Filter Flies Identification Guide

Filter Fly Identification Guide

Physical Characteristics

Filter flies are small insects measuring 2 to 5 millimeters in length, roughly the size of a grain of rice. Their most recognizable feature is their fuzzy, moth-like appearance caused by dense hair covering both their bodies and wings. This distinctive coating gives them a grayish, dusty look that often causes homeowners to mistake them for tiny moths.

When at rest, filter flies hold their broad, leaf-shaped wings in a roof-like position over their bodies. The wings display a pattern of parallel veins and are covered with microscopic hairs that give them a slightly iridescent quality under certain lighting. Their color ranges from light gray to tan or brown, sometimes with faint mottling on the wings.

Filter flies have prominent compound eyes and long, beaded antennae consisting of 13 to 14 segments, each covered with fine hairs. Their overall appearance is somewhat heart-shaped when viewed from above due to their wing positioning. Unlike many other flies, filter flies are weak, clumsy fliers that make short hopping flights rather than sustained flight.

Common Species

The most frequently encountered species in homes and buildings is Clogmia albipunctata, commonly known as the bathroom moth fly. This cosmopolitan species thrives in the warm, moist conditions found in bathroom and kitchen drains throughout North America.

Another common species is Psychoda alternata, known as the trickling filter fly. This species is particularly abundant around sewage treatment facilities and can become a significant nuisance in areas near wastewater treatment plants.

The family Psychodidae includes approximately 3,000 species worldwide, though only a handful commonly infest buildings. Unlike their tropical relatives in the subfamily Phlebotominae (sand flies), North American filter flies do not bite and pose no direct threat to human health.

Filter Fly Behavior and Biology

Filter flies undergo complete metamorphosis through four life stages: egg, larva, pupa, and adult. The entire life cycle takes 8 to 24 days depending on temperature, with warmer conditions accelerating development.

Female filter flies lay clusters of 30 to 100 eggs directly on the surface of organic film in drains or other breeding sites. The eggs are pale and slightly elongated, hatching within 32 to 48 hours under favorable conditions. Larvae emerge as tiny, translucent, legless grubs that immediately begin feeding on the surrounding organic matter.

The larval stage lasts 9 to 15 days as the larvae grow through four instars. Filter fly larvae are well-adapted to low-oxygen environments and can survive submersion in water. They feed on bacteria, fungi, algae, and decaying organic particles within the biofilm. Mature larvae are approximately 4 to 5 millimeters long with a slightly flattened body.

Pupation occurs within the same environment where larvae developed. The pupal stage is brief, lasting only 20 to 40 hours before adult flies emerge. Adults live approximately 14 days but may survive longer in ideal conditions. They feed minimally as adults, primarily on flower nectar, polluted water, or not at all.

Breeding Habitats

Filter flies earned their name from their abundance around trickling filter beds in wastewater treatment facilities. These biological treatment systems provide ideal breeding conditions with constant moisture and abundant organic matter. However, filter flies readily breed in numerous other environments:

• Bathroom sink, shower, and bathtub drains
• Kitchen sink drains and garbage disposal units
• Floor drains in basements, laundry rooms, and garages
• Air conditioning condensate lines and drip pans
• Sump pump basins
• Septic tank vents and damaged septic systems
• Sewage ejector pits
• Areas with water leaks behind walls or under floors
• Compost containers and organic waste bins

The organic biofilm that builds up inside drain pipes provides everything filter fly larvae need. This slimy layer consists of bacteria, soap scum, hair, grease, and other organic debris. Even a thin layer of this material can support filter fly reproduction.

Treatment Methods for Filter Flies

Effective filter fly control focuses on eliminating breeding sources rather than simply killing adult flies. Chemical insecticides provide only temporary relief because they do not address the underlying biofilm where larvae develop.

Locating Breeding Sites

Before treatment, identify all locations where filter flies are breeding. Place strips of clear tape over suspected drains overnight, leaving a small gap for air circulation. Sticky side down, the tape will capture any adult flies emerging from the drain. Check the tape each morning for several consecutive days, as emergence can be inconsistent.

Do not overlook less obvious breeding sites. Air conditioning systems, condensate pumps, and floor drains in utility areas are frequently missed. Water stains on walls or ceilings may indicate hidden moisture problems that harbor filter fly populations.

Mechanical Removal

The most effective treatment for filter flies is physical removal of the organic film where larvae develop. Use a stiff-bristled drain brush or pipe brush to scrub the interior surfaces of affected drains. Focus on the drain opening, the trap, and as far into the pipe as you can reach.

For garbage disposals, run the unit with ice cubes and coarse salt to scour interior surfaces. Follow with hot water to flush loosened debris. Bathroom drains benefit from removing the stopper mechanism and cleaning accumulated hair and residue.

Floor drains require removing the grate and scrubbing the trap and visible pipe surfaces. A wet/dry vacuum can help remove sludge from the trap before scrubbing.

Biological Drain Treatments

Enzymatic and bacterial drain cleaners complement mechanical cleaning by breaking down organic matter within pipes. These products introduce beneficial microorganisms that consume the organic film filter fly larvae depend on. Apply according to product directions, typically in the evening when drains are least used.

Biological treatments work gradually over several days and are most effective as ongoing maintenance after initial mechanical cleaning. They are safe for plumbing and septic systems when used as directed.

Avoid relying on bleach or caustic chemical drain cleaners as primary treatments. While they may kill some larvae on contact, they do not effectively penetrate or remove the gelatinous biofilm. Repeated use of harsh chemicals can also damage pipes and disrupt beneficial bacteria in septic systems.

Preventing Reinfestation

After eliminating an active infestation, implement ongoing practices to prevent filter flies from returning:

• Run water through all drains at least once weekly, including basement floor drains and guest bathroom fixtures
• Clean drain stoppers, strainers, and overflow openings regularly
• Apply enzymatic drain treatments monthly as preventive maintenance
• Repair plumbing leaks promptly and address condensation issues
• Ensure air conditioning drip pans drain completely and clean them periodically
• Keep floor drain traps filled with water to maintain the seal against sewer gases and flies

Professional Assessment

Filter fly infestations that persist after thorough drain cleaning may indicate breeding sources in inaccessible locations. Cracked sewer lines, damaged septic components, or buildup deep within main drain lines may require professional inspection with a sewer camera to locate and address. Moisture problems within walls or beneath floors often need professional remediation to resolve completely.

References

• Drain Flies - NC State Extension
• Drain Flies or Moth Flies - University of Kentucky Entomology
• Moth Flies in the Home - Penn State Extension
• Moth or Drain Flies - UC IPM