Drain Flies

Drain flies, sometimes called moth flies, are common but often overlooked insects associated with moist, highly organic areas such as sink drains, sewage treatment facilities, storm drains, dung and rotten vegetation. The immature stage or larva is considered beneficial, an essential part of the cycle that breaks down waste into water-soluble compounds. However, the adults flies can sometimes become numerous enough in the vicinity of breeding areas to be annoying to people.

A general description of their life cycle is given; eggs hatch into small pale larvae which are the most resistant life stage, being capable of surviving temperature extremes and habitats often low in oxygen.

The larvae have strong jaws for feeding on fungi, bacteria, algae and other microorganisms present in moist, decaying organic matter. Larvae feed on the liquid or slime layers which develop around debris in drains, sewage treatment beds and standing water. However, when food is scarce they become cannibalistic.

Following a feeding period, larvae pupate and soon emerge as adult flies. Adults live only three to four days without food, but they can survive for weeks if nectar or other liquid carbohydrate sources are available.

Adults are small and very hairy, with a pair of pointed wings. They are frequently mistaken for small moths since the wings are held roof-like over the body when at rest. They are weak fliers and often appear to be jumping or hopping. Flight and mating activities normally occur in the evening hours when they are attracted to lights. Each female can produce about 100 eggs and, under optimum conditions, development can be completed in two weeks.

Problem Areas

In the home

           Drain flies may become abundant around bathroom or kitchen sinks. Their presence can become annoying enough to make control desirable. Pest strips containing dichlorvos or household insecticides labeled for flying insect control and containing pyrethrins, dichlorvos, resmethrin, allethrin, or tetramethrin provide some relief by killing adults. Infestations, however, can be eliminated only by removing the larval food source. The larvae usually breed in sink and bathroom drain traps, in spite of the hot water, soap and other debris that flow through them. Thorough cleaning of drain traps with drain cleaners (such as drano) and long-handled brushes will remove hair and trapped debris. Also, remove standing water in saucers or pans underneath house plants containers.

Around the home

Determine the source of infestation by looking for concentrations of adult flies around potential larval habitats such as cooling towers, air conditioners, bird feeders or other places containing standing water and vegetation. In lake houses, most of which are usually vacant, the p-trap in drains will dry out from non-use. The drain fly will enter into the interior of the structure from the septic tank.

To test for drain flies, check unused drains in the floor of the basement, washing machines, and shower, and sinks. A quick simple test is to place a hand over the drain to feel if there is a ‘back draft’ occurring, if air is felt coming up your drain that is a good indication the p-trap is dry.

To Fix a Dry P-Trap

•  Fill empty drains with water
•  Add safe ethyl alcohol based anti-freeze to the drains in ‘winterized’ homes
• Clean, remove or treat the breeding area. Infestations can also be located in a neighbor’s yard, shallow pools or sewage treatment facilities, particularly those upwind from the nuisance area. In these cases, contact those people responsible for infested areas. 

Modified from a publication by

B. Drees and J. Ownes, Texas A&M University