Quick answer: Cluster flies in house walls and attics are overwintering flies, not a sanitation problem. They breed in lawns and fields as parasites of earthworms, gather on sun-warmed walls in September and October, and squeeze into attics and wall voids for the winter. On warm days they wake up and blunder into upper-floor windows. Vacuum them up, seal the sunny side of the house, and time any exterior treatment for early fall.
Key takeaways
- Cluster flies develop inside earthworms in the soil, so they say nothing about your housekeeping and they cannot breed indoors.
- They enter through gaps around eaves, siding, window frames and vents on the sunny side of the house in early fall, then wake on warm days all winter.
- They are slightly larger and much slower than house flies, with golden hairs on the thorax, and a vacuum handles the ones you can see.
- Indoor sprays do little once the flies are in the walls; sealing before they gather and a well-timed exterior treatment are what shrink next year’s numbers.
What are cluster flies, and why are they in my house?
Cluster flies are outdoor flies that use your house as a winter shelter. They are in your house because it is warm, it faces the sun, and it has gaps.
Their life cycle is nothing like a house fly’s. According to Oregon State University Extension, each female lays her eggs in the soil, and the larvae are internal parasites of earthworms. As many as four generations develop each summer in the lawns, pastures and fields around us. No garbage, no drains, no dead animal in the wall.
Then the weather turns. OSU notes that in fall the adults seek shelter to spend the winter and crawl into buildings through cracks and gaps around eaves, sash cords, window sills and baseboards. They settle in attics and wall voids by the hundreds, which is where the name comes from. OSU is also clear on the point that matters most to homeowners: cluster flies do not breed in buildings. In spring they leave to return outdoors and breed.
How do you tell cluster flies from house flies and blow flies?
Watch how it flies and look at its back. A cluster fly is slow and clumsy, a bit bigger than a house fly, and dark gray with golden hairs behind the head. A house fly is quick and striped. A blow fly is shiny metallic blue or green.
| Fly | Look | Behavior | Where it breeds | What it tells you |
|---|---|---|---|---|
| Cluster fly | Slightly larger than a house fly; dull dark gray with golden-yellow hairs on the thorax and light patches on the abdomen | Sluggish, easily swatted; found at upper-floor windows and in attics from fall to spring | Soil, as a parasite of earthworms | Overwintering flies got in through the sunny side of the house; not a cleanliness issue |
| House fly | 4 to 8 mm; gray with four dark lengthwise stripes on the thorax | Fast, erratic; active around kitchens and trash in warm weather | Garbage, pet waste, manure and decaying food | A sanitation or trash-handling issue nearby |
| Blow fly | 8 mm or larger; metallic blue, green or bronze | Loud, fast; often a sudden burst of many flies at once | Animal carcasses, meat waste and excrement | A sudden indoor swarm often means a dead rodent or bird in a wall, attic or chimney |
The cluster fly description comes from OSU Extension and Penn State Extension, which notes the flies are typically sluggish in flight and easily swatted or captured. The house fly and blow fly details come from OSU Extension’s Pacific Northwest guide to nonbiting flies, which also notes that blow flies find dead animals quickly, including rodents and birds. That last point is why a winter blow fly swarm deserves a rodent inspection, while a cluster fly swarm does not.
Why do cluster flies show up on warm winter days?
Because they never really left. The flies that entered in October are dozing in your attic and wall voids, and a sunny afternoon warms those spaces enough to wake them.
OSU Extension notes that cluster flies become active whenever temperatures rise above 54 F, from early fall to mid-spring. Penn State adds that they gather in large numbers at windows within the home on warm winter days. A woken fly crawls toward light, finds a gap around a ceiling fixture, a window casing or the attic hatch, and ends up buzzing weakly against the glass in an upstairs bedroom. Then the sun goes down, the wall cools, and the next warm day repeats the show.
That is why the flies appear in the same room again and again, usually a south- or west-facing room on the top floor, and why you find dead ones on the sill every few days from November through March.
How to get rid of cluster flies in house walls and attics
Deal with the flies you can see, then close the doors they used. In that order, and with realistic expectations about what an indoor spray can do.
- Vacuum them. OSU recommends a vacuum for dead, dying or sluggish flies, and it beats a spray every time. Empty the canister outdoors.
- Skip the indoor fog. Penn State notes it is not advisable to use an insecticide inside once the flies have reached the wall voids or attic. You cannot reach them, and dead insects piling up in a wall bring their own problems.
- Seal the sunny side. Caulk around window and door frames, siding joints, utility pipes and where trim meets the wall, and repair torn screens.
- Screen the attic. Gable vents, soffit vents and the attic hatch are the big doors. Fine mesh over vents and weatherstripping on the hatch close them.
- Get the timing right. Sealing done in September, before the flies gather, protects this winter. Sealing done in January protects next winter.
An exterior treatment can help the houses that get hundreds of flies a year. Penn State notes it should be applied by a licensed pest control operator in the fall just before the flies congregate. That is the same early-fall window we use for boxelder bug treatments, and for good reason: boxelder bugs, stink bugs, lady beetles and cluster flies all pick the same warm walls in the same weeks. Our guide to boxelder bugs in Oregon covers that overlap.
Why farmland-edge homes in the Willamette Valley get the most cluster flies
Cluster flies need earthworms, and the Willamette Valley is earthworm country. Pastures, grass seed fields, orchards and irrigated lawns surround Salem, and the homes that back up to them in Woodburn, Dallas and McMinnville sit right next to the breeding grounds. In our experience those properties see the heaviest fall flights, especially when the house has a big south- or west-facing wall with older wood siding.
Our climate finishes the job. Winters here run mild, often 35 to 50 F, with plenty of afternoons that pass the 54 F mark OSU describes. A house in a colder climate might get one or two winter emergences; a house in Keizer can get many more. Add the unscreened gable vents and settled window frames common in homes built between the 1950s and 1980s, and you have a fly hotel.
One more pattern from the field: the houses that get cluster flies tend to get them again the next fall. The flies are not returning to a nest, because there is none. The sunny wall, the fields next door and the gaps are simply still there. That is also why cluster flies belong on the same fall checklist as the other winter bugs we see in Salem.
When to call a professional
Call when the flies come by the dozens every warm day, when you cannot tell whether you have cluster flies or blow flies, or when sealing has not made a dent. A big blow fly swarm in winter is a call today, because it usually means a dead animal in the structure.
Our technician starts by confirming the species, then walks the sun-facing elevations and the attic to find where the flies are getting in and where they are clustering. You get a written list of the gaps worth sealing. For heavy pressure, we schedule an exterior treatment of the entry elevations for early fall, using EPA-registered products applied according to the label, and we tell you exactly what to expect, including any re-entry times. Cluster flies fall under our general pest services, which also cover the boxelder bugs and stink bugs that share their calendar.
AAA Pest Exterminators has been family-owned and operated in Salem since 1997, we offer family- and pet-considerate options, and we never tie you to a long-term contract.
Frequently asked questions
Are cluster flies a sign of a dirty house?
No. Cluster flies develop inside earthworms in the soil outdoors, according to OSU Extension, and they do not breed in buildings. They are in your house because it is warm and sunny and has gaps around the eaves, siding or windows. A spotless home next to a pasture will get more of them than a messy apartment downtown.
Do cluster flies bite or spread disease?
Cluster flies are nonbiting flies, and because they breed in soil rather than garbage or carcasses they are not the sanitation concern that house flies and blow flies can be. Their main offenses are the buzzing, the smudges they leave on windows, and the dead flies that pile up on sills and in light fixtures.
Will cluster flies go away on their own?
Yes. OSU Extension notes that in spring the flies leave their hibernation sites to return outdoors and breed. The ones that emerge indoors during winter die within days. Left alone, an infestation runs its course by mid-spring, but the same flies’ offspring will look for the same gaps next September unless you seal them.
Why are the flies only in my upstairs rooms?
Because they overwinter above you. Cluster flies crawl upward on the warm side of the house and settle in the attic and the wall voids of the top floor. When a warm day wakes them, they follow light through the nearest gap, which puts them in an upstairs bedroom or bathroom window rather than the kitchen.
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