An annoying and troublesome pest that concerns numerous homeowners is the moth fly, also commonly known as the drain fly, filter fly, or sewage fly. Moth flies are frequently found indoors on windows, sinks and walls. The source of the fly infestation is generally from sinks and floor drains, or from nearby sewage plants or waste disposal areas. Polluted waters and wet organic materials are preferred breeding areas of the fly.

The adult fly (Fig. 1) is about one-tenth inch long (about one-third the size of a housefly). It has a dark gray body and lighter colored wings. The body and wings are densely covered with long hairs which give the body a fuzzy or hairy appearance, hence the name “moth fly”.

Eggs are deposited throughout the spring and summer in moist, decomposing organic materials. The gelatinous organic deposits which accumulate in drains provides an ideal breeding site. Eggs hatch into the maggots, which feed in this organic matter. Under favorable conditions, maggots mature in about two weeks and form a hard shell (puparium) within which the pupa forms and from which the adult fly emerges.

Figure 1. Moth fly adult

Control varies with the origin of the moth fly infestation. The most economical and sanitary way is to remove the organic material in which they breed. This requires cleaning sinks, drain pipes, and traps with a stiff wire brush or by steam. Some control of the maggots can be obtained by flushing infested drains with a caustic household cleaner, or by running the hot water down the drain for a period of time. To determine which drain is infested, place a jar over the outlet or tape a plastic bag over the opening of the overflow outlet. Check the container at intervals for adult flies.