Drain flies are a common nuisance in/around homes and commercial buildings, including sewage treatment plants.
Adult flies may become so numerous indoors that they congregate at windows, around light fixtures, and around showers, bathtubs, sinks and floor drains.
They can annoy people sitting indoors or outdoors. Since these flies often originate in unsanitary conditions, there is the possibility of disease transmission that can affect human health.
Adult drain flies are small (1/6 to 1/5 inch long), fuzzy, dark colored insects with the body and wings densely covered with hairs. Their wings appear large for their body, are held roof-like over the body when at rest, giving them a moth-like appearance. They are weak fliers and fly only a few feet at a time. Larvae are legless, about 3/8 inch long, wormlike and gray, with both ends somewhat darker.
Drain flies breed in polluted, shallow water or highly moist organic solids. You will find the eggs, larvae and pupae in the muck, slime, or gelatinous film often accumulating on the sides of drains and condensate pipes for air conditioners, in the sewage filtration tanks, septic tanks and moist compost.
Other breeding sites include dirty garbage containers, rain barrels, and tree holes or in low lying areas adjacent to buildings where storm water collects and algae or mold grows. The female flies deposit laid eggs in irregular masses of 10 to 200 directly in/on moist organic debris. These eggs usually hatch in less than 2 days and the larvae feed on the decaying organic matter, microorganisms, algae and sediment in the debris and mature in 9 to 15 days. Larvae pupate in/on the surface of this material and new adults emerge in 1 to 2 days.
Correcting Drain Fly Problems
The key to solving a drain fly problem is to find and eliminate the source, i.e., find the areas of excess moisture and a buildup of organic debris.
Areas that are likely to be a source of drain fly infestations indoors include:
• Toilets (particularly if they are not used frequently). Be sure to check the toilet tank, as well.
• Sink and bathtub/shower drains
• Floor drains in commercial buildings and basements
• Condensation lines for icemakers
• Loose ceramic floor tiles where water may collect
The most effective method to correct (or to prevent) drain fly problems is to clean toilets, drain pipes and traps to eliminate any gelatinous rotting, organic matter, thereby eliminating the larval food source. Many of the commercially-available drain and toilet bowl cleaners can be used for this purpose. Many of these cleaners are biodegradable and pose less of a hazard to the environment. If you have a septic system, read the product labels carefully to make sure that the product is compatible with the system.
Alternative methods include cleaning pipes and traps with a good, stiff, long-handled brush. It is best to remove the drain trap and use a plumber's "snake" in clogged drains to dislodge the gelatinous material in the drains. If you use mechanical cleaning methods, you should also flush the lines with boiling water and bleach to remove any material left behind. Caustic drain cleaners may also be used, although they are not necessarily as effective as other cleaning methods.
IMPORTANT: NEVER rinse a drain with bleach after using a caustic drain cleaner. Mixing of these two chemicals in the drain line may produce chlorine gas, which is extremely hazardous to anyone who inhales it.