn |
| Young grow up in | Earthworms, outdoors | Rotting rubbish and manure |
The hairs and wings come from Penn State and UConn. Our own records back up the timing column. iNaturalist research-grade records on card #124 show the house fly near its peak from April to November, with only about three times as many records in July as in January.
The other common mix-up is with blow flies. English Heritage notes that blow flies are larger and often come from a dead animal such as a mouse or pigeon in the roof. A sudden burst of large, shiny flies in one room is worth investigating: iNaturalist research-grade records on card #128 show the common green bottle peaking from June to September, about 16 times its January level, so a cloud of them in midwinter is unusual. If you are not sure which fly you have, a photo in the identifier will tell you, with a confidence level and a safety read.
Are they harmful?
To people, no. The University of Maine says they do not bite people or animals and are not attracted to rubbish. Penn State says they do not breed indoors, will not damage the house, and are not known to carry diseases of medical importance. Both Penn State and English Heritage mention small dark spots of excrement on windows and walls, and the main complaint is the slow buzzing.
The one indirect risk is the dead. Penn State and English Heritage both warn that dead cluster flies make good food for carpet beetles, and UConn adds larder beetles, which can then move on to woollens and stored food.
What actually helps
- Vacuum the ones you can see. The University of Maryland Extension suggests swatting or vacuuming any flies that appear.
- Clear the dead. Sweep sills and the loft floor in spring so the bodies do not feed other insects.
- Seal in summer, not in October. Maryland and Penn State both put prevention first: caulk cracks around windows, doors, siding, pipes and fascia boards, and repair screens. Do it before late summer, when Maryland says they start arriving. English Heritage suggests bristle strips for draughty sash windows.
- Leave the loft alone in winter. A cluster in the roof space is not hurting anything and will leave in spring. Cotswold District Council notes that sprays miss the ones hidden in cracks.
- Mind the bats. The same council points out that in the UK it is illegal to use insecticide where it could come into contact with bats, and old lofts are where both live.
- Do not bother with the lawn. UConn says treating lawns and gardens, or reducing earthworms, generally does not work, and the earthworms are worth more than the flies.
Front desk note: a cluster fly in January is not lost, just early. It thinks your radiator is April. Open the window, point it at the sky, and let it get on with its life somewhere with earthworms.
For the rest of the autumn arrivals, see our guide to bugs that come into the house in autumn, which covers the ladybirds and stink bugs that often turn up at the same windows, and the flies section of the Field Guide.
Sources
- Penn State Extension: Cluster flies
- UConn Home and Garden Education Center: Cluster flies
- University of Maine Cooperative Extension: Cluster flies
- University of Minnesota Extension: What insect is this? Cluster fly
- University of Maryland Extension: Cluster flies
- English Heritage: Pest fact sheet 6: cluster flies (PDF)
- Cotswold District Council: Cluster fly fact sheet (PDF)
- Seasonal pattern: iNaturalist research-grade observations, as shown on card #124 and card #128
Questions
What are the slow flies on my windows in autumn and winter?
Most likely cluster flies, a group of flies in the genus Pollenia. They spend the summer outdoors in fields and lawns and come into buildings in autumn to spend the winter. On warm days, or when the heating comes on, they wake up and crawl out to the nearest window, where they buzz slowly and are easy to catch.
Why do cluster flies come back to my house every year?
Cotswold District Council notes that cluster flies tend to return to the same property year after year, and that nobody knows for certain why one building is chosen over a similar one next door. Sunny, light-coloured walls and old roosts may play a part. It is not a sign of poor hygiene; houses close to grass, fields and earthworms are simply nearer the flies' summer home.
How do I tell a cluster fly from a house fly?
Cluster flies are a little bigger than house flies, dull grey, and slow. Penn State Extension describes golden-yellow hairs on the thorax, and UConn notes that a resting cluster fly folds its wings so they overlap over its back. A house fly is busier, keeps its wings apart, and is about all summer rather than turning up in crowds in autumn.
Do cluster flies breed in the house or carry disease?
No. The University of Minnesota Extension says they do not reproduce indoors, and Penn State Extension says they are not known to carry diseases of medical importance to people. Their young grow up inside earthworms outdoors, not in rubbish, so the flies in the loft are adults waiting for spring.
How do I get rid of cluster flies?
For the ones you can see, a vacuum cleaner does the job, as the University of Maryland Extension suggests. For next year, seal gaps around windows, eaves, pipes and fascia boards in summer, before they arrive. Remove dead flies from sills and lofts, because Penn State notes they can feed carpet beetles, and in the UK never use insecticide in a loft where bats may roost.
