Drain Flies (Moth Flies)
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.
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 fly resting on side of wall. This is one of the common gray
forms that holds its wings flat.
Drain flies do not bite humans but may become a nuisance by their presence in large populations. Persistent
Drain Flies (Moth Flies)—page 2
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.
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.
A black and white species of drain fly that holds its wings rooflike over the body.
This fact sheet is a revision of the fact sheet “Drain
Flies,” originally written by William F. Lyon.
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