Drain flies (sewer flies)

Drain flies, sometimes called sewer flies or moth flies, are small fuzzy insects of the family Psychodidae that breed in the gelatinous film of organic sludge lining drains, traps, gullies and grease residues. Adult flies are weak fliers and tend to be seen resting on walls near a kitchen sink, bathroom basin or floor gully. Their presence usually indicates that a section of pipework, an overflow or a trap is harbouring a build-up of decomposing organic matter. Engineers usually address common causes by cleaning traps, removing sludge from waste pipes and jetting the affected line. Persistent infestations can also point to a hidden leak under a floor or behind a wall where biofilm has developed. Routine maintenance and regular flushing of less-used fittings helps reduce recurrence.