Drain flies: remove the breeding film, not just the adults

What a drain fly looks like

Drain flies, also called moth flies, are small dark flies with hairy bodies and wings that give them a fuzzy, moth-like appearance. They are often found resting on walls near sinks, showers, floor drains or other wet organic sources.

Confirm which drain is producing them

Do not treat every drain just because adults are in the room. Monitor suspected openings separately. Extension guidance describes covering or monitoring a drain opening so emerging adults can be linked to the source.

• Check sink, shower, tub and floor drains one at a time.
• Inspect overflow openings and traps where organic buildup can persist.
• Look under cabinets and around pipe penetrations for leaks.
• If flies persist after accessible drains are cleaned, investigate hidden wet organic material or a plumbing/sewer problem.

The durable control step

Physically remove the gelatinous organic film in which larvae feed. A stiff brush or appropriate drain-maintenance method that actually removes the buildup is more useful than repeatedly killing adults. Some biological or enzyme drain products are designed to break down organic film; use them only according to their directions and never improvise chemical mixtures.

What commonly fails

• Spraying adult flies while leaving the breeding source untouched.
• Assuming boiling water, bleach or a generic drain chemical removed the larval film.
• Cleaning one sink when the source is a floor drain, overflow, condensate line or hidden leak.
• Ignoring sewage odor, damp flooring or recurring moisture that points to a building problem.

When to escalate to plumbing or building inspection

Persistent large numbers, activity from several drains, sewage odor, evidence of a broken line or unexplained dampness can indicate more than a dirty drain. In that situation, finding the structural moisture source matters more than adding pesticide.