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Drain Flies (Moth Flies or Filter Flies)

  • Bayer CropScience, RTP, NC


3 pp., 2 illustrations Drain flies can be a common problem in homes. They live and reproduce in drains and septic tank field lines. The first step in controlling these pests is identifying the source of the infestation. Then there are cleaning products and insecticides that can control them.
Drain Flies
(Moth Flies or Filter Flies)
Chris Sansone, Rick Minzenmayer and Bastiaan M. Drees*
Small flies in the home can be a common
problem. Scientifically, the word “fly” refers
to insects in the order Diptera, which typi-
cally have only one pair of wings. Instead of a
second pair of wings these insects have thread-
like, knobbed structures called halteres. The
Diptera have complete metamorphosis (egg,
larva, pupa and adult) and the larvae of many
groups of flies are commonly called maggots.
Small flies that can infest homes include drain
flies, fruit flies, fungus gnats, mosquitoes and
midges. They can all enter from outdoors, but
fruit flies, fungus gnats and drain flies can repro-
duce indoors and survive there indefinitely. The
first step in controlling an infestation is to deter-
mine which of these pests is the problem.
Fruit flies are usually yellowish with clear or
lightly banded wings. They are most often found
around fruit and vegetable peelings, rotting fruits
and other similar foods. Eliminating the food
sources usually controls these flies. Fungus gnats
usually have dark wings and are not fuzzy in
appearance. Fungus gnat larvae feed on decaying
plants or fungi and usually develop in the pot-
ting media of houseplants. The easiest way to
control them is to let the houseplant soils dry
out between waterings because the larvae do not
survive in dry conditions.
Drain flies are also called moth flies because
of their fuzzy appearance. They are dark gray to
black and found near sinks and tubs. This fly
belongs to the family Psychodidae.
Biology and Habits
Adult drain flies are small (1/6to 1/5inch
long), dark, and densely covered with hairs.
They hold their large wings 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. They are most active in the
*Extension Entomologist, Extension Agent–Integrated Pest
Management, and Professor and Extension Entomologist,
The Texas A&M University System.
Adult drain fly.
Both mated and non-mated females lay eggs.
Egg masses contain 30 to 100 eggs, which hatch
in less than 48 hours into small (3/8inch), legless
larvae that are pale in the middle and darker on
the ends.
Drain fly larvae and pupae live in the thin
film found in drains and septic tank field lines
and on filter stones. The larvae feed on fungi,
bacteria, algae and other microorganisms found
in the liquid or slime layers that develop around
debris in drains, sewage treatment beds and
standing water. When food is scarce, they can
become cannibalistic. Larvae can survive temper-
ature extremes and habitats low in oxygen. The
larval stage lasts 8 to 24 days and the pupal stage
1 to 2 days.
Adults live only 3 to 4 days without food, but
can survive for 1 to 3 weeks if nectar or other
liquid carbohydrate foods are available.
Drain fly larvae are actually beneficial because
they break down organic waste into water-
soluble compounds. Adult flies, however, are a
nuisance. They cannot bite, but they may
become so numerous indoors that they congre-
gate at windows and around light fixtures, show-
ers, bathtubs, sinks and floor drains.
The key to solving a drain fly problem is to
find and eliminate the source—that is, the areas
where excess moisture and organic debris have
built up.
Outdoors, these areas may 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;
areas where algae or mold grows on the soil
or foundation; and
beneath air conditioning units on the roofs
of commercial buildings or where units
adjoin a building.
Infestations also may originate in a neighbor’s
yard or in nearby shallow pools or sewage treat-
ment facilities, particularly those upwind from
the home. If so, contact the people responsible
for infested areas.
Indoors, infestations may occur in:
toilets (particularly if they are not used fre-
sink and bathtub/shower drains;
floor drains in commercial buildings and
condensate lines for icemakers; and
loose ceramic floor tiles where water may
If there are lots of adult flies inside a drain the
drain is probably a breeding site. To check for
breeding sites, place a length of tape across
drains (or cracks in the floor) without totally cov-
ering the opening. (If the opening is totally cov-
ered, there will be no air flow and flies will not
emerge.) Check the tape periodically. If you see
flies stuck to the tape, you have found a source
of infestation.
Manual Cleaning
The most effective way to correct (or prevent)
drain fly problems is to clean toilets, drainpipes
and traps to eliminate any gelatinous, rotting,
organic matter. This eliminates the larval food
source. Clean pipes and traps with a stiff, long-
handled brush. After a thorough scrubbing,
flush the lines with boiling water to remove any
material left behind. There is no benefit to treat-
ing drains with chlorine bleach or ammonia.
Pupa Adult
Drain fly life cycle.
Chemical Drain Cleaners
There are bacterial drain treatments that
biodegrade organic matter. Follow the label
directions carefully for best results.
Many common drain and toilet bowl cleaners
are effective. If you have a septic system, read
product labels carefully to make sure a product
is compatible with the system.
Caustic drain cleaners also can be used,
although they may not be as effective as other
cleaning methods. IMPORTANT: NEVER rinse
a drain with bleach after using a caustic
drain cleaner. Mixing these two chemicals in
the drain line may produce chlorine gas, which
is extremely hazardous to anyone who inhales it.
Once the fly breeding areas are cleaned, the
use of insecticides should not be necessary.
However, if quick control is needed until breed-
ing areas can be cleaned, insecticides are useful.
The only product labeled for use in drains is
Gentrol®. Its active ingredient, hydroprene, acts
as an insect growth regulator on the larvae. This
product does not control the adult flies.
Aerosol sprays containing pyrethrins (or other
“flying insect” sprays) will temporarily control
adult drain flies indoors, but they are not a real
solution to the problem. If the drain flies are
coming from outside, applying any common
residual spray for flies around the outsides of
windows will give temporary relief until the
source of the infestation can be found and elimi-
nated. Always read the label and follow
directions and safety precautions.
Photos by Bastiaan M. Drees, Extension Entomologist, The Texas A&M University System
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