Sewer Flies Identification Guide
Psychodidae (Family)
Sewer flies are small, fuzzy, moth-like insects that breed in the organic sludge found in drains, sewage systems, and septic tanks. Also called drain flies or moth flies, they commonly emerge from bathroom and kitchen plumbing.
Taxonomy
Sewer Flies Coloration
Common color patterns to help identify sewer flies
Sewer Flies
Seasonal Activity
When sewer flies are most active throughout the year
Where Sewer Flies Are Found
Hover over states to see their names. Green regions indicate where sewer flies have been reported.
Sewer Fly Identification Guide
Physical Characteristics
Sewer flies are small insects about 2 to 5 millimeters long. That is roughly the size of a gnat. Their most notable feature is their fuzzy, moth-like look. Dense hair covers both their wings and bodies. This texture makes them easy to spot once you know what to look for.
When resting, sewer flies hold their broad wings in a roof-like position over their bodies. Many people mistake them for tiny moths because of their fuzzy coating and wing shape. Their color ranges from pale gray to tan or brownish-gray. They often have mottled patterns on the wings. They also have long, beaded antennae covered with fine hairs.
Sewer flies are weak fliers. They make short, hopping flights of just a few inches. Most of their time is spent resting on walls, ceilings, and surfaces near where they breed. When disturbed, they fly weakly and land again nearby.
Common Species
The species most often found in North American homes is Clogmia albipunctata. It is often called the bathroom moth fly or filter fly. This species does well in the humid conditions inside home and building plumbing systems.
Other species in the Psychodidae family include Psychoda alternata and Psychoda cinerea. These have similar habits and looks. The drain flies found in homes do not bite. They should not be confused with sand flies, which are biting flies found mainly in tropical regions.
Sewer Fly Behavior and Biology
Life Cycle
Sewer flies complete their life cycle in 8 to 24 days. Warmer temperatures speed up their growth. Females lay 30 to 100 eggs in or near the slimy film that lines drain pipes. These eggs hatch within 32 to 48 hours in good conditions.
The larvae are small, legless, and flat with a pale, see-through look. They live within the organic slime layer. They feed on bacteria, fungi, algae, and other tiny organisms found in drain buildup. This larval stage lasts 9 to 15 days. The larvae then form a pupa for 20 to 40 hours before becoming adults.
Adult sewer flies live for about two weeks. During this time, they mate. Females look for good spots to lay eggs, which keeps the infestation going. Adults eat very little. They mainly consume nectar, honeydew, and polluted water.
Habitat Preferences
Sewer flies live in moist areas with decaying organic matter. Their favorite spots include:
- Bathroom sink, shower, and bathtub drains
- Kitchen sink drains and garbage disposals
- Floor drains in basements, laundry rooms, and garages
- Sewer lines and septic tank areas
- Condensate drains and drip pans from HVAC systems
- Sump pump pits
- Sewage treatment plants
- Any area where organic sludge builds up in standing water
The key factor in all these spots is organic buildup where larvae can feed and grow. This film is made of bacteria, soap scum, hair, food bits, and other debris that coats pipe surfaces over time.
Activity Patterns
Sewer flies are most active in the evening and at night. During the day, they rest in dark, damp areas near where they breed. They are drawn to light. You will often see them near windows or light fixtures in bathrooms and kitchens at night.
Indoor populations can stay active all year. This is because drains stay warm and humid no matter the outdoor weather. But you may notice more of them in warmer months. That is when they grow faster and their numbers can build up quickly.
Treatment Methods for Sewer Flies
Getting rid of sewer flies means removing their breeding sites. Simply killing the adults you see will not solve the problem. The following methods can help with sewer fly issues.
Locating Breeding Sites
First, find all active breeding spots. The tape test works best. Place a strip of clear packing tape over suspected drains at night. Leave a small gap for air. Check the tape each morning for trapped flies. Repeat for a few nights since flies may not emerge every night.
Common breeding sites include air conditioning drip pans, sump pumps, broken tile grout around tubs, and areas with water damage behind walls. Any spot with moist organic buildup can hold sewer flies.
Mechanical Cleaning
Removing the organic film by hand is the most important step. Chemical treatments alone will not work. You need to scrub.
Use a stiff drain brush or pipe brush to scrub inside the drain pipes. Focus on the drain opening and the first few inches of pipe where most buildup occurs. For garbage disposals, run ice cubes and coarse salt through the unit to scour the inside.
Remove floor drain covers and scrub the trap and visible pipe surfaces. Flush all cleaned drains with hot water after scrubbing.
Biological Drain Treatments
Enzymatic and bacterial drain cleaners can help break down organic residue in areas you cannot reach. These products contain microorganisms that eat the organic matter sewer fly larvae need.
Use these treatments as the label directs. Apply them at night when drains are not in use. They work slowly over several days. They work best as upkeep after you have already done the scrubbing.
Environmental Modifications
To prevent sewer fly breeding, take these steps:
- Run water through all drains weekly, especially ones you rarely use
- Fix plumbing leaks right away
- Make sure floor drains have working water traps
- Clean HVAC drip pans and ensure they drain well
- Fix moisture problems in basements and crawl spaces
- Keep drain stoppers and strainers clean
Professional Assessment
If sewer flies remain after you clean the drains well, the breeding source may be hard to reach. Broken sewer lines, cracked septic tanks, damaged vent stacks, or buildup in main drain lines may need a plumber with a camera scope to find and fix. Ongoing moisture problems from hidden leaks may also need professional help.
References
Other Flies
Explore other species in the flies family
Commonly Confused With
Sewer Flies are often mistaken for these similar pests
Where Sewer Flies Are Found
Hover over states to see their names. Green regions indicate where sewer flies have been reported.
Common Questions about Sewer Flies
Why are sewer flies coming up from my drains?
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Sewer flies breed in the organic film and sludge that accumulates inside drain pipes, P-traps, and sewage systems. They emerge as adults through drain openings seeking mates. If you see them near a specific drain, that drain likely has significant organic buildup providing a breeding habitat.
Are sewer flies a sign of a sewage problem?
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Not necessarily. While sewer flies can indicate issues like broken sewer lines or septic problems, they most commonly breed in normal household drain buildup. However, a sudden large infestation or flies appearing in unusual locations may warrant a plumbing inspection.
Can sewer flies make you sick?
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Sewer flies do not bite and are not known to transmit diseases to humans. They are primarily a nuisance pest. However, their bodies and wings can become airborne allergens that may trigger reactions in sensitive individuals, especially in cases of heavy infestations.
What is the difference between sewer flies and drain flies?
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Sewer flies and drain flies are the same insect. The names are used interchangeably. They are also called moth flies, filter flies, or sewage gnats. All these names refer to members of the family Psychodidae that breed in organic waste in plumbing systems.
Why do I have sewer flies but no clogged drains?
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Sewer flies do not require a clogged drain to breed. They need only a thin layer of organic film on pipe surfaces, which can build up even in normally functioning drains. Slow drains, infrequently used drains, and drains without proper P-traps are especially susceptible.
Will bleach kill sewer flies in my drains?
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Bleach can kill adult sewer flies and some larvae on contact, but it does not effectively remove the organic biofilm where eggs and larvae develop. The biofilm protects the developing insects, so mechanical cleaning is necessary for lasting control.
How long does it take to get rid of sewer flies?
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With proper treatment addressing all breeding sites, you should see improvement within one to two weeks. Complete elimination typically takes two to three weeks as remaining eggs and larvae develop into adults. Persistent infestations may indicate untreated breeding sites.
Can sewer flies come from the toilet?
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Sewer flies rarely breed in regularly used toilets because flushing removes organic matter. However, they can emerge from toilets with faulty wax seals, unused toilets where water has evaporated from the trap, or from floor drains near toilet installations.
With five years of hands-on experience in the pest control industry, George Schulz is a registered technician with the Virginia Pest Management Association and a proud third-generation professional in a family business that's been protecting homes for over 57 years. He manages and trains a team of service pros while also leading internal research efforts—recently spearheading a deep-dive review of thousands of documents on pest control materials to hand-pick the most kid and pet friendly, most effective solutions tailored specifically for homes in the DC metro area.