Why are shiny flies in my house in winter?

A heated NH home sustains blow fly larval development even when outdoor temperatures are below freezing, so a metallic fly swarm in winter is a near-certain sign of a dead animal inside the structure. After fall rodent season — when mice enter seeking warmth and sometimes die in wall voids — bottle fly outbreaks in December or January are common in Manchester and Nashua homes. Outdoor bottle flies are essentially inactive once temperatures drop; indoor flies in cold months almost always trace to an indoor carcass, not an outdoor intrusion.

Does this mean my house is dirty?

No. A bottle fly outbreak is a carcass problem, not a housekeeping verdict. A single dead mouse hidden in a wall void — something that can happen in any NH home after fall rodent season, regardless of how clean it is — produces enough carrion to sustain a full fly emergence. The flies arrived because something died in an inaccessible space, not because of anything you did or failed to do in the kitchen. Once the carcass is located and removed, the outbreak ends.

Where is the dead animal coming from?

Common locations in NH homes: wall voids between the basement and first floor (the most frequent source after rodent season), chimney interiors where birds or squirrels enter through uncapped flues, attic insulation, and crawlspaces. The fly cluster — especially where adults press against a window or where buzzing is loudest — points directly toward the carcass location. Systematically check those areas with a flashlight, listening for buzzing in specific wall sections, and inspect gaps around electrical outlets and plumbing penetrations for signs of odor or fly activity.

Are bottle flies dangerous?

Bottle flies do not bite or sting. As mechanical vectors, they can transfer bacteria they picked up from the carrion source onto food and prep surfaces, which is a genuine contamination concern. Lucilia sericata is also the primary cause of 'flystrike' in sheep (a serious livestock concern), and rare human wound myiasis is documented — but this is not a realistic risk for healthy people in ordinary home situations. The practical risk indoors is food contamination from adults moving between the carcass and your kitchen. Remove the source and the risk goes away.

Will spraying or fogging get rid of bottle flies?

Only temporarily. Spraying kills the adult flies in the room but has no effect on larvae developing on the hidden carcass. New adults will keep emerging from the source until development is complete or the carcass is removed. Extension sources are consistent on this point: the remedy is finding and removing the root cause, not spraying the symptoms. Use a fly spray only as short-term knockdown while you locate the source — never as a standalone solution.

How do I find the dead animal?

Start where the flies concentrate — especially against a window or around a specific section of wall. Listen for buzzing inside the wall by placing your ear against the drywall. Check attic insulation for signs of fur and decomposition. Inspect chimney interiors through the fireplace opening with a flashlight. If you smell a sweet-sour odor near a particular section of wall or floor, that is the most reliable locator. If you cannot find it yourself after checking accessible areas, a pest control technician with structural experience and a moisture meter can often locate a wall-void carcass faster.

How long until the flies are gone after I remove the carcass?

Any larvae already present will still complete development after carcass removal — you may see adult emergence for up to two weeks. Light traps catch remaining adults. If flies continue beyond two to three weeks after confirmed carcass removal and thorough cleaning, a second hidden source is likely: another dead animal in a different location, or rarely, a drain with decaying organic buildup that is sustaining a separate population. Re-inspect and, if needed, have a pro assess for hidden secondary sources.

Do bottle flies bite?

No. Bottle flies do not have biting or piercing mouthparts — they sponge fluids. If a fly that looks similar is biting you around the ankles or legs, it is almost certainly a stable fly (Stomoxys calcitrans), which has an identical gray look to a house fly but carries a forward-projecting bayonet proboscis. Stable flies are common near livestock, barns, and decaying vegetation. Metallic green or blue flies do not bite.

How is a bottle fly different from a flesh fly?

Bottle flies are shiny metallic green or blue; flesh flies (Sarcophagidae) are dull gray, non-metallic, with three dark thoracic stripes and a distinctive checkerboard-patterned gray-and-black abdomen. Both breed in carrion, so both indoors mean a hidden dead animal — the detective logic is the same. The visual difference is clear in good light: metallic sheen versus dull gray checkerboard. Flesh flies also have a unique behavior: females deposit live larvae directly on the source rather than eggs.

Bottle fly · Lucilia sericata

Flies & GnatsNH resident

Bottle Flies in New Hampshire — ID, Signs & Control

Also called Greenbottle · Bluebottle · Blow fly

Bottle flies — the green bottle (Lucilia sericata) and blue bottle (Calliphora vomitoria) — are blow flies named for their metallic 'bottle-glass' sheen. They breed in carrion, and a sudden indoor swarm, especially in cold months with windows shut, almost always means a hidden dead animal. They are not a 'dirty house' verdict — one dead mouse is enough.

At a glance

What it is: A blow fly; green/blue 'bottle' species, family Calliphoridae
Species: Lucilia sericata (green) / Calliphora vomitoria (blue); family Calliphoridae
Size: ~10–14 mm (3/8–1/2 in); larger than a house fly; Animal Diversity Web
Breeds in: Carrion; indoors = a hidden dead animal
Eggs: ~200 per cluster; on fresh carrion; Animal Diversity Web
Generations: 3–4 per year; temperate climate; Animal Diversity Web

The short of it

Are bottle flies the same as blow flies?

Yes — 'bottle fly' is the common name for the metallic blow fly species named for their brilliant bottle-glass sheen: the green bottle (Lucilia sericata) and the blue bottle (Calliphora vomitoria), both in the family Calliphoridae. Knowing they are the same family matters because the entire biology and control logic is shared: bottle flies breed in carrion, not in kitchens, and an indoor burst of these shiny flies points to one thing.

A winter swarm of shining green or blue flies indoors, with outdoor fly activity near zero and windows closed, is a near-certain sign of something dead inside a heated NH structure — a mouse in a wall void after rodent season, a bird in a chimney, an animal in the attic. This is not a 'dirty house' verdict. A single hidden mouse produces enough carrion to sustain a full fly emergence. The fix is locating and removing the carcass, not spraying.

One search confusion this page resolves: many readers think 'bottle fly' is a separate pest from 'blow fly.' It is not. The carcass-detective logic applies equally to both.

Identify it

How to identify a bottle fly

The metallic green or blue sheen is unmistakable. No common NH house fly looks like this. If the fly is shiny green or bright metallic blue, you are looking at a bottle fly — and the carcass-detective logic immediately applies.

Species profile

Bottle fly

Scientific name: Lucilia sericata (green bottle) / Calliphora vomitoria (blue bottle)
Family: Calliphoridae
Size: ~10–14 mm (3/8–1/2 in)
Coloration: Brilliant metallic green (Lucilia) or metallic blue (Calliphora); iridescent in light
Habitat: Carrion, garbage, feces; outdoors near dead animals; indoors when a carcass is present.
Active season: April–October outdoors in temperate areas; any season indoors near a carcass
Diet: Larvae develop in carrion; adults feed on nectar, pollen, and carrion fluids.
Reproduction: Female lays ~200 eggs per cluster on fresh carrion; 3–4 generations per year in temperate climate (Animal Diversity Web).
Lifespan: Egg-to-adult cycle approximately 2–3 weeks in warm conditions.

Lucilia sericata recorded throughout North America including New Hampshire; cosmopolitan distribution.

The tells — key ID features

Brilliant metallic green or blue sheen — the signature tell; iridescent and visible even in dim light.
Slightly larger than a house fly — noticeably stouter.
Clear wings with light-brown veins (green bottle).
Loud buzz; bounces against window glass indoors when seeking light.
Reddish eyes.

Size, to scale

The insect

~10–14 mm (3/8–1/2 in)

Compared to

About 1/2 inch — a shiny green or blue 'bottle' of a fly, clearly larger than a house fly

Looks bigger than it is

The metallic sheen catches the eye before the size does — it is the diagnostic feature.

Is it actually this? — the confusion species

Other blow flies

How to tell — Bottle flies are a subset of blow flies (Calliphoridae). Bronze or dull-green blow flies (Phormia, Calliphora with duller coloration) share the family but look slightly different; all share the same carcass-breeding logic.

Flesh fly

How to tell — Flesh flies (Sarcophagidae) are dull gray with three thoracic stripes and a checkerboard abdomen — entirely non-metallic. Same carcass-detective logic applies, completely different look.

Common house fly

How to tell — House flies are smaller, dull gray with four thoracic stripes and no metallic sheen. They breed in garbage and manure, not carrion. No overlap in appearance.

What you'll notice

Signs of a problem

Bottle flies signal their presence — and the presence of a carcass — by where they appear and when. These are the signs to look for.

Shiny green or blue flies at a window with windows closed

What it means — A hidden carcass is almost certainly inside the structure; adults are seeking light to exit.

Flies or maggots emerging from a wall void, chimney, or vent

What it means — The carcass is in or near that specific location — this is where to begin the search.

Metallic flies appearing indoors in cold months when outdoor flies are absent

What it means — The breeding source is inside the heated structure; outdoor intrusion at those temperatures is essentially impossible.

A sweet-sour decomposition odor accompanying the fly appearance

What it means — Active decomposition; the odor and flies together confirm an actively feeding larval mass on a carcass.

How much it matters

The real risks, calibrated

Bottle flies do not bite or sting. The risks are contamination and the underlying carcass that produced them.

Overall risk

Moderate

Mechanical food and surface contamination

Moderate

Onset: Ongoing while flies are active

Adults move from the carrion source to food-contact surfaces, mechanically transferring bacteria including Salmonella and E. coli.

Flystrike / myiasis (livestock and rare human cases)

Low risk

Onset: Uncommon in healthy humans

Lucilia sericata is the primary agent of flystrike in sheep; rare human wound infestation is documented but uncommon in ordinary household situations.

Carcass odor and biohazard

Moderate

Onset: Days after death

Active decomposition produces persistent odor and biohazardous material until the carcass is removed and the site sanitized.

What you can do

DIY — what works, what backfires

Bottle fly control is carcass-detective work. The shiny flies are a symptom; the dead animal is the problem that needs solving.

What actually works

Locate and remove the dead animal: check wall voids by listening for buzzing and inspecting gaps near electrical outlets and plumbing; check the chimney interior, attic, crawlspace, and behind large appliances.
Disinfect the site thoroughly with an enzymatic cleaner after removing the carcass to break down residual organic material and reduce odor.
Use light traps or sticky fly traps to catch remaining adults after the carcass is removed.
Seal chimney openings with a quality cap and inspect roof-line gaps to prevent animals from entering and dying inside.
Conduct rodent exclusion before fall to eliminate the most common bottle fly source in NH homes — a mouse dying in a wall void.

What backfires

Don't spray the room while the carcass remains — it kills the visible adults but has no effect on developing larvae; new adults will keep emerging from the hidden source.
Don't assume the flies mean your house is dirty — a single hidden mouse is enough to produce a full outbreak; this is a carcass problem, not a housekeeping problem.
Don't ignore the outbreak hoping it resolves on its own — without carcass removal, adults will keep emerging until development is complete.
Don't skip rodent exclusion after resolving the immediate carcass — repeat outbreaks next season are common without it.

Local Context

Bottle flies in New Hampshire homes

The green bottle fly (Lucilia sericata) is recorded across the United States, including New Hampshire, with documented presence across the Northeast. Outdoors, adults are active from roughly April through October in temperate climates. Indoors in NH, bottle flies appear any time of year when a dead animal is present in a heated structure. Manchester, Nashua, and Concord homes that dealt with fall rodents — mice that entered seeking warmth and died in wall voids — can produce bottle fly outbreaks well into winter. The heated interior sustains larval development even when outside temperatures are well below freezing.

Peak season

April–October outdoors; any season indoors near a carcass

You might actually have

Other blow flies (same family Calliphoridae — the family-level page)
Flesh flies (gray, checkerboard abdomen, also carcass-breeding)
House flies (dull gray, garbage/manure breeders, not metallic)

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ManchesterNashuaConcordDerryBedfordSalem

Hillsborough · Rockingham · Merrimack Counties

NH-licensed applicator · #782664
Same-day service
Family-owned since 2017

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Southern New Hampshire

When it's past DIY

When to call a pro for bottle flies

If a shiny fly swarm keeps going and the dead animal cannot be located — especially in a wall void, ceiling cavity, or attic space that is not safely accessible — a licensed technician can find the source, handle removal, and sanitize the affected area.

Signals it's past DIY

When to call a pro

A metallic fly swarm persists and you cannot locate the carcass after checking accessible areas.
The suspected carcass is inside a wall void or structural space that requires opening walls or accessing confined spaces.
Flies keep returning after previous removal — suggesting multiple animals or a recurring attractant.
You need professional sanitizing and odor remediation for a space you cannot easily clean.

Keep them out — prevention

NH-specific

Rodent exclusion before fall

Sealing foundation gaps, utility penetrations, and roof-line entry points before October is the most effective way to prevent bottle fly outbreaks in NH winter — no dead mice in the walls means no carrion, no larvae, no flies.

Cap chimneys and seal roof vents

Birds and squirrels that enter through uncapped chimneys or damaged soffits are a leading second source. Install a chimney cap and inspect for roof-line gaps annually before fall.

Check after any rodent activity

After a season with mouse trapping or signs of rodent activity, do a walkthrough 2–3 weeks later — a dead mouse in a forgotten snap trap or in a void is enough to trigger an outbreak.

Not sure it's a bottle 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.

Get a Free Inspection (603) 785-0118

Common Questions

Frequently asked questions

Sources(5)

[1]
UF/IFAS — Black Blow Fly and Blow Flies (IN903)
biology, family identification
extensionverified 2026-06-30
[2]
Animal Diversity Web — Lucilia sericata
generations per year, eggs per cluster, active season
academicverified 2026-06-30
[3]
UW Insect Diagnostic Lab — Blow Flies Indoors
carcass-detective logic, source-not-spray guidance
extensionverified 2026-06-30
[4]
UNH Cooperative Extension — Managing Small Flies Indoors
NH source-first fly control guidance
extensionverified 2026-06-30
[5]
Iowa State Extension — Blow and Flesh Flies
carcass indicator, indoor fly appearance
extensionverified 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.

Verify license

Last updated 2026-06-30

New Hampshire pest control

Dealing with bottle flys in your home?

Anchor Pest Services identifies and handles bottle 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

Bottle Flies in New Hampshire — ID, Signs & Control

Are bottle flies the same as blow flies?

How to identify a bottle fly

The tells — key ID features

Size, to scale

Other blow flies

Flesh fly

Common house fly

Signs of a problem

Shiny green or blue flies at a window with windows closed

Flies or maggots emerging from a wall void, chimney, or vent

Metallic flies appearing indoors in cold months when outdoor flies are absent

A sweet-sour decomposition odor accompanying the fly appearance

The real risks, calibrated

Mechanical food and surface contamination

Flystrike / myiasis (livestock and rare human cases)

Carcass odor and biohazard

DIY — what works, what backfires

What actually works

What backfires

Bottle flies in New Hampshire homes

When to call a pro for bottle flies

Rodent exclusion before fall

Cap chimneys and seal roof vents

Check after any rodent activity

Not sure it's a bottle fly?

Frequently asked questions

Dealing with bottle flys in your home?

Your next step

How to Get Rid of Bottle Flies

How to Get Rid of Blow Flies

Chemical Fly Control: A Supplement, Not the Fix

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

Blow Flies in New Hampshire — ID, Signs & How to Get Rid of Them