Drain Flies

Pest Expert Overview: Drain Flies: Fungus Gnats, Fruit Flies, and Moth Flies in Bryan & College Station

“Drain fly” is one of the most commonly misused names in pest control.

Customers frequently use the term for any tiny fly found around a sink, shower, or floor drain. Unfortunately, pest management professionals often do the same thing.

Drain fly is not a useful identification. Several unrelated groups of small flies may be found around drains, and the correct method of eliminating them depends on which fly is present and where its larvae are actually developing.

Indoor gnats and other small flies are usually one of three types:

• Fungus gnats
• Fruit flies
• Moth flies

All three are in the Order Diptera, but they belong to different families and have different appearances, behaviors, feeding habits, and preferred breeding materials.

For all three groups, control is achieved by locating and removing the larval food source.

A fly resting beside a drain did not necessarily come from that drain. Kitchens and bathrooms provide moisture, light, food odors, and convenient places for adult flies to collect. Identifying the adult and locating the larval breeding site are much more important than where the fly happened to land.

Spraying the adults or pouring something down every drain may temporarily reduce the number of flies being seen. It will not solve the problem if they are developing in potting soil, fermenting fruit, food hidden by a child, a leaking sewer line, or another overlooked source.

Identifying Small Flies Found Around Drains

Fungus gnats, fruit flies, and moth flies are all small, but they do not look or behave alike when examined carefully.

Fungus Gnats

Fungus gnats are slender, dark  colored flies with long legs and antennae. They resemble tiny mosquitoes and are usually about 1/8 inch long or smaller.

Adults are weak flyers and frequently run or skip across a surface before making a short flight. This skipping behavior is one of the quickest ways to distinguish them from fruit flies.

Fungus gnats are in the Family Sciaridae. Several species may occur indoors, and identifying the exact species is rarely necessary for residential control.

Their larvae are slender and nearly transparent, with a distinct shiny black head. They normally develop in moist soil, fungi, algae, and decomposing plant material.

Fruit Flies

The flies commonly called fruit flies inside homes are more accurately called vinegar flies. Most belong to the Family Drosophilidae.

Adult fruit flies are approximately 1/8 inch long and have compact, rounded bodies rather than the slender, long  legged appearance of fungus gnats. They are usually tan, yellowish-brown, or brown. Many common species have bright red eyes, although eye color is not reliable for every species.

Fruit flies are stronger and more active flyers than fungus gnats or moth flies. When disturbed, they normally take off and fly rather than running or skipping across the surface first.

They often hover around fruit, garbage containers, recycling bins, beverage areas, sinks, and food preparation surfaces.

Their larvae are small, cream  colored maggots that develop in moist, fermenting organic material.

These household fruit flies should not be confused with the larger agricultural fruit flies that infest developing fruit on plants.

Moth Flies

Moth flies are the insects most accurately described as drain flies, although even moth flies do not always originate in a drain.

They are in the Family Psychodidae and are also called filter flies, sewage flies, or sewer flies.

Adults are approximately 1/8 inch long and have broad, hairy wings. Their bodies and wings are covered with fine hairs, giving them the appearance of tiny moths.

When resting, the wings are usually held roof  like over the body. Moth flies are weak flyers and often make short, hopping flights before landing on a nearby wall.

They are commonly found resting on bathroom walls, shower stalls, kitchen cabinets, or surfaces near floor drains.

Moth fly larvae develop in wet accumulations of decaying organic material. Inside buildings, they may be associated with the gelatinous film that forms inside drains, sewer lines, grease traps, condensate lines, and other persistently wet areas.

Capturing Small Flies for Identification

Capturing one of the flies on a piece of clear Scotch tape is often the easiest way to have it identified.

Try not to crush the insect. A clear photograph taken through the tape may provide enough detail for us to determine whether it is a fungus gnat, fruit fly, moth fly, or another type of small fly.

Customers can email or text the photograph to Venus Pest Company.

Life Cycle

Fungus gnats, fruit flies, and moth flies undergo complete metamorphosis and develop through four stages: egg, larva, pupa, and adult.

The adults are the stage noticed flying through the house, but the larvae are the stage that determines whether the problem continues.

Females lay eggs in moist material suitable for larval development. Depending on the fly, this may include wet potting soil, fermenting fruit, drain buildup, sewage, fungi, algae, or decomposing organic debris.

The larvae feed and develop within that material. Mature larvae eventually pupate, and new adults emerge.

Under warm indoor conditions, some species can complete development in only a week or two. This allows a small amount of overlooked organic material to produce flies continuously.

Killing adult flies does not remove the eggs, larvae, or pupae already present in the breeding site. Successful control requires finding and correcting the material supporting larval development.

Where Are the Flies Coming From?

The location where adult flies are most visible provides a clue, but it does not prove where they are breeding.

Adults are attracted to light and often collect around windows. They also rest near sinks and drains because those areas are humid and provide water.

The most likely breeding sites differ among the three groups.

Fungus Gnat Breeding Sites

Fungus gnats are most commonly associated with overwatered potted plants.

Their larvae feed primarily on fungi and decomposing organic material in moist soil. Overwatering keeps the soil wet and encourages the fungal growth on which the larvae feed.

Common sources include:

• Overwatered houseplants
• Potting soil that remains continually wet
• Water standing in plant saucers
• Root rot and decomposing plant roots
• Damp bags of potting soil
• Moss poles and decorative plant material
• Organic debris beneath plant containers
• Moisture beneath sinks or appliances
• Water  damaged walls, flooring, or cabinets
• Accumulations of fungi or algae

A large potted plant may produce hundreds of fungus gnats while the adults are noticed primarily around the nearest sink, window, or computer screen.

When weather conditions are suitable for the plant, affected plants can be moved outside for several days. Outdoor airflow and warmer conditions often allow the soil to dry more quickly, which helps interrupt fungus gnat development.

The plants can be brought back indoors after the soil has dried and adult activity has declined.

Moving a plant outside will provide only temporary relief if it is immediately returned to the same watering schedule. Reduce watering and allow the upper portion of the soil to dry between waterings whenever the plant will tolerate it.

Fungus gnats do not always originate in houseplants. Forgotten food, damp organic debris, plumbing leaks, and damaged sewer lines containing soil, roots, or fungal growth can occasionally support them.

Fruit Fly Breeding Sites

Fruit flies can develop in a surprisingly small amount of fermenting food. The source may be anywhere inside the house and sometimes immediately outside it.

Common sources include:

• Overripe or damaged fruit
• Potatoes and onions stored in cabinets
• Forgotten trash containers
• Food hidden by children
• Old food inside lunch boxes or ice chests
• Open food containers
• Juice and soft  drink spills
• Empty beer, wine, and soda containers
• Recycling bins
• Garbage disposals
• Mop heads and cleaning rags
• Food or liquid beneath appliances
• Residue beneath garbage  container liners
• Dirty beverage dispensers
• Bar drains and soda equipment

Fruit flies can develop in organic material inside drains, but drains are not typically the original source in residential infestations.

A single rotting potato beneath a pantry shelf can produce a substantial infestation. So can an old piece of fruit in a lunch box, food hidden in a bedroom, or a small amount of juice that leaked beneath a refrigerator.

During wet years, outdoor fruit fly populations may also become unusually high. Adults enter through doors and small openings and then collect around sinks and drains because those areas provide moisture.

Their presence beside a drain does not prove they developed there.

Moth Fly Breeding Sites

Moth flies are the group most consistently associated with drains and sewage.

Their larvae develop in wet, decaying organic material, including the gelatinous film that accumulates on the sides of drains and sewer pipes.

Common sources include:

• Sink drains
• Shower and bathtub drains
• Floor drains
• Garbage disposals
• Grease traps
• Sewage ejector pits
• Condensate lines and drain pans
• Sump pits
• Septic systems
• Wet mop areas
• Sewage treatment equipment
• Clogged or damaged sewer lines
• Plumbing leaks beneath slabs or inside walls
• Sewage  contaminated soil beneath a building

Moth fly larvae do not simply live in clean water. They develop in wet organic material attached to a surface.

A small number may develop in an infrequently cleaned drain. Persistent or heavy moth fly activity should receive more attention.

In our experience, a substantial moth fly infestation frequently indicates a sewage leak, damaged sewer line, failed plumbing connection, or another concealed accumulation of sewage. This is particularly likely when moth flies continue after accessible drains have been thoroughly cleaned or when they appear in an area without an obvious drain.

Pesticide applications cannot correct these conditions. The plumbing defect must be located and repaired, and sewage  contaminated material may need to be removed.

Confirming Whether a Drain Is the Source

A simple test can help determine whether flies are emerging from a particular drain.

Place a clear cup or container upside down over the drain without completely blocking airflow. A clear plastic bag may also be positioned over the opening.

Check the covering the following morning. Flies trapped beneath it indicate that the drain is probably one of the breeding sources.

Do not simply place a piece of sticky tape across the drain opening. Flies approaching from elsewhere in the room may land on the tape, making it difficult to determine whether they emerged from below.

The test may need to remain in place for several nights. A negative result from one night does not conclusively eliminate the drain, particularly when adult emergence is intermittent.

Each drain should be tested separately. Treating every drain without determining which one is producing flies creates work but does not necessarily provide useful information.

Why They are Pests?

Fungus gnats, fruit flies, and moth flies do not normally bite or damage the structure of a home. Most residential infestations are primarily a nuisance.

Their presence can still be important because it indicates that moist, decomposing organic material is present somewhere in or around the building.

Fruit flies can move between garbage, fermenting material, food, and food  contact surfaces. They are particularly concerning in restaurants, bars, kitchens, and other facilities where food is prepared or served.

Moth flies develop in drains, sewage, and other contaminated materials. Although they are not normally considered a major disease threat in homes, flies emerging from unsanitary material should not be allowed to collect in kitchens or food  handling areas.

Fungus gnat larvae primarily feed on fungi and decomposing organic material, but they may also feed on fine plant roots. Large populations can injure seedlings, young plants, or plants already weakened by excessive moisture and root disease.

The insects may be less important than the condition producing them. Persistent flies can indicate overwatered plants, leaking plumbing, poor drainage, sewage contamination, or hidden food residue.

A continuing infestation should not be dismissed as a few harmless bugs around the sink. The real problem is determining what is allowing them to reproduce.

Drain Fly Prevention and Control 

Fungus Gnat Control

The first step in fungus gnat control is inspecting every potted plant in the affected area.

Watch for adults running or skipping across the soil or flying upward when a pot is moved. Yellow sticky cards placed near the soil can help identify active plants and capture adults.

When weather conditions permit, move affected plants outside for several days. The increased airflow and warmer conditions may dry the soil more quickly and help reduce the infestation.

Additional control steps include:

• Reduce how often affected plants are watered.
• Allow the upper portion of the soil to dry between waterings.
• Empty water from plant saucers.
• Improve drainage through the potting medium.
• Remove dead leaves and decomposing plant material.
• Replace badly decomposed or waterlogged soil.
• Correct leaking irrigation devices.
• Discard severely damaged plants when appropriate.
• Store unused potting soil where it will remain dry.

Products containing Bacillus thuringiensis israelensis, commonly abbreviated Bti, may be applied to potting media according to label directions. Bti targets fly larvae and can be useful when drying the soil alone is insufficient.

Sticky traps capture adults and help monitor activity. They will not eliminate larvae developing in the soil.

Spraying adult fungus gnats flying around a window provides little long  term benefit if the plants remain overwatered.

When no houseplant source is found, inspect for plumbing leaks, water  damaged materials, damp crawl spaces, forgotten food, and damaged sewer lines that may contain soil or roots.

Fruit Fly Control

Fruit fly control begins with finding every source of fermenting organic material.

Discard or refrigerate overripe fruit. Inspect potatoes, onions, tomatoes, and other produce stored outside the refrigerator.

Look inside trash containers, lunch boxes, ice chests, cabinets, bedrooms, and other areas where food may have been forgotten.

Clean beneath and behind appliances rather than limiting sanitation to visible countertops.

Additional control steps include:

• Empty and clean garbage containers.
• Rinse cans and bottles before placing them in recycling.
• Clean beneath garbage  container liners.
• Wash mop heads, rags, and floor  cleaning equipment.
• Remove residue from garbage disposals.
• Clean beverage machines, drip trays, and bar equipment.
• Repair leaking syrup, juice, or beer lines.
• Remove food debris from cracks and beneath cabinets.
• Keep compost containers sealed and empty them regularly.

Fruit fly traps can capture adults and reduce the immediate nuisance. Venus Pest Company offers traps that may help reduce the remaining adults once the source has been identified and removed.

Commercial traps and homemade vinegar traps work because adult flies are attracted to fermentation odors.

Traps do not eliminate the infestation if fermenting material remains available elsewhere. In a heavily infested kitchen, a trap may compete poorly with the actual breeding source.

Adult sprays provide only temporary relief and should not be used as a substitute for sanitation.

After the source is removed, adult fruit flies may remain for several days. The population should decline quickly unless another breeding site was overlooked or additional flies are entering from outdoors.

Moth Fly Control

Moth fly control requires removing the wet organic film in which the larvae are developing.

Begin by confirming which drain or plumbing fixture is producing flies. Inspect beneath removable drain covers and use a stiff brush to physically scrub the interior surfaces.

Cleaning should reach as far into the drain as practical, including around:

• Drain covers
• Strainers
• Overflows
• Garbage  disposal splash guards
• Floor  drain bowls
• The upper portion of drainpipes
• Cracks and joints that collect organic material

Foaming or microbial drain  cleaning products may help break down organic buildup when used according to label directions. A product that coats the sides of the drain is generally more useful than a liquid that immediately runs through the middle of the pipe.

Bleach, boiling water, and ordinary chemical drain openers are not dependable moth fly treatments. They may kill some exposed larvae but frequently fail to remove the organic material supporting development.

Do not mix bleach, drain cleaners, acids, ammonia, or other cleaning chemicals. Dangerous gases and chemical reactions can result.

Drains that are rarely used should be flushed periodically to maintain water in the plumbing trap. However, adding water will not eliminate an infestation developing in organic buildup.

Continued moth fly activity after accessible drains have been cleaned may indicate:

• A cracked drainpipe
• A broken sewer line
• A dry or damaged plumbing trap
• A leaking wax ring beneath a toilet
• Sewage beneath a slab
• A leaking condensate system
• An inaccessible accumulation inside commercial equipment

These problems require plumbing or mechanical correction. Pest control products cannot repair a sewer line or remove sewage beneath a building.

Preventing Small Fly Problems

Prevention depends on removing the conditions needed by the larvae.

To reduce future problems:

• Avoid overwatering indoor plants.
• Keep drains and garbage disposals free of organic buildup.
• Refrigerate ripe fruit when practical.
• Inspect stored potatoes, onions, and produce regularly.
• Clean recycling and garbage containers.
• Empty lunch boxes and ice chests after use.
• Repair plumbing and condensate leaks promptly.
• Remove food and beverage spills beneath equipment.
• Keep mop heads and cleaning tools clean and dry.
• Maintain floor drains and plumbing traps.
• Inspect seldom  used rooms and fixtures for moisture.
• Correct sewer and drainage problems rather than repeatedly treating the flies.

Good sanitation means more than wiping visible surfaces. Fly larvae frequently develop underneath, inside, or behind the areas that receive routine cleaning.

Professional Drain Fly Control

Calling all small flies “drain flies” leads to a lot of unnecessary drain treatments.

Fungus gnats may be developing in an overwatered plant. Fruit flies may be coming from a forgotten potato, an old lunch box, food hidden by a child, or an outdoor population during a wet year. Moth flies may indicate organic buildup inside a drain or a concealed sewage leak.

The adult fly should be identified before the breeding source can be located efficiently.

Capture a fly on clear Scotch tape and send us a close photograph whenever possible. A professional inspection can then focus on the most likely sources rather than treating every drain and hoping the flies disappear.

For all three groups, control is achieved by removing the material on which the larvae are feeding. Adult sprays and traps may temporarily reduce the flies you see, but they do not eliminate an active breeding source.

If you are dealing with fungus gnats, fruit flies, moth flies, or another small fly problem, Venus Pest Company can help. Founded by Aggie Entomologist Dr. Scott Lingren, we provide expert, eco-friendly Integrated Pest Management solutions in Bryan and College Station. To get started, fill out the contact form, call/text 979.774.0486 or email [email protected]. Don’t let nuisance pests take over your home!