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Drain Flies

Also commonly known as moth flies or filter flies, drain flies are a common household pest.

Small flies at only 1/6 to 1/5 of an inch long drain fly adults are covered in thick dark hairs and are typically dark gray to black in color.  They hold their single pair of wings high above their body when at rest.  Primarily active in the evenings, drain flies are only capable of flying short distances of only a few feet in a single attempt.  Drain flies live both on the exterior and interior and can reproduce and survive indefinitely indoors as long as food supplies are plentiful.

Indoor Fly Infestations

Indoor infestations typically occur in or around toilets (especially those that are infrequently used), condensate lines for icemakers including those on home refrigerators, and any areas containing drains such as sinks, bathtubs, showers, floor drains and appliance drains.  Infestations can however occur in any area where moisture and decaying organic material are present.  Adults only survive a matter of a few days without food, unless nectar or a similar liquid substance is available in which they may survive 1 to 3 weeks.  Adults DO NOT bite and are more of a nuisance than anything.  Adult drain flies utilize drains as breeding sites.  Egg masses contain 30 to 100 eggs and have a hatch time of fewer than 48 hours.

Emerging larvae are legless, roughly 3/8 of an inch in length, and darker at each end than they are in their center.  Larvae and pupae live in the slime layer in drains, septic tank lines and on filter stones.  They feed on a variety of microorganisms, fungi, bacteria, and algae.  When food supplies are decreased they will often feed on each other as well.  Their ability to survive environments low in oxygen and extreme temperatures make it difficult to drown larvae even with boiling water being poured into the drain.  One benefit to drain fly larvae is that they break down organic wastes found within the drains.  This benefit is rarely enough to convince residents to allow the infestation to continue, however.   Larval stages last 8 to 24 days followed by a pupal stage lasting a mere 24-48 hours.

Drain Infestations

Eliminating a drain fly infestation starts with identifying the source(s) of the infestation (breeding and feeding sites).  Drains can be confirmed or eliminated as a breeding source by completely covering the drain with tape and monitoring the tape for the presence of adult flies.  Seeing flies stuck to the tape indicates that the drain is a breeding ground for the drain flies.  Drains should be regularly cleaned to remove the slime layer in which drain flies breed and feed.

Manual cleaning using a stiff long-handled brush will aid in eliminating the organic matter from within the drain.  Boiling water can be used as a rinsing agent for this type of cleaning.  Bacterial drain treatments are available over-the-counter to biodegrade organic matter, although most common drain and toilet cleaners will work.

Make sure that chemical agents are compatible with your pipes and waste system prior to application.  The use of ammonia and/or chlorine bleach not only has no benefit it can when mixed with caustic cleaners can form chlorine gas or other toxins that are extremely hazardous upon inhalation.  Insecticides can typically be avoided in remedying a drain fly infestation simply by cleaning the drains.  In the case of extremely high infestations, consult a pest control professional to help successfully remove all traces of Drainfly activity.