Drain flies are fuzzy, moth-like flies 1/16–1/4 inch long that hold their wings like a roof, rest on walls near drains and breed in the slimy film inside them (Clemson HGIC; University of Kentucky). Fungus gnats are slender, dark, mosquito-like flies that stay near houseplants and breed in moist potting soil (UC IPM). To be sure: tape a clear bag over the drain overnight — flies trapped inside mean the drain is the source (University of Kentucky).
Drain flies vs gnats: the differences
The word "gnat" gets used for almost any tiny fly, so drain flies are often called gnats too — the University of Kentucky itself describes them as "small, dark-winged, non-biting gnats". But the insect most people mean by a houseplant gnat is the fungus gnat, and the two are easy to separate once you know what to look for. Drain flies are fuzzy and moth-like; fungus gnats are slim and mosquito-like.
| Drain fly (moth fly) | Fungus gnat | |
|---|---|---|
| Family | Psychoda and related genera (Psychodidae) | Bradysia and Orfelia spp. (Sciaridae) |
| Size | 1/16–1/4 in (about 2–4 mm) | 1/16–1/8 in (about 2–4 mm) |
| Colour | Grey-brown to black, fuzzy | Dark grey to black; light grey to clear wings |
| Body and wings | Hairy, moth-like; oval wings held like a roof | Slender, mosquito-like, long legs and antennae |
| How it moves | Weak flight in short hops; sits on walls near drains | Weak, erratic flight near pots and windows; runs over the soil |
| Where you see it | Bathrooms, kitchens, floor drains, seldom-used toilets | Houseplants, windows, computer screens |
| Attracted to | Light in the evening | Light; moist potting soil |
| Bites people? | No | No |
| Breeds in | Gelatinous film inside drains and sewers; water standing a week or more | Moist potting soil and organic matter |
| Larvae | Long, skinny, legless, 1/8–3/8 in; darker head and tail with dark bands; a breathing tube (Clemson HGIC) | Shiny black head, whitish-to-clear legless body; may leave slime trails on the soil (UC IPM) |
Turn the question around — gnats vs drain flies — and the answer is the same: a hairy fly sitting still on a bathroom wall is a drain fly; a delicate dark fly fluttering up from a flowerpot is a fungus gnat. The rest of this page goes through each clue, then shows you how to prove which drain is the source.
Are drain flies gnats?
Are drain flies gnats? Only in everyday speech. Drain flies belong to the family Psychodidae, which is why Clemson Extension lists their common names as "moth flies, drain flies, filter flies, sewer flies". Fungus gnats belong to the Sciaridae. That's why people search for "drain gnats", "sink gnats" or "bathroom gnats": they are describing a drain fly by where it lives. Treat a "drain gnat" as a drain fly, and the fix — cleaning the drain — follows naturally.
Drain flies vs fungus gnats: wings, flight and larvae
Wings: fuzzy roof vs clear and narrow
The wings settle most cases. A drain fly's body and wings are covered with fine hairs, and its large oval wings are held "roof-like over the abdomen" (University of Kentucky) — from above it looks like a tiny grey heart or a moth. The University of Kentucky adds that the wings are covered with scales, so a drain fly "disappears in a cloud of fine dust" when you swat it. A fungus gnat's wings are light grey to clear, narrow and smooth, folded flat over a slender body (UC IPM).
Flight: short hops vs weak flutter
Neither is a strong flier. Drain flies have "weak flying ability, often moving in short hops", resting on walls near their breeding site and making short hopping flights when disturbed (University of Kentucky). Clemson notes they're most active in the evening and may be attracted to light. Fungus gnats are "relatively weak fliers" that stay near potted plants and run across the soil or rest on leaves; they're attracted to light, so you notice them at windows first (UC IPM). This moth fly vs gnat difference is visible from across the room: one sits motionless on the tiles, the other drifts around the plant.
Drain fly vs fungus gnat larvae
If you can find the larvae, the answer is certain, because they live in completely different places:
| Drain fly larva | Fungus gnat larva | |
|---|---|---|
| Where | In the slimy film inside drains, sewers and anywhere water stands a week or more (Clemson; UKy ENTFACT-615) | In moist potting soil, mostly the top inch (UF/IFAS) |
| Look | Grey, wriggling; pale underside, darker head and tail (UKy; Clemson) | Clear to white with a shiny black head (UC IPM) |
| Breathing | Through a tube at the tail; can trap air bubbles and stay submerged for 1 day or more (UKy ENTFACT-615) | In the air spaces of the soil |
| Food | Decaying organic matter, bacteria, mould, algae (UKy ENTFACT-657) | Fungi, organic matter and fine roots (UC IPM) |
| Larval stages | Three, over 8–24 days depending on temperature (UKy) | Four instars (UC IPM) |
To check soil, push a few chunks of raw potato, cut side down, into the pot and lift them after a few days; fungus gnat larvae gather underneath (UC IPM). To check a drain, use the tape test below — the larvae themselves are hidden in the pipe.
That's fungus gnats vs drain flies in one line: soil larvae with black heads, or drain larvae with breathing tubes.
Drain gnat vs fruit fly
The other fly you'll see around a sink is the fruit fly, which complicates the drain gnat vs fruit fly question: both can come out of the same drain. Fruit flies breed in drains, garbage disposals and anywhere a "moist film of fermenting material" builds up (University of Kentucky), while drain flies breed in the gelatinous biofilm itself (Clemson HGIC).
- Look: the fruit fly is smooth, with clear wings, a tan front, a dark rear and usually red eyes; the drain fly is fuzzy and grey-brown with moth-like wings.
- Behaviour: fruit flies hover and dart around food and the bin; drain flies sit on the wall and hop.
- Season: fruit flies are especially common in late summer and autumn (University of Kentucky); drain flies often show up after a vacation, when a drain or toilet hasn't been used (University of Kentucky ENTFACT-615).
So sink gnats with red eyes around the fruit bowl are fruit flies; fuzzy ones on the splashback are drain flies. For a full three-way comparison with fungus gnats, see fruit fly vs gnat vs drain fly.
The drain tape test
How to tell if flies are coming from the drain? Cover it and see what emerges. Extension services describe three versions of the same test:
- Clear plastic bag, overnight. Tape a clear plastic food storage bag over the drain opening overnight. "If flies are breeding in these areas, the adults will emerge and be caught in the bag" (University of Kentucky ENTFACT-621).
- Oiled cup, several days. Coat the inside of a clear plastic cup very lightly with vegetable oil or petroleum jelly and turn it upside down over the drain for several days; emerging adults stick to it (University of Kentucky ENTFACT-615).
- Tape or a clear bowl over the drain shows whether flies are breeding in the drain's biofilm (NC State Extension).
Read the result simply: flies trapped = that drain is a source. No flies after several days means the source is elsewhere — the University of Kentucky's advice is to "expand the search": other drains, a seldom-used toilet, the drip pan under the fridge, or wet spots outside where air-conditioning units drain. Test every drain that has flies nearby, because more than one can be breeding. The calculator's second mode turns your list of drains into a night-by-night checklist.
The test also answers the bathroom question. Bathroom gnats that collect in a cup over the shower drain are drain flies (or fruit flies, if they have red eyes); if the cups stay empty and there's a plant on the windowsill, test the soil instead.
How to get rid of drain flies vs fungus gnats
Because they breed in different places, how to get rid of drain flies vs gnats are two different jobs. Both start with the breeding site, not the adults: "the use of pesticide is usually not necessary if the breeding source is located and removed" (University of Kentucky).
Drain flies
- Brush out the drain. "The most effective control method is to clean pipes and traps thoroughly to remove accumulated slime" (University of Kentucky ENTFACT-615). A stiff or pipe brush gets the film a cleaner can't (Clemson HGIC).
- Then flush with very hot water. Enzyme cleaners, baking soda or vinegar can help loosen the film first (University of Kentucky ENTFACT-657). Hot water on its own gives only short-term control (ENTFACT-615).
- Run water through seldom-used drains and toilets. Small infestations usually end once water flows normally again (University of Kentucky).
- Don't pour insecticide or bleach down the drain. The University of Kentucky advises against insecticides in drains, and NC State notes bleach and standard cleaners won't kill the larvae.
Fungus gnats
- Let the soil surface dry between waterings and empty saucers (UC IPM).
- Improve drainage with more perlite or sand in the mix (UC IPM).
- Yellow sticky traps on skewers catch adults; potato chunks trap larvae (UC IPM).
- Repot in pasteurised potting mix if a plant is badly infested (UC IPM).
How quickly they disappear afterwards differs too: drain flies live only days as adults — see the drain fly lifespan calculator for the date yours should be gone — while fungus gnats need a full soil-drying cycle; the gnat lifespan calculator covers those.
How this tool works
Which is it? Each answer adds points to every insect whose sourced description matches: where you see them and the wings count 3 points each, movement and larvae 2 each (10 in total). Confidence is "high" when the best match scores 8 or more and leads by 3; "medium" with a lead of 2; otherwise "low". "Not sure" and "None seen" answers add nothing and cap confidence at medium. The fruit fly is included because it shares drains with drain flies.
Drain tape test. For each drain you tick, the planner lists the test (a clear bag overnight, or an oiled cup for 2–4 days — the University of Kentucky's "several days"), a day-by-day check-off list and how to read the result. Nothing is calculated from a guess: the steps are the ones in the extension fact sheets above.
Frequently asked questions
How can I tell drain flies from gnats?
Drain flies are fuzzy and moth-like, hold their wings like a roof and sit on walls near drains. Fungus gnats are slender, dark and mosquito-like and flutter around houseplants (Clemson HGIC; UC IPM).
Are drain flies gnats?
Not scientifically. Drain flies (moth flies) are Psychodidae; fungus gnats are Sciaridae. People call drain flies "drain gnats" or "sink gnats" because of where they live.
What's the difference between drain fly and fungus gnat larvae?
Drain fly larvae are grey, slender and live in drain slime and water, with a breathing tube at the tail. Fungus gnat larvae are clear with a shiny black head and live in potting soil.
What are the tiny gnats in my bathroom?
Usually drain flies breeding in a sink, shower or floor drain or a seldom-used toilet. Tape a clear bag or an oiled cup over each drain to find out which one.
How do I know if flies are coming from my drain?
Tape a clear plastic bag over the drain overnight, or invert a lightly oiled clear cup over it for several days. Flies caught inside mean that drain is breeding them (University of Kentucky).
Is a moth fly the same as a drain fly?
Yes. Moth flies, drain flies, filter flies and sewer flies are names for the same family of fuzzy little flies (Clemson HGIC).
Drain gnat vs fruit fly: how do I tell them apart?
Fruit flies are smooth with clear wings and usually red eyes and hover around food. Drain flies are fuzzy with moth-like wings and rest on walls. Both can breed in a dirty drain.
Does boiling water kill drain flies?
Hot water helps, but on its own it gives only short-term control. Scrub the drain with a stiff brush to remove the slime first (University of Kentucky; Clemson HGIC).
Do drain flies or fungus gnats bite?
Neither bites people (Clemson HGIC; UC IPM).
Why do I get drain flies after a vacation?
Drains and toilets that sit unused build up a slime layer that drain flies breed in. Running water through them regularly helps (University of Kentucky).
Should I pour bleach down the drain for drain flies?
No. Bleach and standard cleaners won't kill the larvae (NC State), and the University of Kentucky advises against pouring insecticides down drains. Brush out the film instead.
The identifier gives a best match from sourced descriptions, not a lab identification. For a stubborn problem, your local cooperative extension service can identify a specimen. We give no pesticide advice, and for fleas on pets please talk to your vet.