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
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
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!