Drain flies (also known as
"moth flies" or "filter 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.
 Identification
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 mothlike 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.
Life Cycle and Habits
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 lowlying areas adjacent to buildings where stormwater 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
 Non-Chemical
Control
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.
Outdoors -
Areas that are likely
to be a source of moth fly infestations outdoors include:
-
Damaged
or faulty septic lines
-
Areas where
rainwater tends to pool or where air conditioning condensate lines
drain, particularly if the area is shaded and algae or mold is growing
on the soil or foundation.
- For commercial buildings
- check the area (usually a concrete or gravel covered) beneath air
condition cooling units on roofs or adjoining the building.
If a local source of the
flies cannot be found, it is possible that the flies are being carried
on wind currents from a nearby sewage treatment plant or another faulty
septic system.
Indoors -
 Areas
that are likely to be a source of moth 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
- Condensate lines for icemakers
- Loose ceramic floor tiles
where water may collect
One way to determine if the
flies are coming from a particular drain is to cover the drains with
glue boards (elevated on cardboard "collars" to keep them
from sticking to the floor or drain. There is also a simple method that
you can use at home. Take a translucent container (e.g., a margarine
container) and coat the inside with a thin layer of petroleum jelly.
Place the container upside down over the drain and leave it in place
overnight or for a few days, but check it daily. Emerging flies should
be trapped in the petroleum jelly. In commercial buildings, the source
of the problem may be more difficult to identify. Pest control companies
often use blacklight traps indoors to determine areas where the flies
are most numerous and then proceed with a thorough inspection of the
area.
The most effective method
to correct (or to prevent) drain fly problems is to clean toilets, drainpipes
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 (i.e., will not
disrupt the microbial action in the septic tank). Routine cleaning of
floor and sink drains is important particularly in commercial establishments.
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.
Chemical Control
Indoors, aerosol sprays of
pyrethrins (or other "flying insect" sprays) will kill adult
drain flies, giving temporary control, but they are not a real fix for
the problem. If the drain flies are coming from an outdoor source, exterior
applications of any common residual sprays for flies can be made around
windows for temporary relief until the source of the infestation
can be found and eliminated. Examples of such pesticides can be found
in the
NC Agricultural Chemicals Manual
.The insect growth regulator methoprene (Strike®)
is registered for use in municipal wastewater treatment facilties to
control midges and drain (filter) flies.
Always read the label and follow directions and safety precautions.
Some information in this publication
was taken from W. Lyon, Ohio State
University
Drain fly image - Jeff Hahn - University of Minnesota
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