Drain Flies

What Are They?

Also Known as Moth Fly
The common names drain fly, sewer fly or moth fly are used interchangeably for the organism scientifically named Psychoda alternata. These names refer to the preferred breeding ground of the insect as well as its appearance.

Though they do not bite or otherwise physically harm humans or plants, these flies breed in the buildup of gelatinous material found in drainage pipes and in sewage. Therefore, their presence exposes individuals to a number of bacteria and pathogens.

What Does a Drain Fly Look Like?

Drain flies physically resemble moths, but grow an average of 1/8 of an inch long. These flies can be black, gray, or brown in color.

Drain Fly
Image credit: Matt Bertone, NC State
drain fly picture
Image credit: Michigan State


Physically resembling a small moth, the drain fly boasts a distinctively fuzzy appearance due to the dense layer of hair covering the body and wings.


Roughly a third of the size of the common house fly, adult drain/moth flies generally measure only about two millimeters in length.


The bodies of adult drain flies vary in color from light gray or tan to brown or black and feature a pair of long antennae, each divided into 12 to 16 segments.

group of dead drain flies
dead drain flies

Do Drain Flies Bite?

Compared to other fly species, many of which are known to bite humans and spread disease, drain flies are relatively harmless.  Drain flies do not bite humans and are not considered threats to transmit diseases. The non-biting insects merely serve as a household nuisance.

Where Do You Find Them?

Drain flies are also frequently found around toilet bowls and plumbing fixtures, as drain fly larvae require aquatic environments. Weak fliers, adult drain flies rarely travel far from breeding sites and often congregate on the walls of basements, bathrooms, and kitchens.

Found in Drains
As one of the common names of the insect suggests, drain flies are often found emerging from the sinks and bathtubs of homes. The insects breed and spend the early stages of their life cycle in the organic residue attached to the walls of drains.

Emergence from Drains
Upon reaching adulthood, drain/moth flies emerge from the drain and are frequently observed crawling or resting on walls and other surfaces near the breeding site. When flying, drain flies move erratically and only travel short distances of up to a few feet before landing.

Though frequently encountered indoors, moth flies also live outside in shady areas near water. Clogged gutters, storm drains, birdbaths, and swamplands (where decomposing organic matter accumulates) make ideal outdoor habitats for drain flies.

What Do They Eat?

Like all true flies, Psychoda alternata adults lack the mouthparts necessary for chewing and ingesting solid foods. Instead, adult drain flies feed on liquefied carbohydrates such as nectar. During the larval and pupal stages of the life cycle, moth flies consume the organic matter contained in the layers of film that line the walls of drains.

Life Cycle & Reproduction


Drain flies breed and lay eggs in decaying organic material, which serves as a food source for the newly hatched offspring. Adult females deposit eggs, which hatch within 48 hours, in batches of 30 to 200. After hatching, drain/moth flies spend up to two weeks as larvae before pupating.


Drain fly eggs are very tiny and range in color from brown to cream, while the emerging larvae have eyeless, legless bodies measuring nearly a half-inch long and featuring dark bands on each segment.


In between the larval stage of development and adulthood, drain flies become pupae, which are brownish in color and look like grains of rice.

The pupal stage lasts between 20 and 40 hours, with drain flies emerging as adults upon completion.


From egg to adult, the entire life cycle concludes within one to three weeks, depending on the temperature of the environment. Newly formed adults typically live for an additional two weeks.

Problems Caused by Drain Flies

The presence of drain/moth flies generally serves as a minor annoyance.  Adult drain flies rarely infest homes in large numbers and typically prove easy to eliminate due to their weak flying abilities.

Signs of Infestation

On occasion, however, drain flies cause more serious infestation problems when an indoor breeding site is overlooked and permitted to endure. When drain flies increase in number indoors, unsightly congregations of the insects often appear around bathtubs, sinks, floor drains, light fixtures, and windows.

Though not typically known as transmitters of disease, drain flies breed in unsanitary conditions and have the potential to transfer the bacteria and microorganisms found in drains to other household surfaces and food.

Because drain flies only occasionally populate indoor areas in large numbers, the continued presence of the insects over the course of several weeks generally indicates a chronic infestation. Adult drain flies typically rest on walls or ceilings near drains and sinks during the daytime hours and become more active in the evening. Drain flies are also frequently found on and around windows, as the insects often are attracted to light.

Learn how to get rid of drain fly infestations.


  • May notice tiny adult flies gathered in bathrooms, kitchens, or basements.
  • Look for collections of rainwater or drainage outdoors that may attract drain flies.
  • Be alert for thin, whitish-brown larvae collected around drains.

Drain flies vs fruit flies