Why do flies suddenly appear inside on a warm winter day?

Those flies are overwintering cluster flies that have been dormant in your wall voids and attic since fall. When indoor or outdoor temperatures reach the mid-50s°F — whether from a sunny afternoon or your furnace running — they wake up and crawl toward the nearest light source, usually a window. It looks alarming because dozens can appear at once, but it is not a new infestation. It is the same population that entered your walls the previous August or September, simply responding to warmth.

Are these the same as house flies?

No. House flies are smaller, faster, and breed in garbage, manure, and decaying matter, staying active outdoors from spring through fall. Cluster flies are noticeably larger and darker, move sluggishly, and have short golden hairs on the thorax that give them a slightly fuzzy look. Unlike house flies, cluster flies never breed in filth or food waste — their larvae live exclusively outdoors as parasites of earthworms. If you're seeing slow-moving flies at a window in January, they're cluster flies, not house flies.

Do cluster flies breed in my house?

No. Cluster flies do not breed, feed, or lay eggs indoors at any point in the year. Their larvae develop only outdoors, where they parasitize earthworms in lawns and fields over a roughly 27–39-day cycle. The flies you see indoors in winter are strictly overwintering adults sheltering from the cold — they pose no health hazard, do not contaminate food, and will not multiply inside your walls no matter how long they remain there.

When do cluster flies come out or first come in?

Cluster flies begin entering New Hampshire homes in late August, as the last outdoor generation of the season seeks a place to overwinter. Entry continues through September and October, peaking on sunny days as flies aggregate on south- and west-facing walls before slipping into gaps around windows, siding, and soffits. They then reappear — or 'come out' — indoors on warm winter or early-spring days once temperatures climb into the mid-50s°F, which triggers them to wake and move toward light.

Why does this happen at my house every year?

Homes near lawns and fields with healthy earthworm populations tend to attract heavier cluster fly numbers, since earthworms are the only host for their larvae. Sunny south- and west-facing walls also play a role, absorbing more fall afternoon heat and drawing flies looking for a warm entry point. Once a building has been used successfully as an overwintering site, the flies tend to return to that same structure annually, which is why some houses see recurring, heavy clusters year after year.

Are cluster flies the same as fruit flies?

No, and the two are easy to tell apart. Fruit flies are tiny, only about 2–4 millimeters long, have reddish eyes, and breed in fermenting fruit, drains, or other moist organic waste — they're a kitchen problem active whenever a food source is present. Cluster flies are much larger, roughly 3/8 to 1/2 inch, move slowly, and only appear indoors in cold months as overwintering adults waking at windows. If the flies you're seeing show up specifically in fall or winter, they're cluster flies, not fruit flies.

Should I fog or spray the attic to kill them?

No. Once cluster flies are already inside a wall void or attic, killing them with an insecticide leaves dead carcasses that attract secondary pests like carpet beetles and larder beetles, trading one nuisance for another. The recommended approach is exclusion, not treatment after the fact: seal entry points the following summer before flies begin aggregating. Indoors, simply vacuum up sluggish flies at windows rather than crushing them, which can leave stains and odor.

How do I keep cluster flies out of my house?

The most effective window is summer through late August, before the fall generation starts seeking shelter. Caulk and seal cracks around windows, eaves, soffits, siding, and utility line penetrations, focusing extra attention on south- and west-facing walls where flies aggregate most heavily. Screening attic and soffit vents helps too. Because cluster flies tend to return to the same building annually, a thorough exclusion pass one summer can meaningfully reduce the size of the clusters you see the following fall and winter.

Daniel Brady · NH-Licensed Pesticide Applicator · License #782664

|Reviewed by Daniel Brady

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

Overwintering Invaders

Flies go into wall voids, attics, and spaces under siding for the winter, entering as adults beginning in late August and settling in through the first frost. They are cluster flies (genus Pollenia), not house flies, and once inside they do not breed, feed, or pose a health hazard. On warm winter days — roughly mid-50s°F or warmer — furnace heat or sun on the walls rouses them, and they crawl toward windows in a sluggish, harmless swarm before settling back down.

At a glance

Peak indoor-entry: Late Aug–OctLast-generation adults seek shelter starting late August, continuing through the fall entry window.
Winter location: Wall voids, attics, under sidingFlies cluster in the thousands between walls in protected overwintering sites.
Wake-up trigger: ~Mid-50s°FFurnace heat or sunny days rouse dormant flies toward windows.
Breeds indoors?: No — larvae parasitize earthworms outdoorsNo feeding, breeding, or health hazard indoors; larvae develop only outdoors.

Peak indoor-entry
Late Aug–Oct
Last-generation adults seek shelter starting late August, continuing through the fall entry window.
Winter location
Wall voids, attics, under siding
Flies cluster in the thousands between walls in protected overwintering sites.
Wake-up trigger
~Mid-50s°F
Furnace heat or sunny days rouse dormant flies toward windows.
Breeds indoors?
No — larvae parasitize earthworms outdoors
No feeding, breeding, or health hazard indoors; larvae develop only outdoors.

The timing answer

The short of it

The verdict · overwintering invaders

Dormant in walls until spring

Cluster flies you see in winter entered your walls last fall and are simply waking on warm days — they don't breed or feed indoors.

NH activity calendar

When are they active — the whole year at a glance

Cluster flies follow a predictable New Hampshire calendar: entirely outdoors all summer, then a sharp fall entry window before settling into winter dormancy indoors.

12-month activity · New Hampshire

Peak · Sep, OctYou are here · Aug

Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

Dec

PeakActiveBuildingDecliningDormantCurrent season · Summer

Best window to act · Summer through late August — seal cracks around windows, eaves, soffits, siding, and utility lines, and screen vents before flies begin aggregating.

What else peaks thenSep Oct

Peaks reflect the fall entry window, anchored to Manchester's first frost (~Oct 19, 50% probability — NOAA 1991–2020 Climate Normals).

Where they go in winter

Winter behavior

The single most-asked question about these flies is where they go in winter — and the answer is: nowhere. They're already inside.

Overwintering strategy

Hides indoors

Adults overwinter dormant inside wall voids, attics, and false ceilings — mainly on sun-warmed south/west walls — gathering in large, sluggish clusters. They do not die off, breed, or feed indoors.

Move in
Late August through the first frost (~Oct 19), with the heaviest aggregation on sunny walls in September.
Hides indoors
Adults overwinter dormant inside wall voids, attics, and false ceilings — mainly on sun-warmed south/west walls — gathering in large, sluggish clusters. They do not die off, breed, or feed indoors.
Spring exit
On warm days (mid-50s°F or warmer) or with furnace heat, flies crawl to windows and light; survivors leave outdoors in spring to lay eggs in soil near earthworm burrows.

Season by season

A year in the life

Cluster fly behavior in New Hampshire shifts sharply by season, from entirely outdoor breeding to a hard fall entry and a quiet, dormant winter indoors.

01
Spring
Overwintered adults leave the walls and attic, move outdoors, and lay eggs near earthworm burrows in the soil. Homeowners often notice a late-winter or early-spring 'second wave' of sluggish flies at windows — these are the same individuals from fall, not a new hatch.
02
Summer
Cluster flies are entirely outdoors. Larvae develop inside earthworms over roughly 27–39 days, completing up to four generations across the warm months with no indoor presence at all.
03
Fall
The critical window. Last-generation adults begin entering homes in late August, aggregating heavily on sunny south- and west-facing walls through September and October, then slipping into wall voids and attics ahead of the first frost.
04
Winter
Flies sit dormant in walls and attics for months. Warm spells and indoor heating periodically rouse a few, producing the classic sluggish window-crawling that many NH homeowners see on mild January or February afternoons.

New Hampshire context

Cluster flies in southern New Hampshire

Cluster flies are one of the most common fall home-invaders in Manchester, Nashua, and Concord because NH's mix of lawns, fields, and older wood-frame construction gives them both the earthworm hosts they need in summer and the sun-warmed wall cavities they favor for winter shelter. Once a building is used as an overwintering site, the same flies — via aggregation pheromones — tend to return to it year after year, which is why some homes see heavier clusters annually than their neighbors.

Key local data

Manchester's first hard frost averages October 19 (50% probability) — the tail end of the cluster-fly entry window (NOAA 1991–2020 Climate Normals).

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Service Area Map

Southern New Hampshire

When to act

Timing your treatment

Because cluster flies are already sealed inside by the time most homeowners notice them, timing prevention for the following fall is what actually works.

The right window

Summer through late August — seal cracks around windows, eaves, soffits, siding, and utility lines, and screen vents before flies begin aggregating.

Large annual clusters at windows every fall

Hundreds of flies indoors on warm winter days

Flies returning to the same rooms year after year

Buzzing or crawling sounds in the attic

Visible masses on sun-side walls in September

General Pest Control

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Learn more about this pest

Common Questions

Frequently asked

Sources & references

UNH Extension — Cluster Flies Fact Sheet
entry timing, overwintering site, no indoor breeding, don't-fog guidance
UNH Extension — House Fly Fact Sheet
cluster fly vs. house fly distinction
UConn Home & Garden — Cluster Flies
mid-50s°F wake trigger, clustering behavior
University of Kentucky Entomology — EF624
no indoor reproduction, larval biology
Orkin — Cluster Flies
late-August entry timing corroboration
University of Illinois Extension — Overwintering Insects
secondary-pest carcass note
Garden.org — Manchester, NH Frost Dates
~Oct 19 Manchester first-frost anchor (NOAA 1991–2020)

Editorial disclaimer: This content is provided for informational purposes only and does not constitute pest control advice. Every property is unique — consult a licensed professional for guidance specific to your situation. Anchor Pest Services is licensed in New Hampshire (#782664).

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 Jul 2026

Anchor Pest Services

Whatever the season — we handle the timing for you.

Same-day service across Southern New Hampshire. NH-licensed #782664, family-owned since 2017. We track the local calendar so you don't have to guess the right window.

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Keep Exploring

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

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

The short of it

When are they active — the whole year at a glance

Winter behavior

A year in the life

Spring

Summer

Fall

Winter

Cluster flies in southern New Hampshire

Timing your treatment

Frequently asked

Why do flies suddenly appear inside on a warm winter day?