Why are there flies in my house in winter?

Almost certainly cluster flies. These are fall invaders that push into wall voids and attic spaces in September and October to overwinter. On warm winter days — even a sunny day in January — the warmth wakes them from their dormant state and they work toward light, ending up bumbling at your windows. Because their overwintering site is inside your walls, they appear indoors on every warm day all winter. They are not breeding indoors; they are not coming in from outside; they are waking up from inside the structure. UNH Extension has documented this pattern specifically in New Hampshire homes.

Are cluster flies filth flies? Will cleaning help?

No and no. Colorado State Extension states explicitly that cluster flies 'are not a type of filth fly that develops in carrion, decaying plant matter or garbage.' Their larvae are parasites of earthworms out in your lawn — your kitchen has absolutely nothing to do with their presence. Cleaning the house, taking out the garbage, and wiping down counters will not reduce a cluster fly problem by a single fly. The only thing that works is exclusion: sealing exterior gaps and screening attic vents before the fall entry push, ideally before early September.

Why do more flies appear every single day?

Because they are in your walls. The cluster flies overwintering inside your wall voids and attic wake up in small batches on each warm day and work toward light. It can look like they are multiplying or coming in from outside — neither is true. They are not breeding indoors (cluster flies do not reproduce inside structures) and they are not infiltrating from outdoors in winter. The daily appearance will continue throughout warm spells all winter until the overwintering population is exhausted or until the following spring when they leave.

Are cluster flies dangerous?

No. Cluster flies are harmless to humans and pets. They do not bite, do not sting, do not contaminate food the way house flies do, and carry no known human disease. At most, a very large overwintering mass in an attic leaves minor spotting on surfaces. The key public health message here is anti-fearmonger: if you find large sluggish flies at your windows in winter and someone tells you it is a sanitation or disease problem, that information is wrong. Cluster flies are a nuisance seasonal pest — nothing more.

How do I tell a cluster fly from a house fly?

Four features separate them: (1) Cluster flies have golden/yellowish tangled hairs on the thorax — house flies have four sharp dark stripes and no golden hairs. (2) Cluster flies are larger and darker. (3) Cluster flies' wings overlap at rest; house fly wings do not. (4) Cluster flies are sluggish and bumbling; house flies are fast and erratic. If the fly is quick and buzzing around food, it is a house fly. If it is large, slow, and pressing against a sunny window in January, it is a cluster fly.

Are cluster flies breeding in my attic?

No. Cluster flies do not reproduce indoors. The adults in your attic or walls are in a dormant overwintering state (diapause); they entered in the fall and are not laying eggs inside the structure. Their larvae only develop in earthworms in outdoor soil — that biology requires the lawn and the earthworms in it, not anything indoors. What you are seeing in the attic is the overwintering population emerging on warm days, not a breeding colony. This also means 'treating' the attic to kill larvae makes no sense — there are none.

How do I keep cluster flies out next year?

Seal exterior gaps and screen attic vents before early September — UNH Extension recommends completing this before approximately September 10. Pay particular attention to south- and west-facing walls (where flies congregate on warm fall days), gaps in siding, areas around window and door frames, utility penetrations, and gable and soffit vents. Even a thorough amateur sealing job substantially reduces cluster fly pressure the following winter. For comprehensive coverage, a professional exterior inspection in August identifies entry points that are easy to miss.

Do cluster flies damage the house?

No structural damage — cluster flies do not chew wood, destroy insulation, or create entry holes. At most, very large overwintering populations leave minor spotting (excrement) on window sills or attic sheathing. The nuisance is the flies themselves appearing on warm days, not any property harm. They are considerably less damaging than other fall invaders like carpenter ants, which can damage wood, or rodents, which can damage wiring. Cluster flies are purely a nuisance pest.

Why are cluster flies a problem every fall and winter?

NH homes with unsealed gaps will see cluster fly activity repeat annually because adult flies actively seek sheltered overwintering sites each fall, and a home that worked as a harborage last year is known by the surviving population as a viable site. The behavior is instinct-driven by cooling temperatures in September. Without sealing the entry points, the cycle repeats every year. The only way to break the annual pattern is completing systematic exterior sealing before early September — then maintaining that seal with annual touchups as caulking and weather stripping age.

What is the best time of year to deal with cluster flies?

Late summer — August through early September — is the optimal window for exclusion work. That is when the adult flies are still outdoors and the fall entry push has not yet begun. UNH Extension advises completing sealing before approximately September 10. Once flies are in the walls, your options that season narrow to vacuuming those that emerge at windows. If you are dealing with a winter infestation now, plan the exclusion work for the following late summer — that is when the intervention actually stops the problem.

Cluster fly · Pollenia rudis

Flies & GnatsNH resident

Cluster Flies in New Hampshire — Why They're in Your House in Winter

Also called Attic fly · Loft fly

Cluster flies are fall occasional invaders — not filth flies. Their larvae parasitize earthworms in your lawn; the adult flies overwinter in wall voids and attics, then emerge sluggishly on warm winter and early-spring days to cluster at sunny windows. They are harmless and do not contaminate food. Cleaning your kitchen does nothing. The only fix is exclusion before they get in — ideally before early September.

At a glance

What it is: A fall occasional invader — NOT a filth fly; Colorado State Extension
Species: Pollenia rudis; family Polleniidae (formerly placed near Calliphoridae)
Size: 3/8–1/2 in; larger and darker than a house fly; Colorado State Extension
Larvae: Parasitize earthworms in the lawn; UNH Extension; Colorado State Extension; CT Agricultural Experiment Station
Breeds indoors?: No — overwinters in diapause; does not reproduce inside the structure; Colorado State Extension
Seal-up deadline: Before ~September 10; UNH Extension

The short of it

Why are there flies at my window in the middle of winter?

Cluster flies (Pollenia rudis, family Polleniidae) are one of the most misunderstood flies in New Hampshire — and the misunderstanding drives a lot of wasted effort. They are not filth flies. They do not breed in your kitchen, your garbage, or any food source. Colorado State Extension states plainly that cluster flies 'are not a type of filth fly that develops in carrion, decaying plant matter or garbage' — their larvae are parasites of earthworms out in the lawn.

What they do is this: adults push into wall voids, attic spaces, and other sheltered spots in the fall, enter a dormant state (diapause), and overwinter there. On warm winter days and in early spring, they wake up and work their way toward light — ending up sluggishly bumbling at sunny windows, apparently trying to get back outside. This is why you see them in January or February despite having done nothing to attract flies.

UNH Extension specifically notes that these large sluggish flies 'often invade New Hampshire homes in fall and turn into wintertime pests,' noticeable on warm winter days at windows. Because they are not a filth fly, sanitation is irrelevant. The only lever is exclusion: seal exterior gaps and screen attic vents before the fall push — UNH advises before approximately September 10.

Identify it

How to identify a cluster fly

Four features separate cluster flies from house flies and from the metallic blow/bottle flies they might be loosely grouped with: golden thoracic hairs, larger sluggish body, overlapping wings at rest, and the fall-to-winter timing indoors.

Species profile

Cluster fly

Scientific name: Pollenia rudis
Family: Polleniidae (historically placed near Calliphoridae; modern taxonomy separates them)
Size: 3/8–1/2 in (larger than a house fly)
Coloration: Dull grayish-brown; distinctive golden/yellowish tangled hairs on the thorax
Habitat: Larvae in lawns/fields (in earthworm hosts); adults overwinter in attics, wall voids, and sheltered gaps.
Active season: Invade fall; emerge on warm winter and early-spring days; summer activity outdoors
Diet: Larvae: parasitic on earthworms. Adults: feed minimally during overwintering; some flower nectar outdoors.
Reproduction: Up to 4 generations per summer (UNH Extension); larvae are obligate earthworm parasitoids.
Lifespan: Adults overwinter in diapause; do not reproduce indoors.

Common throughout the northeastern United States and New Hampshire; introduced from Europe.

The tells — key ID features

Golden/yellowish tangled hairs on the thorax — the single best ID tell vs. the house fly.
Slightly larger and darker than a house fly, moving sluggishly.
Wings overlap at rest — house fly wings do not overlap.
Clusters at sunny windows and in attic spaces on cool days.
Bumbling, slow movement compared to the quick, erratic house fly.

Size, to scale

The insect

3/8–1/2 in

Compared to

Noticeably bigger than a house fly — roughly a cooked grain of rice and then some

Looks bigger than it is

The sluggish movement is as diagnostic as the size — cluster flies are lazy, bumbling fliers, not quick like house flies.

Is it actually this? — the confusion species

Common house fly

How to tell — House fly is smaller, faster, has four sharp dark thoracic stripes (not golden hairs), and wings do not overlap at rest. House flies are active filth flies; cluster flies are seasonal invaders. If it is quick and buzzing around food or garbage, it is a house fly.

Blow / bottle fly

How to tell — Blow and bottle flies have a brilliant metallic blue/green/bronze sheen. Cluster flies are dull grayish-brown — no metallic sheen at all.

What you'll notice

Signs of a problem

Cluster fly signs follow a predictable seasonal pattern tied to temperature, not to any food or sanitation condition in the home.

Sluggish, large flies clustering at sunny windows on warm winter days

What it means — Classic cluster fly emergence from within the walls or attic — the overwintering adults are waking and seeking light to exit.

Clusters of flies in the attic or in wall voids

What it means — The overwintering harborage — this is where the flies are spending the cold season in diapause.

More flies appearing each warm day despite removing yesterday's

What it means — Flies are emerging from inside the structure, not breeding there — a large overwintering population is present in the walls or attic.

Large flies appearing on sunny exterior walls in fall (September–October)

What it means — The fall entry push — flies are gathering on sun-warmed siding before entering gaps to overwinter. This is the window to seal.

How much it matters

The real risks, calibrated

Cluster flies are harmless. This is the most important fact on this page — anti-fearmonger framing is warranted.

Overall risk

Low risk

Nuisance only — no health risk

Low risk

Onset: During overwintering season

Cluster flies do not bite, do not contaminate food the way filth flies do, and carry no known disease risk to humans. They are a nuisance, not a health hazard.

Minor spotting and odor in heavy infestations

Low risk

Onset: Over the winter season

Very large overwintering masses may leave minor spotting (excrement) on window sills or attic surfaces and produce a faint sweetish odor.

What you can do

DIY — what works, what backfires

Cluster fly control is about exclusion before they get in — not about sanitation, not about spraying after they are already inside the walls. Here is what actually works.

What actually works

Seal cracks in siding, around windows, door frames, and utility penetrations before early-to-mid September — UNH Extension advises before approximately September 10, ahead of the fall entry push.
Screen attic and gable vents with fine mesh before early September to block the primary overwintering route.
Vacuum sluggish cluster flies from windows and surfaces — use a vacuum with a bag, and empty the bag outdoors immediately to prevent them from escaping back into the house.
On warm fall days, inspect sunny exterior walls for fly congregation — this tells you exactly where the entry gaps are while you still have time to seal them.

What backfires

Don't rely on cleaning or sanitation — cluster fly is not a filth fly; your kitchen has nothing to do with their presence, and cleaning won't reduce them.
Don't spray the interior once flies are already inside the walls — spraying rooms does not reach the overwintering population in voids; it only kills those that have already emerged.
Don't expect a fix this winter — if cluster flies are already in the walls, interior treatment helps little; the real solution is planning exclusion for next fall.
Don't confuse the daily re-emergence with breeding indoors — cluster flies do not reproduce inside your home; the daily appearance is from the overwintering population waking up, not from new egg laying.

Local Context

Cluster flies in New Hampshire homes

UNH Extension identifies cluster flies as a common New Hampshire fall invader that becomes a wintertime window pest. They are found statewide across NH in homes with accessible wall voids, attic spaces, or gaps in siding and window frames. The fall entry push begins as temperatures cool in September, well ahead of Manchester's first hard frost (typically mid-to-late October per NOAA 1991–2020 normals). Seal-up work must be completed before early September to be effective — after they are inside the walls, interior treatment is difficult and exclusion that fall season is already lost.

Peak season

Invade in fall (September); emerge at windows on warm winter and early-spring days

You might actually have

Common house fly (filth fly, four thoracic stripes, faster — the most important ID contrast)
Blow/bottle flies (metallic — cluster fly is dull grayish-brown)
Asian lady beetles and boxelder bugs (same fall-invader overwintering behavior)

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Southern New Hampshire

When it's past DIY

When to call a pro for cluster flies

Cluster fly exclusion requires systematically sealing every exterior gap before fall — a task that benefits from a professional exterior inspection to find entry points that are easy to miss. For very large overwintering populations in attics, a licensed technician can also treat voids with approved products timed to intercept flies before they fully enter.

Signals it's past DIY

When to call a pro

Recurring heavy winter infestations despite your own sealing attempts — a professional exterior inspection identifies gaps you missed.
Exclusion work timed correctly before fall entry — mid-to-late August through early September is the optimal window.
Very large overwintering populations in attic spaces that are difficult to address with vacuuming alone.
You want a whole-structure gap assessment to prevent cluster flies, Asian lady beetles, and other fall invaders in one service.

Keep them out — prevention

NH-specific

Seal before September — the most important step

UNH Extension advises sealing exterior gaps before approximately September 10. This is the annual deadline. Caulk gaps in siding, seal around windows and doors, and install tight-fitting screens on attic and gable vents before the fall push.

Pay attention to sun-facing walls

Cluster flies congregate on south- and west-facing walls on warm fall days before entering. These sun-warmed surfaces show you exactly where the entry points are — inspect these sides of your home first when sealing.

Treat it as an annual task

NH homes with established cluster fly pressure will see the behavior repeat every fall until all entry points are sealed. A once-annual late-summer inspection and sealing pass is the long-term fix.

Not sure it's a cluster fly?

A licensed NH technician can confirm the species and tell you honestly whether it's a DIY fix or a job for a pro — free inspection, same-day service across Southern New Hampshire.

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Common Questions

Frequently asked questions

Sources(5)

[1]
UNH Extension — Cluster Flies Fact Sheet
NH wintertime window pest, 4 generations, seal before Sept 10
extensionverified 2026-06-30
[2]
Colorado State Extension — Cluster Flies and Other Winter Flies
not a filth fly; earthworm parasite; diapause; size
extensionverified 2026-06-30
[3]
CT Agricultural Experiment Station — Cluster Fly (Pollenia)
earthworm life cycle, biology
govverified 2026-06-30
[4]
UNH Extension — Seasonal Insect Invaders in New Hampshire Homes
fall-invader timing and exclusion in NH
extensionverified 2026-06-30
[5]
NOAA/NWS Gray–Portland — NH Climate Normals
Manchester frost normals (context for fall seal-up deadline)
govverified 2026-06-30

Written & reviewed by

Verified

Daniel Brady

NH-Licensed Pesticide Applicator · License #782664 (NHDAMF, RSA 430)

Daniel Brady is the New Hampshire–licensed applicator behind Anchor Pest Services, a family-owned pest control company serving the Manchester area since 2017.

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Last updated 2026-06-30

New Hampshire pest control

Dealing with cluster flys in your home?

Anchor Pest Services identifies and handles cluster fly problems across southern and central New Hampshire. Family-owned since 2017, NH license #782664.

Get a Free Inspection (603) 785-0118

NH-licensed applicators (#782664)
Same-day service · free inspection
Family-owned since 2017

Keep Exploring

Cluster Flies in New Hampshire — Why They're in Your House in Winter

Why are there flies at my window in the middle of winter?

How to identify a cluster fly

The tells — key ID features

Size, to scale

Common house fly

Blow / bottle fly

Signs of a problem

Sluggish, large flies clustering at sunny windows on warm winter days

Clusters of flies in the attic or in wall voids

More flies appearing each warm day despite removing yesterday's

Large flies appearing on sunny exterior walls in fall (September–October)

The real risks, calibrated

Nuisance only — no health risk

Minor spotting and odor in heavy infestations

DIY — what works, what backfires

What actually works

What backfires

Cluster flies in New Hampshire homes

When to call a pro for cluster flies

Seal before September — the most important step

Pay attention to sun-facing walls

Treat it as an annual task

Not sure it's a cluster fly?

Frequently asked questions

Dealing with cluster flys in your home?

Your next step

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Where Do Flies Go in the Winter? NH Cluster Fly Timing

What Attracts Cluster Flies to Your House?

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What is the best way to get rid of cluster flies?

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