Drain Cleaning & Unclogging
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Plumbing guide
Drain Flies in Peoria, AZ: Causes and How to Stop Them
Small, fuzzy flies that cluster around a sink, a shower or a floor drain are almost always hatching inside the pipe. Swatting the adults changes nothing until the film they breed in is gone.
By the Peoria Express Plumbing team ·
What drain flies are
Moth flies, the other name for drain flies, are tiny gray insects with fuzzy wings no bigger than a pinhead. They tend to sit on the wall beside a drain and hop in short awkward bursts. They do not bite, and seeing them is no sign of a dirty home.
The adults lay eggs in the slimy film that coats the inside of a drain. The larvae feed on that film, and a new generation can appear in a few weeks, so a handful of flies can turn into a cloud before anyone notices where they came from.
Confirm the source before you treat it
Lay a strip of clear tape over the suspected drain at night, sticky side down, and check it in the morning. Flies that tried to leave will be stuck to it. A cup turned upside down over the drain works the same way. Test every drain in the house, since the one with flies is rarely the one you expected.
Pay attention to drains that rarely get used. A guest bathroom, a laundry sink and a floor drain in the garage are all common breeding spots, because the film builds up without regular flow to rinse it away.
Why pouring bleach does not solve it
Bleach runs through the pipe in seconds and never scrubs the wall where the larvae live. It also does nothing for the film above the water line, under the rim of the drain opening.
The fix is mechanical. Scrub the inside of the drain with a long brush, scrape the underside of the strainer and the overflow opening, and flush the pipe with hot water. Repeat for a few days, because eggs in the film survive a single cleaning.
Dry traps and floor drains
Every drain has a curved section, the trap, that holds a plug of water to block sewer air. In a drain that is seldom used, that water evaporates. In a hot, dry climate like ours, it can vanish in a few weeks, leaving an open path from the sewer to the room.
Once a month, pour a quart of water into drains that rarely get used, and top it with a spoonful of mineral oil to slow evaporation. Certain floor drains have a trap primer that refills the trap on its own, and a plumber can fit one.
Telling small flies apart
Several tiny flies look alike at a glance. The place you find them is the best clue.
| Insect | How it looks | Where it shows up | Response |
|---|---|---|---|
| Drain fly | Fuzzy body with moth-like wings | Near bathroom, kitchen and floor drains | Scrub the slime layer in the pipe, then flush |
| Fruit fly | Tan body, red eyes | Around produce, bins and sticky spills | Clear the food source and clean the bin |
| Fungus gnat | Thin, dark, a weak flier | Potted plants with wet soil | Water less often |
When the flies point to something bigger
If the flies return after a thorough scrubbing, the breeding site may be further down the line. A cracked pipe, a slow main-line clog or a leaking trap under the slab can hold the moisture and film that the larvae need.
Warning signs include a sewer smell, slow drains in more than one fixture or damp spots near the drain. A sewer camera inspection shows the condition of the pipe behind the wall or the floor.
Keeping them away
Rinse drains regularly, keep the strainers clean and avoid letting food scraps collect in the kitchen sink. A drain that carries grease will build film quickly, so wipe pans before washing.
Before a summer trip, run water in every drain, and do the same when you get back. Consider professional drain cleaning if you have had repeated infestations in the same fixture.
Further reading
The Utah State University Extension guide on drain (moth) flies explains that the larvae develop in the scum lining drains and sewer lines. It recommends finding the breeding site and scrubbing the film away rather than relying on boiling water or sprays.
Common questions
Drain fly questions
They breed in the slimy film inside a drain. Any drain with standing water and organic buildup can become a source.
Not always. Most cases come from film in a single drain. If they persist after scrubbing, ask a plumber to look at the line.
Clean the source thoroughly and expect to repeat it for a week or two, since eggs and larvae survive a single pass.
Related services
Sewer Camera Inspection
Send a camera through the line to see the real condition before you spend on a fix.
Hydro Jetting
Wash grease, mineral scale and fine roots off the pipe wall with pressurized water.
More plumbing guides
Sewer smell in the house
Sewage odor most often traces to a dried-out trap, a worn seal or a vent fault. Follow the smell to its source.
Toilet won't flush: causes and fixes
When a flush does nothing, a handful of causes explain it. Work out yours and see what to try before calling.
Tree roots in the sewer line
Hairline cracks are enough for roots to get into a sewer line. Know the early signs and the repair choices.
Flies that keep coming back?
Phone the line if a thorough scrubbing did not solve it. A dispatcher can send a contractor to check the drain and the line behind it.