How to get rid of drain flies: find the drain that is producing them

First, check the fly rather than the room

Drain flies, also called moth flies, have hairy wings and a fuzzy outline. Their short, weak flights often look like hops. Examine a resting insect with a magnifier or a sharp close-up photo. A bathroom location alone is not an identification.

Scroll across to read every column.

Test each suspect drain separately

Start with sink, tub, shower and floor drains in the affected room, including seldom-used fixtures. More than one opening may be involved. Use a clean, empty clear cup or jar, vegetable oil or petroleum jelly, a label and a flashlight.

1. Choose a period when the fixture can remain unused. Do not cover a drain needed for active equipment discharge or emergency overflow. Tell everyone using the room that the fixture is out of service.
2. Lightly coat the inside of the cup with oil or petroleum jelly. Invert it over the entire drain opening, with its rim seated on the surrounding surface. Label the drain and start time.
3. Leave it for 24 hours, then look for flies caught inside. Photograph the result before removing the cup. A fly on the outside does not count as evidence of emergence.
4. Remove every cover before using fixtures. Mark positive drains for cleaning; if none catches flies but the problem continues, repeat a properly seated test and broaden the search below. One empty trap is not an all-clear.

The busy bathroom problem: test one opening at a time

The 24-hour cup test works only if that fixture can remain unused. In a one-bathroom home, test the sink on a day when another handwashing sink is available, then test the shower separately. Label the covered fixture so nobody fills it against the cover. Keep required floor drainage and equipment discharge paths open.

Record the actual start and finish times. A six-hour test squeezed between showers is a shorter observation, so do not compare its count directly with a 24-hour sink test. An empty short test is a reason to retest when practical, not proof that the drain is clear.

Use a fresh cup at each opening and photograph the inside before cleaning. Flies stuck outside the cup came from the room. Flies inside identify that opening as a route, which helps choose where to work first.

Remove the film, not just the visible flies

Use the brushing steps for an accessible sink or shower drain without a garbage disposal. A disposal needs its own baffle-cleaning procedure, with power disconnected; keep hands out of the grinding chamber.

For an accessible sink or shower drain, gather gloves, splash protection, a flashlight, a small waste bag and a stiff-bristle drain brush suited to the opening. Keep tools away from food preparation. If drain-opening chemicals were recently used or may remain, stop before brushing or dismantling anything; follow that product’s handling instructions or tell the plumber what was added.

1. Remove only a strainer or stopper designed for owner removal. Follow its release method rather than forcing a fixed cover. Lift out reachable hair and debris and put it in the waste bag.
2. Brush the coated inner walls and the underside of the removed part to dislodge the gelatinous film. Work around the circumference; a clear hole down the middle can still have breeding material on its sides. Remove loosened debris rather than merely pushing a wad farther down.
3. Inspect the sink’s overflow opening too. Sludge can collect in overflow passages. Clean only reachable areas with a suitable non-damaging tool; inaccessible passages or a trap that needs removal call for fixture-specific help.
4. Refit the parts, remove test covers, and run the faucet or shower normally. Check that water drains freely and no disturbed part leaks. If it remains slow or backs up, resolve the blockage before repeating pest treatments.

What a cleaner or spray can actually do

A drain treatment formulated with bacteria or enzymes can help break down organic deposits. Choose one labeled for the drain and plumbing involved, then follow its dose, contact time and rinse instructions. No universal overnight schedule applies to every product. Thick deposits still need physical removal.

Do not pour insecticide into a drain. Bleach, vinegar and boiling-water treatments do not reliably remove the protected breeding material. Do not combine cleaners, especially bleach and ammonia. Swat adults as needed; fewer flies on the wall after spraying would show adult knockdown, not successful drain cleaning.

Check for new emergence after cleaning

Repeat the same cup test on the cleaned drain during another unused period, starting with a clean cup. Record the drain, date, catch and cleaning completed. Compare like with like: a room count alone mixes newly emerging flies with adults already present. The finish check is repeated empty emergence traps plus the disappearance of remaining adults without a rebound; continued catches mean more source work is needed.

Scroll across to read every column.

What a useful drain log looks like

This is an invented example showing how to interpret observations; the numbers are not expected catch rates. Each catch covers the same 24-hour period.

The sink’s improvement does not clear the floor drain. Conversely, two adult flies on a wall do not undo two empty sink checks. Keep source catches separate from room sightings. University of Maryland Extension notes that adults can live up to two weeks, so a falling room count can lag behind source removal. Continued emergence or a rebound means the source work is unfinished.

Scroll across to read every column.

If the drains test negative, widen the search

In a guest bathroom unused for a long time, an evaporated trap can let flies enter from the sewer system. Restore water by using the fixture normally, then recheck. Flies arriving through an opening do not by themselves prove larvae are breeding in its reachable section.

Look for wet debris in plant saucers, dirty trash containers and accessible appliance condensate pans. Follow appliance instructions before reaching near powered equipment. Persistent dampness at a toilet base, below a sink or around damaged shower surfaces needs a leak investigation rather than another bottle of drain treatment.

If positive catches persist after thorough accessible cleaning, or flies remain numerous with no exposed source, ask a plumber and pest professional to investigate hidden leaks or wet material. Do not open a wall, remove a toilet or enter a contaminated crawlspace just to hunt for flies. Give them the photographs and drain-by-drain results.

If flies are far more numerous outside than inside, check window and door screens. An outdoor source requires a different response from repeatedly cleaning a negative indoor drain.

Keep the source from rebuilding

Remove hair and visible slime during routine strainer care, keep little-used fixtures in use, and correct leaks that keep debris wet. An odor-free drain is not proof that breeding has stopped; use the emergence checks and the overall decline in flies. Existing adults can linger for a week or more after source removal, so there is no dependable same-day finish line.