Species: Drain fly or moth fly (Psychoda phalaenoides)
What to look for
- Size. Colorado State gives the adult moth fly as 2 mm (about 0.08 inch). Maryland gives drain flies as 1/16 to 1/4 inch (1.6 to 6.4 mm) long.
- Look. Maryland describes them as delicate and fuzzy, pale brownish gray to black. Colorado State says they are often grayish and superficially resemble a tiny moth.
- Wings. Kentucky says the wings are covered with scales, so they disappear in a cloud of fine dust when the fly is swatted or mashed.
- Behavior. Kentucky describes them resting on walls or ceilings and making short hopping flights if disturbed, rather than flying around a room.
- The larva. Maryland says larvae live in the gelatinous film or slime in drains and traps, feeding on algae, bacteria and fungi. Kentucky notes the gray, wriggling larvae are sometimes visible swimming in standing water.
- Where it came from. Maryland lists kitchen and bathroom drains, floor drains in basements, sewer leaks, dirty garbage cans, saucers under potted plants, clogged storm drains and condensate pans under refrigerators and air conditioners.
Where and when
Kentucky extension says drain flies are often a temporary problem that shows up after a vacation or a stretch of travel, because the larvae need water standing for a week or more; normal household use of toilets and drain traps usually clears them. Maryland gives egg to adult development as 7 to 28 days, and Kentucky gives the cycle as roughly 10 to 15 days at about 70 degrees Fahrenheit.
Does it matter?
Kentucky extension calls drain flies non-biting gnats and treats them as a nuisance whose real signal is a wet breeding site somewhere. Seeing many over several weeks usually means a permanent one. Both Kentucky and Maryland put the remedy in the same place: clean the pipe and trap thoroughly to remove the accumulated slime the larvae live in, rather than treating the adults.
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