Drain Cleaning & Unclogging
Open a blocked sink, tub, laundry drain or main line, and find what keeps causing the stoppage.
Homeowner guide
Drain Flies in Louisville, CO: Causes and How to Stop Them
If tiny fuzzy flies keep gathering around a bathroom sink, a shower or a basement floor drain, the larvae are probably living in the slime on the inside of the pipe. Killing adults alone will not end it.
By the Flatirons Express Plumbing team ·
What you are looking at
Drain flies, sometimes called moth flies, are small gray or tan insects with hairy wings held in a roof shape over the body. They fly poorly and tend to flutter a short distance, then land. They do not bite or sting.
Adults lay eggs in the gelatinous film that builds up in slow or seldom-used drains. Larvae feed there, and a new generation can appear in about a week or two, so a few flies turn into a swarm quickly.
A simple overnight test
Cover each suspect drain with a clear cup turned upside down, or tape a piece of plastic wrap across the opening. Leave it overnight. Flies caught under the cover in the morning show which drain is the source.
Test every drain in the area, including the shower, the tub overflow and the basement floor drain. A guest bath that sees little use is a frequent culprit, since stagnant water and film build up without any flushing action.
Scrub, then flush
A brush that fits the pipe will do more than any liquid. Scrub the upper walls of the drain opening, where the film clings above the waterline, then run hot tap water with a squirt of dish soap. Repeat for several days in a row to catch each new hatch.
Biological cleaners that use enzymes or bacteria digest organic buildup and are gentler on pipes than harsh chemicals. Use them at night, when the drain sits idle, and never combine two different cleaning products.
Which small fly is it?
Several tiny flies look alike. Notice where yours collect and the table points to the likely type.
| Insect | Appearance | Usual hangout | Best response |
|---|---|---|---|
| Drain fly | Furry gray body with wings held like a tent | Near sink, shower and floor drains | Scrub the slime lining the pipe, then flush |
| Fruit fly | Tan body, often with red eyes | Over-ripe produce, garbage and sweet spills | Clear out the food and scrub the container |
| Fungus gnat | Slender, dark and a feeble flyer | Soggy potting soil | Water plants less often |
Floor drains and dry traps
Each fixture drain holds a small plug of water in its trap. That water stops sewer gas from rising. When it evaporates, gas and insects have an open path. Basement and laundry floor drains dry out most often, and the dry Front Range air speeds it up.
Add water to unused drains every few weeks. A little cooking oil poured on top slows evaporation. A plumber can fit a trap primer that tops up a floor drain by itself, which is a lasting answer for a drain you rarely use.
Signs of a deeper problem
Flies that return after careful cleaning, along with gurgling, slow drains or a sewage smell, suggest trouble farther down. A sagging pipe section holds water and waste, and a cracked line gives insects a way in.
A camera inspection reveals the pipe condition. Heavy buildup that a brush cannot reach responds to professional drain cleaning or hydro jetting.
Why Colorado homes see them in winter
Heating dries out indoor air, and a drain that sits unused for weeks loses its trap water faster than you might expect. Houses with a finished basement bathroom that is rarely used often see flies appear in the coldest months.
If you are away for part of the winter, pour water into each drain before you leave and again when you return. The small habit keeps the trap sealed and denies flies a place to start.
What not to bother with
Foggers and sprays kill flying adults but leave the larvae alone in the pipe, so the problem returns in days. Boiling water scalds the surface but does not reach the film clinging above the waterline. Pouring bleach in can damage some pipe materials and fittings, and it gives no lasting result.
Spend that effort on scrubbing and on the overnight test instead. If one drain keeps producing flies after a week of work, treat it as a clue about the line behind it and ask for a plumber's opinion.
A weekly routine that takes five minutes
Pick one day a week. Run hot water for thirty seconds in every drain you rarely use, brush the sink overflow, and wipe the rim of the drain opening with a damp cloth. Pour a cup of water into basement floor drains and any laundry standpipe.
Keep a note on the calendar for the first month. Most households find that the flies disappear after two or three weeks of steady attention, and the habit is easy to keep once the problem is gone. If a particular drain still produces flies, mark it and mention it to a plumber.
Habits that keep them away
Run water through every drain at least weekly, wipe the sink overflow holes with a bottle brush and keep grease out of the kitchen drain. Rinse the disposal with cold water and a little dish soap. Fix cabinet leaks, since constant dampness feeds the film.
Not every tiny fly is a drain fly. Fruit flies prefer ripe produce, and fungus gnats live in overwatered plant soil. Matching the fly to the source saves time, which is why the table above is worth a look.
Quick answers
Drain fly questions
They do not bite. They may carry bacteria from the film where they breed, which is a reason to clean the source rather than ignore them.
Adults tend to be most active at dusk and rest on walls near drains during the day, so evening sightings near a sink or tub are common.
It may kill a few adults but leaves the film on the pipe wall. Scrubbing and an enzyme cleaner work better.
Sometimes, when a trap is dry or a line is damaged. A camera look can show whether the sewer is part of the story.
Services that connect to this topic
Hydro Jetting
Blast grease, scale and roots out of a pipe with pressurized water when a cable left it partly blocked.
Sewer Camera Inspection
Run a camera through the sewer to see its condition before a home purchase or a repair decision.
Keep reading
Why your house smells like sewer
A sewer smell has a short list of causes. Most start with a dry trap or a cracked pipe, and each has a different fix.
A toilet that refuses to flush
Most flush failures are a tank problem or a clog. Check the water level, the flapper and the chain before you call.
Tree roots in your sewer line
Roots find tiny gaps in sewer pipes and grow into blockages. Learn the signs, the fixes and how to prevent a repeat.
Keep seeing flies after cleaning?
Tell the dispatcher which drains are involved and what else you have noticed, and you will be put through to a nearby contractor.