Drain Flies (Moth Flies)

ENT-41
Date: 
04/10/2014
David Shetlar, Department of Entomology

Drain flies (also known as moth flies) sometimes appear suddenly and mysteriously, becoming a nuisance in homes, rest rooms of commercial buildings, sewage disposal plants and agricultural facilities that handle moist animal waste. Adult flies may become so numerous indoors that they congregate at windows, darken lamp shades at night and fall into food. They may also accumulate around showers, bathtubs, sinks and floor drains, especially in the basement. Restaurants, schools and other public buildings may be avoided when drain flies are a nuisance. Outdoors they can be a public nuisance by getting into the eyes, ears and nose of people; getting stuck in fresh paint; and plugging sewage filter beds (intakes and drains). Bronchial asthma can be caused by inhaling fragments and dust of dead flies. Since these flies originate in filthy conditions, there is the possibility of physical transmission of microbes of human health concern.

Identification

Drain fly resting on side of wall. This is one of the common gray forms that holds its wings flat.

Adult drain flies are tiny (1/5 to 1/6 inch long), fuzzy, dark or grayish insects with the body and wings densely covered with hairs. The antennae are long (13 segments), with each segment being bead-like and having a whorl of long hairs. Wings are longer than the body and are often held roof-like over the body when at rest. Some species hold the wings out to the side, which gives them a moth-like appearance—hence the name "moth fly." They are weak fliers and make irregular, hesitating flights covering only a few feet in short, jerky lines. Eggs are tiny, brown or cream-colored and are laid in irregular masses of 10 to 200. Larvae are legless, about 3/8 inch long, wormlike and gray, with both ends somewhat darker.

Life Cycle and Habits

Drain fly larvae grow and feed in polluted, shallow water or in highly moist organic solids. The eggs, larvae and pupae can be found in the bacterial muck, slime or gelatinous film often accumulating on the sides of drains and overflow pipes in homes, sewage disposal beds, septic tanks and wet compost. They have also been found in dirty garbage containers, rain barrels and tree holes. Eggs are laid in and on the moist media, and they hatch in 32 to 48 hours at 70°F. Larvae feed on the decaying organic matter, bacterial films, algae and sediment found in the moist environments. Larvae mature in 9 to 15 days and are considered to be part of the organisms useful in cleaning sewage water and breaking down wastes. Larvae live in the drain pipes or wet organic matter and breath by extending tube-like tails to the surface. Pupae occur in or on the surface of the breeding media, and the new adults emerge after 20 to 40 hours.

The life cycle can be completed in one to three weeks. Adults live about two weeks, with old ones dying and new ones emerging. Adults may feed on flower nectar and polluted water. During the day, adults rest in shaded areas or on walls near plumbing fixtures and on the sides of showers and tubs. Most activity occurs during the evening when these flies are seen hovering about drains and sinks. They may breed in large numbers at sewage filter plants and then may be carried by prevailing wind to nearby homes up to a mile away. Some species have adults that are small enough to pass through ordinary window screening.

Control Measures

Drain flies do not bite humans but may become a nuisance by their presence in large populations. Persistent effort may be needed to eliminate an infestation in homes, restaurants or other buildings. Concentrate on eliminating larval breeding sites from drains in floors, sinks, wash basins, bathtubs, etc. Sometimes the source of the problem is a nearby filter plant, commercial cooling towers or mulch that is being regularly irrigated or is lying in a wet site.

Prevention

A black and white species of drain fly that holds its wings rooflike over the body.

To detect if flies are indeed coming from a drain, cover the opening during a down time with a clear plastic container that has one or more glue boards taped to the sides. Leave in place overnight or for a few days to catch flies. Trapping the flies from a drain will help eliminate the possibility that the flies are actually coming from some other location.

The most effective way to prevent drain fly infestations is to eliminate their breeding places. Inside buildings, this would include cleaning the drain pipes, drain traps and other plumbing system components in an attempt to eliminate the bacterial scum (gelatinous rotting, organic matter) that regularly forms on the surfaces of plumbing. This will eliminate the food source for developing larvae. Cleaning is best done by using an enzyme-action product that is designed to break the sticky bond that bacteria and algae form on the pipes (the so-called scum). There are several of these enzyme cleaners on the market, but they may not be widely available. Products may be purchased online or at a custodial supply company. The use of bleach does not seem to rid pipes of the scum, and the drain fly larvae are surprisingly resistant to bleach.

Alternative management methods include cleaning pipes and traps with a good, stiff, long-handled brush. It is best to remove the drain trap and use a "snake" with a brush tip in clogged drains to clean the pipes of all gelatinous material. If using mechanical means, flush lines with boiling water or caustic drain cleaners to remove any material left behind by the cleaning process. Caustic drain cleaners (such as lye-based cleaners) are not as effective as other means and must never be followed with bleach since chlorine gas can be released if the two mix in the drain line.

Clean dirty garbage containers and empty frequently. Look for wet lint under the washing machine, and empty standing water in containers under houseplant pots. Outside the home, inspect air conditioners, bird baths and above-ground swimming pools for standing water. Perform a water audit of any irrigation systems to ensure that they are not causing standing water issues or are keeping mulch constantly wet.

Pesticide Treatments

While there are numerous "flying insect" aerosols available on the market, these are only effective in knocking down active insects. If these are used, only use products that are registered for indoor use in and around sinks and drains. If the flies are occurring in a kitchen area, the product must state that it can be used in such an environment. These products will not kill the larvae and new flies will soon appear. "Bug bombs" (i.e., total-release aerosols) will only kill flying adults and they leave no residues. These products should not be used around food preparation areas.

There are no insecticides registered for use in drains or sewage systems, so do not simply pour an insecticide down a drain in the hopes that it will kill the larvae. Such action can cause major damage of sewer and sanitation systems. In most cases, the larvae seem to be resistant to such treatments.

Mechanical Systems

Fly paper strips and UV-light insect traps can capture some of the flies, but will not eliminate an infestation.

This fact sheet is a revision of the fact sheet "Drain Flies," originally written by William F. Lyon.

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