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How to identify it
- A little larger than a house fly and dull gray to black. Sizes given by extension services run from about 5/16 inch to 1/2 inch.[2][1][4][5]
- Short, golden-yellow hairs on the thorax give a golden sheen; Penn State says the hairs are thickest on the underside, between the legs.[2][1][6]
- The abdomen is checkered black and gray.[5]
- Wings overlap over the back at rest, and the flies are slow, sluggish and easy to swat.[2][1][3][4]
Look-alikes
Where it lives
Overwinters in attics, upper walls and other voids, especially on south- and west-facing sides of buildings; outdoors in hedgerows, under bark and in rock crevices.[2][1]
Biology and life cycle
Larvae are parasites of earthworms: eggs are laid on soil or near earthworm burrows, and the maggots enter earthworms to feed. A 2020 world checklist of the family says every polleniid whose larvae are known develops inside earthworms.[2][1][3][7]
Clemson says the eggs hatch within three days, the maggot stage lasts 13 to 22 days and the pupal stage 11 to 14 days.[6]
Several generations develop each summer; in late summer and early fall the last generation looks for shelter for the winter.[2][1][3]
Penn State says cluster flies are thought to be native to Europe and may have reached North America in ship ballast soil carrying earthworms. The 2020 world checklist lists the Pollenia species found in North America as introductions.[1][7]
Outdoors, adults are mostly found around fruit and flowers.[6]
On warm days in late winter and spring they crawl out of walls and attics, many heading for the brightest window, as they try to get back outside.[1][3][4][2]
Damage and signs
Health risk
Extension fact sheets describe cluster flies as a nuisance pest. A study of a mass infestation in a German hospital found opportunistic bacteria on the bodies of all 50 flies sampled, and the authors judged such infestations a low but not negligible risk in sensitive places such as hospitals.[1][3][6][8]
Control
Keep them out
Before flies gather in late summer, seal cracks around windows, doors, siding, utility pipes, chimneys and fascia with good caulk, repair screens, and screen attic and crawl-space vents. New Hampshire advises finishing before September 10. Sealing helps, though it rarely solves the problem alone.[1][2][3][5]
Traps and physical control
Indoors, vacuum or swat flies; they're drawn to bright windows in darkened rooms, which makes them easy to collect. Take the vacuum bag outside or seal it in a container, or the flies may crawl back out.[2][1][3][6]
Sticky flypaper can catch flies in rooms or attics with little air movement.[3]
Pesticides
Where sealing isn't practical, a licensed applicator can treat the exterior in late summer or early fall, just before flies gather, concentrating on the sunny south- and west-facing walls.[1][2][4]
Avoid dusting wall voids: the dead flies can feed carpet beetles that later attack woolens and stored goods. Foggers and indoor crack sprays won't stop flies emerging from the walls.[1][4]
Where sources disagree
Size: Minnesota gives 3/8 to 1/2 inch, Montana State about 1/3 inch (8 mm) and Washington State about 5/16 inch; Penn State says only that they are slightly larger than a house fly.[2][4][5][1]
Generations: Minnesota says two or three a year, New Hampshire up to four each summer, and Penn State three to four or more per season in Pennsylvania.[2][3][1]
How long an outdoor treatment lasts: Penn State says sunlight breaks the residue down so it may not kill flies much beyond several days or a week, while Montana State says pyrethroid perimeter treatments can leave a residue for 2 to 3 weeks.[1][4]
Common questions
- Are cluster flies breeding in my house?
- No. They overwinter in walls and attics and become active on warm days, but they breed outdoors in soil with earthworms.[1][2][3]
- Should I spray in spring?
- No. Flies on the outside in spring are leaving the building; killing them doesn't reduce next fall's numbers.[2]
- Can I get rid of where cluster flies breed?
- Not in any practical way. Their larvae live in earthworms, which are almost everywhere, so the useful steps are keeping adults out of the building and removing the ones that get in.[5][6]
Sources
- Penn State Extension, “Cluster Flies.” extension.psu.edu/cluster-flies (accessed 2026-11-11)
- University of Minnesota Extension, “Flies (reviewed 2024).” extension.umn.edu/garden-and-home/home-maintenance/household-insects/flies (accessed 2026-11-11)
- University of New Hampshire Cooperative Extension, “Cluster Flies [fact sheet].” extension.unh.edu/resource/cluster-flies-fact-sheet (accessed 2026-11-11)
- Montana State University Extension, “Cluster Flies (insect fact sheet).” montana.edu/extension/ipm/documents/entomology/ent-fact-sheets-2020-2021/insect_fact_sheet_cluster_flies_ada.pdf (accessed 2026-11-11)
- Washington State University Extension, “Cluster flies (Pestsense).” pestsense.cahnrs.wsu.edu/?p=1161 (accessed 2026-11-11)
- Clemson Cooperative Extension, Home & Garden Information Center, “Cluster Flies.” hgic.clemson.edu/factsheet/cluster-flies/ (accessed 2026-11-11)
- ZooKeys, via PubMed, “Gisondi et al. (2020), The world Polleniidae (Diptera, Oestroidea): key to genera and checklist of species.” pubmed.ncbi.nlm.nih.gov/33061774/ (accessed 2026-11-11)
- International Journal of Hygiene and Environmental Health, via PubMed, “Faulde et al. (2001), Hospital infestation by the cluster fly, Pollenia rudis, and its possible role in transmission of bacterial pathogens in Germany.” pubmed.ncbi.nlm.nih.gov/11279815/ (accessed 2026-11-11)
- iNaturalist, “Pollenia taxon page (order and family).” www.inaturalist.org/taxa/133825 (accessed 2026-10-06)