P&PDL Picture of the Week for
January 25, 2010

Moth Flies - A Product of Their Environment

Tim Gibb, Insect Diagnostician, Department of Entomology, Purdue University

Moth flies must suffer from a severe case of ‘identity crisis’.  They are often called Moth flies – but they are not truly moths at all.  Sometimes they go by equally unflattering names such as drain fly, sewer fly or sewer gnats.  While none of these would make the 2010 list of most popular baby names, each name actually does tell a bit about this common household pest.
Upon close examination, moth flies have only two wings - like all self-respecting flies.  Thus they are in the family Diptera and are related to blow flies and gnats.  In contrast to typical membranous fly wings, however, moth fly wings and bodies are densely covered with tiny gray hairs and so look very much like a moth. 
The adult fly is about one-third the size of a housefly. It has a dark gray body and lighter colored wings that are held in a characteristic tent-like manner over the body. The body and wings are densely covered with long hairs that give the body a fuzzy or hairy appearance.

Eggs are laid in moist, decomposing organic materials. The gelatinous organic material lining drains can provide an ideal breeding site. Eggs hatch into tiny headless maggots that feed in this nasty organic matter for about two weeks before forming a hardened pupa from which the adult fly emerges.

Moth flies are frequently found indoors in bathrooms or kitchen areas where there is a plumbing disorder.  The source of the fly infestation often begins with pipes that have been broken and where enriched organic matter has remained wet for a significant period of time.  Plugged sinks and clogged drains create a similar environment.  Sometimes moth flies originate in and swarm around sewage plants or waste disposal areas out of doors.  Thus, ‘sewer’, ‘drain’, ‘gnat’ and ‘moth’ become good descriptors of where these maggots live and what they look like.

Control depends upon where the flies originate.  Wet and putrid garbage accumulations must be cleaned and dried.  Plumbing must be inspected for breaks or leaks.  Inside homes, moth flies may appear in windows or on surfaces away from their actual breeding sites.  Actually pin pointing a clogged or dirty drain can be difficult.  To determine if a drain is infested, place a container or a sticky trap over the opening and watch for trapped flies.

If a breeding source is discovered, the most economical and safe control is to physically remove the organic material in which the fly breeds. This may require opening and cleaning sinks, drain pipes, and traps with a stiff brush. Several new products are now on the market that contain either bacteria or enzymes designed to breakdown the gelatinous scum in drains. They are either poured around the drain lip or injected as foam and left in place for a set period of time and will eliminate drain fly problems.  These are available through professional pest management services.

Considering where and how these moth fly babies live – it is no wonder that they grow up to be pests!

Click image to enlarge

Moth fly

Purdue Plant & Pest Diagnostic Lab Purdue Cooperative Extension Service