Are blow flies dangerous?

Blow flies do not bite or sting. The health concern is indirect: because they develop in carrion and then walk on food and food-contact surfaces, they can mechanically transfer bacteria like Salmonella and E. coli — the same contamination concern as house flies, but tied specifically to the decomposition source. Remove the carcass and disinfect the site and the contamination risk disappears with it. Rare myiasis (larvae in tissue) is documented in medical literature but is not a realistic concern for healthy adults in ordinary household situations.

Where are blow flies coming from inside my house?

Almost certainly a dead animal somewhere in the structure. Blow flies locate carrion by scent and can arrive within seconds of death — so an indoor swarm with windows closed is a near-certain sign of something dead inside. Common locations in NH homes: a mouse in a wall void after rodent season, a bird or squirrel that entered an uncapped chimney or damaged soffit vent, or an animal that died in attic insulation. The flies' location — especially where you see them clustering or buzzing — is your best clue to the carcass site.

Why do shiny flies suddenly appear in my house in winter?

This is the most important question on this page. A heated NH home in January or February is warm enough to sustain blow fly larval development if a dead animal is present. After fall rodent activity, a mouse that died in a wall void in October can produce a fly emergence in November or December. Because outdoor blow fly activity is essentially zero in NH winter, an indoor metallic swarm with windows shut is a near-certain signal of a carcass indoors — not a random intrusion from outside.

How do I find the dead animal causing blow flies?

Listen for buzzing in specific wall sections. Use a flashlight to inspect gaps around electrical outlets, plumbing penetrations, and baseboards. Check the attic, chimney interior, crawlspace, and spaces behind large appliances. The fly cluster — particularly where they concentrate against a window — is directly between you and the carcass. In heated NH homes, the most common location after fall rodent season is a wall void between the basement and first floor. If you cannot find it yourself, a licensed technician with a moisture meter and structural experience can locate it faster.

Will spraying get rid of blow flies?

No — not while the carcass remains. Spraying or fogging kills the visible adults in the room but does nothing to the larvae still developing on the hidden source. New adults will keep emerging until the carcass is fully consumed or removed. The University of Wisconsin Insect Diagnostic Lab is explicit: 'Rather than spraying an insecticide, the best remedy is to find the root of the issue and clean it up.' Treat spraying as temporary adult knockdown only, never as a substitute for carcass removal.

No. Blow flies do not have biting or piercing mouthparts. They feed by sponging fluids. If you are being bitten by a fly that looks like a blow fly, it is likely a stable fly (Stomoxys calcitrans) — which has an identical gray body but carries a forward-projecting bayonet-like proboscis and does bite, usually around the ankles. Metallic blue/green flies that do not bite are blow flies; biting flies of any color are a different species.

How is a blow fly different from a bottle fly or a house fly?

Bottle flies are blow flies — specifically the named green-bottle (Lucilia sericata) and blue-bottle (Calliphora vomitoria) species within the Calliphoridae family. This page covers the whole family; the bottle-fly page covers those specific species and the 'are they the same?' confusion. House flies (Musca domestica) are dull gray with four thoracic stripes and no metallic sheen — they breed in garbage and manure, not primarily in carrion, and are generally smaller than blow flies.

Do blow flies carry disease?

Yes, as mechanical vectors — meaning they carry pathogens on their bodies from contaminated surfaces to food and food-contact surfaces, without the pathogen replicating inside them the way a mosquito-borne disease does. Because blow flies develop in carrion, bacteria associated with decomposition (including Salmonella and E. coli) can be transferred when they land on food or prep surfaces. The CDC and extension sources confirm this mechanical transmission risk for filth flies as a group. The practical fix is carcass removal and surface disinfection.

How long until the blow fly problem stops after I remove the carcass?

Any larvae already in the structure when you remove the carcass will still complete development. The University of Wisconsin Insect Diagnostic Lab notes that the fly cycle can run approximately two to three weeks, so you may see adult emergence for up to two weeks after removal if larvae were well-established. Light traps help catch remaining adults. If flies continue beyond two to three weeks after confirmed carcass removal, a second hidden source — another dead animal or a rarely-used drain — is likely present.

How is a blow fly different from a flesh fly?

The metallic sheen is the dividing line. Blow flies have a brilliant blue, green, or bronze body; flesh flies (Sarcophagidae) are dull gray, non-metallic, with three thoracic stripes (not four like the house fly) and a distinctive checkerboard-pattern abdomen. Both breeds in carrion and both indoors mean a hidden dead animal — the detective logic is the same — but the visual difference is stark. If the fly is shiny, it is a blow fly. If it is dull gray with a gray-and-black checkerboard belly, it is a flesh fly.

Blow fly · Family Calliphoridae

Flies & GnatsNH resident

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

Also called Carrion fly · Greenbottle · Bluebottle

Blow flies are metallic blue/green/bronze flies slightly larger than a house fly that breed exclusively in carrion. A sudden indoor burst of these shiny flies — especially with windows shut — almost always means a dead animal is hidden inside the structure. The fix is finding and removing the carcass, not spraying.

At a glance

Family: Calliphoridae; blow / carrion flies; Calliphora, Lucilia, Phormia
Size: ~1/4–3/8 in; slightly larger than a house fly; UW Insect Diagnostic Lab
Tell-tale look: Metallic blue/green/bronze sheen; the family hallmark
Breeds in: Carrion; indoors = a hidden dead animal; UW Insect Diagnostic Lab
Indoor meaning: Often a dead mouse, bird, or squirrel; UW Insect Diagnostic Lab
Arrival speed: Within seconds of death; UW Insect Diagnostic Lab

The short of it

Shiny metallic flies indoors? That's a detective clue.

Blow flies (family Calliphoridae) — the metallic blue, green, and bronze flies you may know as greenbottles or bluebottles — are nature's first responders to a dead animal. They locate carrion by scent and can arrive within seconds of death, sometimes from a mile away.

Outdoors, that makes them ecologically essential. Indoors, a sudden cluster of shiny metallic flies, especially when windows are closed, is not a sign of a dirty house — it is a detective clue. Something has died inside the heated structure: a mouse in a wall void after rodent season, a bird down a chimney, a squirrel in the attic.

The University of Wisconsin Insect Diagnostic Lab is direct: blow flies indoors 'often indicates a dead mouse,' and the best remedy is to 'find the root of the issue and clean it up,' not spray. Fogging the room kills the visible adults but does nothing while maggots keep developing on the hidden source. Every blow fly page comes down to one action: find and remove the carcass.

Identify it

How to identify a blow fly

The metallic sheen is the blow fly family's unmistakable signature. No other common NH house fly has it. If the fly in question is shiny blue, green, or bronze, you are looking at a Calliphoridae — and the carcass-detective logic applies.

Species profile

Blow fly

Scientific name: Family Calliphoridae (Calliphora, Lucilia, Phormia spp.)
Family: Calliphoridae
Size: ~1/4–3/8 in (6–10 mm)
Coloration: Metallic blue, green, or bronze; never dull or gray
Habitat: Carrion, garbage, feces; outdoors near dead animals; indoors when a carcass is present in the structure.
Active season: Warm season outdoors (spring through fall); any season indoors when a dead animal is present
Diet: Larvae feed on carrion and decaying tissue; adults feed on nectar, pollen, and carrion fluids.
Reproduction: Female lays batches of eggs directly on fresh carrion; multiple generations per year.
Lifespan: Egg-to-adult cycle approximately 2–3 weeks in warm conditions.

Common throughout New Hampshire and the northeastern U.S.; active warm season outdoors, year-round indoors near a carcass.

The tells — key ID features

Brilliant metallic blue, green, or bronze body — the family hallmark and the single best ID tell.
Slightly larger and stouter than a house fly.
Loud, droning buzz; bounces against window glass indoors when seeking light.
Reddish eyes.
Strong, fast flier outdoors; clusters near windows indoors.

Size, to scale

The insect

~1/4–3/8 in (6–10 mm)

Compared to

A bit larger than a house fly — roughly the size of a small watermelon seed

Looks bigger than it is

The metallic sheen is more diagnostic than the size; the sheen is visible even in poor light.

Is it actually this? — the confusion species

Bottle fly

How to tell — Bottle flies are blow flies — specifically the named green-bottle (Lucilia sericata) and blue-bottle (Calliphora vomitoria) species. Same family, same carcass logic. This page covers the whole Calliphoridae family; bottle-fly covers the named species.

Flesh fly

How to tell — Flesh flies are dull gray with a checkerboard abdomen and three thoracic stripes — not metallic at all. Same carcass logic applies, but the look is completely different.

Common house fly

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

What you'll notice

Signs of a problem

Blow flies telegraph their breeding source by where they appear and how they behave. These are the signs — and what each one means.

A sudden cluster of metallic flies indoors with windows closed

What it means — A dead animal is almost certainly hidden inside the structure — a wall void, chimney, attic, or crawlspace.

Flies or maggots near a wall void, chimney, or attic access

What it means — That location is very likely the carcass site. The flies are working their way toward light after emerging.

Loud buzzing at a window on the inside of the glass

What it means — Emerged adults are seeking light to exit — the carcass is somewhere behind them in the structure.

A sweet-sour odor accompanying the fly appearance

What it means — Decomposition odor accompanies active blow fly breeding; the smell and the flies together confirm an active carcass.

Indoor swarm that peaks and then tapers over two to three weeks

What it means — Adult emergence from a single carcass typically runs its course in two to three weeks (UW Insect Diagnostic Lab); tapering means larvae have completed development, not that the problem is gone.

How much it matters

The real risks, calibrated

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

Overall risk

Moderate

Food and surface contamination

Moderate

Onset: Ongoing while flies are active

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

Rare myiasis

Low risk

Onset: Uncommon

Blow fly larvae can infest open wounds in rare circumstances; this is uncommon in healthy humans but documented in medical literature.

Carcass odor and biohazard

Moderate

Onset: Days after death

Decomposition creates a persistent odor and biohazardous material until the carcass is removed and the site disinfected.

What you can do

DIY — what works, what backfires

Blow fly control is carcass-detective work. The flies are a symptom; the dead animal is the problem. Here is what works — and what wastes time.

What actually works

Find and remove the dead animal: systematically check wall voids (listen for buzzing, use a flashlight at gaps), chimney interior, attic insulation, crawlspace, and spaces behind appliances — especially after fall rodent activity.
Disinfect and deodorize the site after removal: use an enzymatic cleaner on the area to break down remaining organic material and reduce lingering odor.
Use light traps or sticky fly traps for remaining adults after the carcass is removed.
Cap chimneys and seal gaps in soffit and fascia to prevent birds and squirrels from entering the structure and dying inside.
Practice rodent exclusion before fall to prevent mice from entering and dying in wall voids.

What backfires

Don't spray the room while the carcass remains — fogging kills visible adults but does nothing to the larvae developing on the hidden source; the adults will keep emerging.
Don't pour bleach around hoping to mask the odor — it does not reach or neutralize the actual breeding source.
Don't assume the flies will 'just leave' — they will keep emerging until the carcass is fully consumed or removed.
Don't skip the rodent exclusion step after resolving the carcass — a repeat outbreak next season is common without it.

Local Context

Blow flies in New Hampshire homes

Blow flies are present statewide across New Hampshire through the warm season, but they become an indoor problem any time of year when a dead animal is trapped inside a heated structure. After fall rodent season in Manchester, Nashua, and Concord — when mice seek warmth and sometimes die in wall voids or attic insulation — blow fly outbreaks are among the most common winter fly calls. A sealed, heated NH home is warm enough to sustain larval development even in January, so a metallic swarm in February is not unusual if a mouse died in October.

Peak season

Warm season outdoors; any season indoors near a carcass

You might actually have

Bottle flies (same family — green/blue-bottle species)
Flesh flies (gray checkerboard-abdomen carcass flies)
House flies (dull gray, garbage/manure breeders)

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ManchesterNashuaConcordDerryBedfordSalem

Hillsborough · Rockingham · Merrimack Counties

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Family-owned since 2017

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Southern New Hampshire

When it's past DIY

When to call a pro for blow flies

If a metallic fly swarm keeps going and you cannot locate the carcass — especially when it is in a wall void, between floors, or in an attic space you cannot safely access — a licensed technician can use moisture detection and structural knowledge to find and remove it, then sanitize the site.

Signals it's past DIY

When to call a pro

A metallic fly swarm persists and you cannot locate the dead animal after checking obvious areas.
The suspected carcass is inside a wall void, ceiling cavity, or structural space you cannot safely open.
Flies keep returning after each previous removal — suggesting multiple animals or an ongoing attractant.
You need post-removal sanitizing and odor treatment for a difficult-to-reach space.

Keep them out — prevention

NH-specific

Rodent exclusion before fall

Sealing entry points around the foundation, utilities, and roof line before October is the single most effective way to prevent winter blow fly outbreaks in NH homes — no dead mice in the walls means no blow flies.

Cap chimneys and seal roof vents

Birds and squirrels entering through uncapped chimneys or damaged soffit vents are a leading cause of blow fly outbreaks. Install a quality chimney cap and inspect roof-line entry points annually.

Inspect after any rodent activity

If you have seen mice or set traps during rodent season, do a thorough inspection 2–3 weeks later — a dead mouse in a snap trap you forgot, or one that died in a void, is enough to start a blow fly infestation.

Not sure it's a blow 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]
UW Insect Diagnostic Lab — Blow Flies Indoors
carcass-detective logic, dead-mouse indicator, source-not-spray guidance
extensionverified 2026-06-30
[2]
Iowa State Extension — Blow and Flesh Flies
carcass indicator, biology
extensionverified 2026-06-30
[3]
Maine School IPM Factsheet — Flies
biology, flight range
govverified 2026-06-30
[4]
UNH Cooperative Extension — Managing Small Flies Indoors
NH source-first fly control guidance
extensionverified 2026-06-30
[5]
CDC — Flies and Disease
mechanical contamination / pathogen transfer risk
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.

Verify license

Last updated 2026-06-30

New Hampshire pest control

Dealing with blow flys in your home?

Anchor Pest Services identifies and handles blow 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

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

Shiny metallic flies indoors? That's a detective clue.

How to identify a blow fly

The tells — key ID features

Size, to scale

Bottle fly

Flesh fly

Common house fly

Signs of a problem

A sudden cluster of metallic flies indoors with windows closed

Flies or maggots near a wall void, chimney, or attic access

Loud buzzing at a window on the inside of the glass

A sweet-sour odor accompanying the fly appearance

Indoor swarm that peaks and then tapers over two to three weeks

The real risks, calibrated

Food and surface contamination

Rare myiasis

Carcass odor and biohazard

DIY — what works, what backfires

What actually works

What backfires

Blow flies in New Hampshire homes

When to call a pro for blow flies

Rodent exclusion before fall

Cap chimneys and seal roof vents

Inspect after any rodent activity

Not sure it's a blow fly?

Frequently asked questions

Dealing with blow flys in your home?

Your next step

How to Get Rid of Blow Flies

What Attracts Blow Flies to Your House?

How to Get Rid of Flesh Flies

How to Get Rid of Bottle Flies

Bottle Flies in New Hampshire — ID, Signs & Control

Flesh Flies in New Hampshire — ID, Signs & Control

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

Shiny metallic flies indoors? That's a detective clue.

How to identify a blow fly

The tells — key ID features

Size, to scale

Bottle fly

Flesh fly

Common house fly

Signs of a problem

A sudden cluster of metallic flies indoors with windows closed

Flies or maggots near a wall void, chimney, or attic access

Loud buzzing at a window on the inside of the glass

A sweet-sour odor accompanying the fly appearance

Indoor swarm that peaks and then tapers over two to three weeks

The real risks, calibrated

Food and surface contamination

Rare myiasis

Carcass odor and biohazard

DIY — what works, what backfires

What actually works

What backfires

Blow flies in New Hampshire homes

When to call a pro for blow flies

Rodent exclusion before fall

Cap chimneys and seal roof vents

Inspect after any rodent activity

Not sure it's a blow fly?

Frequently asked questions

Are blow flies dangerous?

Where are blow flies coming from inside my house?

Why do shiny flies suddenly appear in my house in winter?

How do I find the dead animal causing blow flies?

Will spraying get rid of blow flies?

Do blow flies bite?

How is a blow fly different from a bottle fly or a house fly?

Do blow flies carry disease?

How long until the blow fly problem stops after I remove the carcass?

How is a blow fly different from a flesh fly?

Dealing with blow flys in your home?

Your next step

How to Get Rid of Blow Flies

What Attracts Blow Flies to Your House?

How to Get Rid of Flesh Flies

How to Get Rid of Bottle Flies

Bottle Flies in New Hampshire — ID, Signs & Control

Flesh Flies in New Hampshire — ID, Signs & Control