Are drain flies dangerous?

No. Drain flies (Psychodidae) do not bite and are not known to transmit pathogens to humans — UF/IFAS is explicit on this point. They are a nuisance pest. In very heavy infestations, shed scales can potentially irritate the respiratory tract of sensitive individuals. The indirect concern is what a persistent drain fly infestation can signal: an underlying plumbing defect or heavily biofilmed drain that should be addressed. But drain flies themselves present no direct health hazard, and there is no need for alarm.

Where are drain flies coming from?

From the biofilm inside a specific drain — almost always. That gelatinous organic film, coating the interior walls of the drain pipe, is where larvae develop. The drain they emerge from may not be the one immediately below where you see them resting; airflow in a building can carry adults several rooms away. The cup/tape test (placing a petroleum-jelly-coated cup over each suspect drain overnight) is the extension-verified way to identify exactly which drain is the source — and University of Arkansas Extension confirms it works reliably.

How do I confirm which drain is the source?

Use the cup test: coat the inside of a clear cup or jar with petroleum jelly (or stretch clear tape across the drain opening), and place it over the suspect drain overnight. Adult drain flies emerging from that drain will be trapped against the inside of the cup or on the tape. University of Arkansas Extension documents this as a reliable method to 'help a homeowner identify a source of infestation and breeding sites.' Test every drain in the affected area — bathroom, kitchen, basement floor drain, and laundry room — not just the most obvious one. The source is often the one you least expect.

Why didn't bleach get rid of my drain flies?

Because bleach does not remove the biofilm. Drain fly larvae live embedded in the gelatinous organic film coating the inside of the drain pipe. Bleach kills bacteria on contact and disinfects, but it does not physically strip that film — the larvae remain protected in it. University of Arkansas Extension is clear that mechanical removal (scrubbing with a stiff brush) is the essential step, with enzyme or bacterial drain gels following to digest remaining organics. Bleach is not a substitute for scrubbing. This is also why repeated bleach pours seem to have no effect — you are killing surface bacteria but leaving the breeding medium intact.

Are drain flies the same as fruit flies?

No — they are easy to tell apart once you know what to look for. Drain flies are fuzzy, moth-like, and sit motionless on walls near drains. Fruit flies are smooth, tan or brown with red eyes in many specimens, and hover actively around fermenting fruit, spills, and recycling. Both are small (~1/8 inch), both appear near kitchen fixtures, and both breed in organic material in drains — but the visual and behavioral differences are clear. If the small fly is fuzzy and sitting on the wall, it is a drain fly. If it is smooth and hovering at the fruit bowl, it is a fruit fly.

Can drain flies come from a sewer break?

Yes. A cracked or broken sewer line under a slab allows decomposing organic material to accumulate in a subsurface void, creating an ideal drain fly breeding site that is unreachable for standard drain cleaning. This is why drain flies that persist beyond two to three weeks of thorough cleaning should raise a flag. The phorid fly is the more classic sewer-break indicator (University of Maryland Extension specifically links ground-floor phorid swarms to broken sub-slab sewer lines), but drain flies can share this source. If cleaning is not resolving the problem, a plumbing inspection is appropriate.

What is the slime in my drain?

Biofilm — a gelatinous layer composed of bacteria, decomposed organic matter (food particles, soap scum, hair, grease), and moisture that accumulates on the interior walls of drain pipes. It is sticky, often dark-colored, and visible when you shine a flashlight into a drain opening. This is the breeding medium drain fly larvae develop in. The biofilm is normal in any drain with regular use — it only becomes a drain fly problem when it thickens enough to provide adequate food and shelter for larval development, which happens faster in less-used or poorly-maintained drains.

How long does it take to get rid of drain flies?

After thorough mechanical cleaning of the source drain, active larvae will still complete their cycle — you may see adult emergence for up to one to two weeks. With consistent cleaning (mechanical scrubbing plus enzyme gel weekly), most drain fly infestations resolve within two to three weeks. If the problem persists beyond that after you have cleaned every drain, the source has not been fully removed — either another drain is producing them (re-test with the cup), or a subsurface plumbing defect is sustaining a breeding site you cannot reach with standard cleaning.

Do drain flies bite or carry disease?

No. Drain flies (Psychodidae) do not bite. They do not have biting or piercing mouthparts and are not known to transmit pathogens to humans, according to UF/IFAS. This distinguishes them from stable flies and mosquitoes, which do bite. In very large infestations, fine wing and body scales may irritate the respiratory tract of sensitive individuals — but that is the extent of the direct health concern. Drain flies are a plumbing-maintenance problem, not a disease-risk problem.

Can I use Bti to treat drain flies?

Yes — Bti (Bacillus thuringiensis israelensis) is a naturally occurring bacterial insecticide that targets fly larvae and is safe for use around humans and pets. UF/IFAS cites research showing Bti reduced drain fly emergence by 79% in one study. It is available in various drain-treatment formulations. Bti treats the larvae in the drain without the corrosive effects of chemical drain cleaners. However, it works best as part of a complete approach: mechanical scrubbing first to remove the bulk of the biofilm, then Bti or enzyme gel to address residual organic material and larvae.

Drain fly · Family Psychodidae

Flies & GnatsNH resident

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

Also called Moth fly · Filter fly · Sewer fly · Bathroom fly

Drain flies (moth flies) are tiny, fuzzy, moth-like flies that rest on walls near sinks and tubs. They breed in the gelatinous organic film — biofilm — coating the inside of drains, not in standing water alone. The lasting fix is mechanically removing that film. The cup-or-tape-over-the-drain test, verified by University of Arkansas Extension, pinpoints exactly which drain is producing them.

At a glance

Family: Psychodidae; moth flies / filter flies; Clogmia albipunctata, Psychoda spp.
Size: ~1/8 in (2–5 mm); Texas A&M AgriLife; University of Arkansas Extension
Best tell: Fuzzy, moth-like; rests on walls near drains; the only small fly that looks like a tiny moth
Breeds in: Biofilm inside drains; not standing water alone; Texas A&M AgriLife
Bite / disease: Non-biting; no known pathogen transfer; UF/IFAS
Bti effect: 79% fewer emerging adults; UF/IFAS, citing Houston et al. 1989

The short of it

Fuzzy flies on your bathroom wall? The source is inside the drain.

Drain flies — also called moth flies or filter flies (family Psychodidae) — are one of the four small flies that people constantly confuse with each other. The easy tell: a drain fly is the only one that looks like a tiny moth. It is fuzzy, with broad leaf-shaped wings it holds tent-like over its body, and it rests motionless on walls and ceilings near drains rather than hovering or scuttling.

They breed in one precise medium: the gelatinous organic film (biofilm) that coats the inside surfaces of drains, not in standing water alone. This distinction changes everything about how you treat them. Bleach, boiling water, and chemical drain cleaners dissolve or disinfect, but they do not physically strip the biofilm layer — the larvae live embedded in it, and bleach does not reach them reliably.

The lasting fix is mechanical biofilm removal: a stiff drain brush scrubbing the pipe walls, followed by a bacterial or enzyme drain gel that digests the remaining organics. And before you clean anything, you need to know exactly which drain is producing the flies — because it may not be the one nearest the flies. The University of Arkansas Extension documents an extension-verified test to do exactly that: tape a clear cup or plastic bag over each suspect drain overnight.

Identify it

How to identify a drain fly

Drain fly is the only small fly in New Hampshire homes that looks like a tiny moth. The fuzzy body, broad moth-like wings held roof-like at rest, and the habit of sitting motionless on walls near drains are definitive. No hovering. No scuttling. No long legs. Just a tiny fuzzy moth on the bathroom wall.

Species profile

Drain fly

Scientific name: Family Psychodidae (Clogmia albipunctata, Psychoda spp.)
Family: Psychodidae
Size: ~1/8 in (2–5 mm)
Coloration: Dark gray to black, densely hairy (fuzzy); wings are moth-like and leaf-shaped
Habitat: The gelatinous biofilm inside drains, sewage, and filter beds; any moist organic film in plumbing.
Active season: Year-round indoors
Diet: Larvae feed on the organic material in biofilm; adults feed minimally.
Reproduction: Egg masses contain 30–100 eggs (Texas A&M AgriLife); cycle approximately 1–3 weeks.
Lifespan: Adults live approximately 1–2 weeks.

Found throughout New Hampshire and worldwide wherever drains with organic buildup exist; indoor populations are independent of outdoor season.

The tells — key ID features

Fuzzy, moth-like body with broad leaf-shaped wings held tent-like (roof-like) at rest — the single best tell; no other common small fly looks like this.
Sits motionless on walls and ceilings near drains, not hovering or scuttling.
Weak flier — travels only a few feet at a time.
Most active in the evening.
~1/8 inch, dark gray.

Size, to scale

The insect

~1/8 in (2–5 mm)

Compared to

About the size of a sesame seed with fuzzy moth-like wings — like a tiny ash fleck that sits on the wall

Looks bigger than it is

The fuzzy texture and wing shape are visible to the naked eye and separate drain flies from all other small flies.

Is it actually this? — the confusion species

Fruit fly

How to tell — Fruit flies are smooth (not fuzzy), tan/brown with red eyes in many specimens, and hover around fermenting fruit, spills, and recycling. They do not sit motionless on walls. Drain fly = fuzzy moth on the wall; fruit fly = red-eyed hoverer at the fruit bowl.

Phorid fly

How to tell — Phorid flies have a distinctive humped thorax when viewed from the side and scuttle rapidly across surfaces on foot. They are not fuzzy and do not look like moths. Both can share drain biofilm as a breeding source, so the behavior and shape tell them apart.

Fungus gnat

How to tell — Fungus gnats are dark, delicate, long-legged, and associated with overwatered houseplant soil — not drains. They fly weakly but do not sit on walls the way drain flies do.

What you'll notice

Signs of a problem

Drain flies announce themselves by location and the cup/tape test. These signs help you find not just the fly but the specific drain producing it.

Fuzzy moth-like flies on bathroom or kitchen walls near a drain

What it means — Drain fly — the source is almost certainly that drain or one nearby. Begin the cup/tape test to confirm exactly which one.

Gelatinous dark slime visible inside a drain when you look with a flashlight

What it means — Active biofilm — this is the breeding medium. This drain is a likely source.

Flies persist after surface cleaning of kitchen or bathroom

What it means — The biofilm inside the drain was not removed; the larvae are still developing and adults keep emerging.

Flies appearing from a seldom-used basement floor drain

What it means — A dry trap in an unused drain allows flies from a connected sewer line to travel through and establish. Running water weekly refills the trap and often eliminates the source.

Cup/tape test: adults trapped under a cup placed over a specific drain overnight

What it means — Confirmed breeding drain — University of Arkansas Extension documents this as an effective extension-verified source test. Test every drain, not just the nearest one.

How much it matters

The real risks, calibrated

Drain flies are a nuisance pest with low direct health risk — but a persistent infestation can signal a plumbing problem worth addressing.

Overall risk

Low risk

Nuisance — non-biting, low direct health risk

Low risk

Onset: Ongoing while present

Drain flies do not bite and are not known to transmit pathogens to humans (UF/IFAS). The primary concern is annoyance from their presence.

Potential respiratory irritation from shed scales

Low risk

Onset: With heavy populations

Drain flies shed fine scales from their bodies and wings; UF/IFAS notes that in large infestations, these scales may irritate the respiratory tract if inhaled, particularly in sensitive individuals.

Plumbing defect indicator

Moderate

Onset: Persistent infestation

Drain flies that persist after thorough biofilm cleaning may indicate a cracked sewer line, a drain defect, or a subsurface organic accumulation that is outside normal plumbing — worth a plumbing assessment.

What you can do

DIY — what works, what backfires

Drain fly control is biofilm removal, not spraying. The larvae live in the biofilm; get rid of the biofilm and they stop. Here is the extension-recommended process.

What actually works

Run the cup/tape test first: place a clear cup or jar coated inside with petroleum jelly over each suspect drain overnight. Adults emerging from the drain are trapped on the inside surface, confirming that drain as the source. University of Arkansas Extension documents this as a reliable extension-verified method — test every drain, not just the most obvious one.
Scrub the drain interior with a stiff-bristled drain brush, working the brush around the inside of the pipe to physically remove the gelatinous biofilm coating the walls. This mechanical step is essential — no liquid treatment replaces it.
Follow with a bacterial or enzyme drain gel: these products digest the remaining organic material in the drain rather than simply disinfecting or masking it. Apply as directed and repeat weekly until the infestation is resolved.
Run water through dry or seldom-used drains (especially basement floor drains) weekly — this refills the trap seal that prevents sewer access and often eliminates that source entirely.
If a cracked sewer line or drain defect is suspected (flies persist after thorough cleaning), schedule a plumbing inspection.

What backfires

Don't rely on bleach or boiling water alone — these disinfect or kill surface bacteria but do not physically remove the biofilm layer the larvae are embedded in. The University of Arkansas Extension is explicit: bleach alone does not reliably eliminate drain fly larvae living in the biofilm.
Don't spray the adult flies — spraying kills visible adults but has no effect on the larvae in the drain; new adults will emerge as long as the biofilm is present.
Don't assume it is just one drain — test every drain in the affected area, including basement floor drains and laundry room floor drains, before treating.
Don't pour chemical drain cleaners (liquid plumber-type products) into the drain — these can damage older plumbing and still do not remove biofilm the way mechanical scrubbing does.

Local Context

Drain flies in New Hampshire homes

Drain flies are a year-round indoor problem across New Hampshire because heated homes keep drains warm and moist regardless of the season. They are especially common in Manchester, Nashua, and Concord homes with basement floor drains, older cast-iron plumbing with rough interior surfaces that accumulate biofilm faster, and seldom-used drains whose trap water evaporates (a dry trap allows flies from a connected sewer to travel through and establish). A winter drain fly outbreak is not unusual and does not indicate anything unique about the cold season — it simply means a drain has enough biofilm to sustain them.

Peak season

Year-round indoors in NH heated homes regardless of outdoor temperature

You might actually have

Fruit flies (smooth, tan, red-eyed, hover at fruit — often confused with drain flies)
Phorid flies (humped back, scuttles — can also use drain biofilm)
Fungus gnats (delicate, long-legged, near houseplants)

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ManchesterNashuaConcordDerryBedfordSalem

Hillsborough · Rockingham · Merrimack Counties

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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 drain flies

If drain flies persist for more than two to three weeks after thorough mechanical cleaning of all identified drains, the source may be a cracked sewer line, a subsurface drain break, or an accumulation in a section of plumbing that is not accessible for home cleaning. A licensed technician can assess and address these hidden sources.

Signals it's past DIY

When to call a pro

Flies persist for more than two weeks after thorough biofilm removal from all accessible drains — possible cracked sewer line or subsurface defect.
A sourceless swarm in a ground-floor or basement area with no obvious drain source (possible broken sewer line under the slab).
A commercial kitchen or food-service location with a drain fly problem facing a health inspection.
Infestation in a multi-drain plumbing system (e.g., a restaurant, older building, or property with a grease trap) where drain access is limited.

Keep them out — prevention

Weekly drain maintenance

Running a stiff brush through sink and floor drains weekly — or using a bacterial/enzyme drain gel on a regular schedule — prevents biofilm accumulation and eliminates the breeding medium before drain flies can establish.

Keep unused drains wet

Seldom-used floor drains in basements, laundry rooms, and garages lose their trap seal through evaporation. Run a cup of water through them weekly to maintain the trap and prevent sewer access.

NH-specific

Inspect NH homes with older plumbing

Older cast-iron drains in Manchester and Nashua homes can develop rough interior surfaces that accumulate biofilm faster than modern PVC plumbing. An annual plumbing inspection in older homes can catch drain defects and heavy buildup before they become a fly source.

Not sure it's a drain 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]
University of Arkansas Extension — Drain Flies
cup/tape source test (extension-verified), biofilm as breeding medium
extensionverified 2026-06-30
[2]
Texas A&M AgriLife Extension — Drain Flies (E-184)
ID, size, biofilm breeding medium, egg masses 30–100
extensionverified 2026-06-30
[3]
UF/IFAS — Moth/Drain Flies (IN1226)
non-biting, no known pathogen transfer, Bti 79% reduction
extensionverified 2026-06-30
[4]
PMC — Drain Fly / Psychodidae Study
biology and distribution corroboration
academicverified 2026-06-30
[5]
UNH Cooperative Extension — Managing Small Flies Indoors
NH source-first fly control guidance
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 drain flys in your home?

Anchor Pest Services identifies and handles drain 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

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

Fuzzy flies on your bathroom wall? The source is inside the drain.

How to identify a drain fly

The tells — key ID features

Size, to scale

Fruit fly

Phorid fly

Fungus gnat

Signs of a problem

Fuzzy moth-like flies on bathroom or kitchen walls near a drain

Gelatinous dark slime visible inside a drain when you look with a flashlight

Flies persist after surface cleaning of kitchen or bathroom

Flies appearing from a seldom-used basement floor drain

Cup/tape test: adults trapped under a cup placed over a specific drain overnight

The real risks, calibrated

Nuisance — non-biting, low direct health risk

Potential respiratory irritation from shed scales

Plumbing defect indicator

DIY — what works, what backfires

What actually works

What backfires

Drain flies in New Hampshire homes

When to call a pro for drain flies

Weekly drain maintenance

Keep unused drains wet

Inspect NH homes with older plumbing

Not sure it's a drain fly?

Frequently asked questions

Dealing with drain flys in your home?

Your next step

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