Overwintering adults: autumn to spring, roughly half a year (Colorado State; Penn State). Egg to adult: about 27–39 days — eggs 3 days, maggots in earthworms 13–22 days, pupae 11–14 days (Clemson). Indoors: they don't breed, and those that wander into warm rooms die quickly (Virginia Tech). Gone by: spring. Seal gaps by: mid-August.
How long do cluster flies live?
The adults that crowd into your attic in autumn live for months — they have to, because they spend the whole winter there. Cluster flies gather on sunny walls from mid-August, move into buildings in autumn and stay until spring (Colorado State University Extension; Penn State Extension). So how long do cluster flies live? An overwintering adult survives from late summer or autumn until the following spring — roughly half a year by the calendar — living on the fat it built up in summer (Virginia Tech notes they leave a greasy spot when swatted because of that stored fat).
Summer generations are a different story. We found no extension service that gives a figure for how long summer adults live, so we don't quote one. What is known is how fast they develop: eggs hatch within three days, the maggots spend 13–22 days inside an earthworm, and pupae take 11–14 days, so the whole cluster fly lifespan from egg to adult takes about 27–39 days (Clemson). Penn State counts three to four or more generations a season; a pest-control company puts the whole life cycle at four to six weeks (Orkin, used here only to fill that gap).
Indoors, their time runs out quickly once they leave their hiding places. Cluster flies "typically do not live long inside homes, which are usually too hot and dry for their survival" (Virginia Tech), and those that wake from hibernation "will often go to the nearest light source, such as a window or glass door" and "tend to die rather quickly" (Montana State University). That's why dead ones pile up on windowsills.
What are cluster flies?
What are cluster flies? They are blow flies of the genus Pollenia — Virginia Tech's fact sheet covers Pollenia rudis — and they get their name from the tight clusters they form in attics and wall voids. Adults are "medium-sized, robust, somewhat bristly flies about 7 mm (0.3 inches) long", brownish-grey "with numerous short yellow hairs on the thorax, a checkered pattern on the abdomen, and large, reddish-brown eyes" (Virginia Tech). Penn State notes they're "slightly larger than the common house fly", and the golden hairs give them a sheen. They're also called attic flies, and buckwheat flies because of a buckwheat-honey smell when crushed (Virginia Tech).
Are cluster flies harmful?
No. Cluster flies "do not bite, are not associated with the transmission of disease, and do not reproduce inside buildings" (Virginia Tech), and they "will not damage your home" (Penn State). They can leave small dark spots, and large numbers of dead flies may smell and attract carpet beetles, whose larvae feed on dead insects (Virginia Tech; Colorado State). So are cluster flies harmful? They're a nuisance, not a hazard.
Cluster fly life cycle and the earthworm connection
Unlike house flies, cluster flies don't breed in garbage or manure. "The immatures are parasites of earthworms" (Montana State University):
- Egg: females lay single eggs in cracks in the soil (Clemson), near the entrances to earthworm burrows (Virginia Tech). They hatch within about three days.
- Larva: each maggot finds an earthworm, burrows in and feeds inside it for 13–22 days (Clemson), eventually killing it (Colorado State). They don't reduce earthworm numbers enough to harm the soil (Virginia Tech).
- Pupa: the grown maggot leaves the worm and pupates in the soil for 11–14 days (Clemson).
- Adult: emerges to mate and lay more eggs. Up to four generations can be completed in a growing season (Colorado State).
That's why cluster flies are most common where there are lots of earthworms: rural and suburban homes near large fields, pastures and lawns (Virginia Tech). Treating a lawn for the larvae isn't effective (Colorado State) — they're inside the worms.
Why cluster flies come inside in autumn
Shortening days tell them winter is coming. "As days shorten in late summer," adults stop reproducing and look for sheltered places to spend the winter; from mid-August they gather on sun-warmed south and west walls, then slip in through window cracks, soffit vents and damaged siding (Colorado State). Homes on hilltops or that stand out in the landscape can attract especially large numbers (Virginia Tech).
| Season | Months | What cluster flies are doing | Source |
|---|---|---|---|
| Spring | March–April | Overwintered adults wake, swarm to windows and leave | Clemson; Colorado State; Penn State |
| Summer | May–July | Breeding outdoors: eggs in soil, maggots in earthworms; several generations | Clemson; Penn State; Colorado State |
| Late summer | August | Adults stop breeding and gather on sunny south and west walls from mid-August | Colorado State |
| Autumn | September–October | They crawl into attics and wall voids to overwinter | Penn State; Virginia Tech |
| Winter | November–February | Dormant in walls and attics; some wander out on warm days | Penn State; Colorado State |
Months are our translation of the seasons in these sources; in the Southern Hemisphere, shift each stage by six months. The calculator does this for you.
Cluster flies in the attic and in winter
Cluster flies in attics are the classic sign: they cluster in the warmest, highest parts of the house, which is why they're called attic flies (Virginia Tech). Cluster flies in winter are dormant, but "on warm winter days they become active and crawl out of wall voids and attics in a confused attempt to go back outside" (Penn State). They don't breed indoors, so the number you see can only go down until spring.
When do cluster flies go away?
In spring. Most overwintering flies "leave in early spring, and few die inside" (Colorado State), and Clemson says they're most noticeable in spring "when they emerge in large numbers to leave the building". When do cluster flies go away for good? Not until you seal the gaps they come in by — otherwise next autumn's generation will find the same way in.
Cluster flies vs house flies
They're easy to confuse at a glance, but they behave very differently:
| Cluster fly | House fly | |
|---|---|---|
| Size and look | About 7 mm, dull grey with golden hairs on the thorax and a chequered abdomen; slow fliers (Virginia Tech) | 6–7 mm, grey with four dark stripes on the thorax (UF/IFAS) |
| Larvae live in | Earthworms, as parasites (Virginia Tech; Clemson) | Manure, garbage, compost and other rotting matter (UF/IFAS) |
| Breeds indoors? | No — they only shelter indoors (Virginia Tech) | Yes, wherever there's waste for the maggots |
| Seen indoors | Autumn to spring, in attics, walls and at windows (Colorado State) | Mostly warm months, around food and bins |
| Adult life | Overwintering adults: autumn to spring | Usually 15–25 days (UF/IFAS); median 21–24 days at 25 °C (Fletcher et al. 1990) |
| Bites or spreads disease? | Doesn't bite; not associated with disease (Virginia Tech) | Doesn't bite, but can carry pathogens to food (UF/IFAS) |
If the flies are breeding in a bin or compost and are around all summer, they're probably house flies — see the house fly lifespan calculator and the house fly life cycle. If they appear in autumn, sluggish and in groups at upstairs windows, they're cluster flies.
How to keep cluster flies out
Timing is everything: once they're in the walls, you can only wait them out. These non-toxic steps come from extension services:
- Seal before they gather: close gaps "by mid-August and certainly no later than Labor Day" (Colorado State) — caulk around windows, doors, siding and utility pipes (Penn State).
- Screen everything: tight window and door screens, fine mesh over vents, weather stripping (Virginia Tech; Clemson).
- Close the inside routes: flies in the walls find their way in around ceiling lights, exhaust fans, baseboards and attic doors (Virginia Tech).
- Vacuum, then empty: vacuum flies from windowsills and floors and empty the canister or bag outside, so dead flies don't feed carpet beetles (Virginia Tech; Clemson).
- Skip indoor sprays: Penn State doesn't advise insecticide indoors once flies are in walls or attics; "bug bombs" aren't effective (Montana State).
Light traps and sticky traps are good choices for the flies that do get in (Virginia Tech).
How this tool works
- Season: each month is matched to a stage using the extension sources above: overwintering November–February, leaving in spring (March–April), breeding outdoors May–July, gathering from mid-August and moving in during September–October, for the Northern Hemisphere. For the Southern Hemisphere the calendar is shifted by six months — our assumption, since the sources are all from the Northern Hemisphere.
- Lifespan: the egg-to-adult time (27–39 days) adds Clemson's stage durations; overwintering adults live from autumn to spring. No figure for summer adults is shown because we couldn't verify one.
- Where you see them changes the advice, not the timing.
Frequently asked questions
How long do cluster flies live?
Overwintering adults live from autumn to spring — about half a year. Egg to adult takes about 27–39 days (Clemson); we found no extension figure for how long summer adults live.
What are cluster flies?
Blow flies of the genus Pollenia, about 7 mm long, grey with golden hairs, whose larvae are parasites of earthworms (Virginia Tech).
Why do cluster flies come inside?
As days shorten in late summer they stop breeding and look for sheltered places to overwinter, such as attics and wall voids (Colorado State).
Are cluster flies harmful?
No. They don't bite, aren't associated with disease and don't breed indoors (Virginia Tech); they won't damage your home (Penn State).
When do cluster flies go away?
In spring — most leave in early spring (Colorado State). Seal gaps by mid-August to stop the next autumn's flies getting in.
Why are there cluster flies in my attic?
Attics are their favourite winter quarters, which is why they're also called attic flies (Virginia Tech).
Do cluster flies come out in winter?
On warm winter days some wake up and crawl out of walls and attics trying to get back outside (Penn State).
What's the difference between cluster flies and house flies?
Cluster flies are slightly larger with golden hairs, breed in earthworms and only shelter indoors; house flies breed in waste and are around in summer.
Why are dead flies on my windowsill?
Usually cluster flies: on leaving hibernation they head for the nearest light and die quickly in warm, dry homes (Montana State; Virginia Tech).
These calculators give estimates for education. Real flies vary with strain, food, humidity and temperature. For a stubborn infestation, your local cooperative extension service can help identify the insect. We give no pesticide advice.