L-2037 6/03

Drain Flies (Moth Flies or Filter Flies)

Eventually, everyone has to deal with small flies (0.2 in or 5 mm) in the home. From a scientific viewpoint the word “fly” refers to insects in the order Diptera which typically have only one pair of wings. The second pair of wings have been reduced to threadlike, knobbed structures called halteres. The Diptera have complete metamorphosis (egg, larva, pupa and adult) and the larva of many groups of flies is commonly referred to as a maggot.

Because identification is the first step in any pest control strategy, pest managers should make sure the small fly causing problems is a drain fly. Other common small flies that survive well indoors are fruit flies and fungus gnats. Other small flies such as mosquitoes and midges can also occur indoors but are usually result of infestations from the outside.

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 sources. Elimination of the food sources will usually control these flies.

Fungus gnats typically have dark wings but do not have the fuzzy appearance of drain flies or moth flies. Fungus gnat larvae (immatures) feed on decaying plant or fungal materials and usually develop in the potting media (soil) of houseplants. The easiest way to control the fungus gnat is to let the houseplant soils dry out between watering because the larvae do not survive dry conditions.

Drain flies, also called moth flies because of their fuzzy appearance, are dark gray to black in color and found near sinks and tubs. The moth fly or drain fly belongs to the family Psychodidae.

Biology and Habits

Many species of psychodid flies are in Texas, so only a general description of their life cycle is given.

Adult drain flies are small (1/6 to 1/5 inch long), fuzzy, dark colored insects with the body and wings densely covered with hairs. Their wings appear large for their body and are held 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. The flies are most active in the evening.

Both mated and non-mated females are capable of laying eggs. Drain flies lay eggs in a mass of 30 to 100. These eggs hatch in less than 48 hours into small pale-colored larvae. Larvae are legless, about 3/8 inch long, wormlike and gray, with both ends somewhat darker than the middle. The larvae and pupae of the drain fly live in the thin film found in drains, septic tank field lines or filter stones. The larvae have chewing mouthparts and feed on fungi, bacteria, algae and other microorganisms present in moist, decaying organic matter. Larvae feed on the liquid or slime layers which develop around debris in drains, sewage treatment beds and standing water. When food is scarce, they can become cannibalistic. The larvae is the most resistant life stage, being capable of surviving temperature extremes and habitats often low in oxygen. The larval stage occurs for 8 to 24 days and the pupal stage occurs for 1 to 2 days. Adults live only 3 to 4 days without food, but they can survive for 1 to 3 weeks if nectar or other liquid carbohydrate food sources are available.

Pest Status and Control

The immature stage (larva) is considered beneficial since the larvae break down organic waste into water-soluble compounds. Adult flies are a nuisance since they cannot bit. The flies may become so numerous indoors that they congregate at windows, around light fixtures, and around showers, bathtubs, sinks, and floor drains. Because these flies often originate in unsanitary conditions, a possibility of disease transmission exists. However, no evidence of transmission has been found.

The key to solving a drain fly problem is to find and eliminate the source, i.e., find the areas of excess moisture and a buildup of organic debris.

Outdoors

• Damaged or faulty septic lines
• Areas where rainwater tends to pool or where air conditioning condensate lines drain, particularly if the area is shaded and algae or mold is growing on the soil or foundation.
• For commercial buildings check beneath air condition cooling units on roofs or adjoining a building.

Infestations can also be located in a neighbor's yard, shallow pools or nearby sewage treatment facilities, particularly those upwind from the nuisance area. In these cases, contact those people responsible for infested areas.

Indoors

• Toilets (particularly if they are not used frequently)
• Sink and bathtub/shower drains
• Floor drains in commercial buildings and basements
• Condensate lines for icemakers
• Loose ceramic floor tiles where water may collect

The presence of many adult flies inside a drain is a good sign that the drain is a breeding site. To check for possible breeding sites, place a length of tape across drains (or cracks in the floor) without totally covering the opening. If the opening is totally covered, there will be no air flow and flies will not emerge. Check the tape periodically. If flies are found stuck to your tape, you have found a source.

Mechanical Control The most effective method to correct (or to prevent) drain fly problems is to clean toilets, drainpipes and traps to eliminate any gelatinous rotting, organic matter, thereby eliminating the larval food source. The most effective method is cleaning pipes and traps with a good, stiff, long-handled brush. After a thorough scrubbing, the lines should be flushed with boiling water to remove any material left behind. There is no benefit to treating drains with chlorine bleach or ammonia. Drain Cleaners Another possibility is the use of a bacterial drain treatment that biodegrades the organic matter. Follow the label directions carefully for best results. Many of the commercially-available drain and toilet bowl cleaners can be used to clean drainage areas. If you have a septic system, read the product labels carefully to make sure that the product is compatible with the system.

Caustic drain cleaners may also be used, although they are not necessarily as effective as other cleaning methods. IMPORTANT: NEVER rinse a drain with bleach after using a caustic drain cleaner. Mixing of these two chemicals in the drain line may produce chlorine gas, which is extremely hazardous to anyone who inhales it.

Insecticides
Once the breeding areas are cleaned, the use of insecticides should not be necessary. However, conditions may exist where a quick knockdown of larval or adult fly problems is needed. Insecticides provide only temporary relief.

The only product labeled for use in drains is Gentrol®. This product contains hydroprene that acts as an insect growth regulator on the larvae. This product does not control the adult flies.

Indoors, aerosol sprays of pyrethrins ( or other “flying insect” sprays) will kill adult drain flies, giving temporary control, but they are not a real fix for the problem. If the drain flies are coming from an outdoor source, exterior applications of any common residual sprays for flies can be made around windows for temporary relief until the source of the infestation can be found and eliminated. Always read the label and follow directions and safety precautions.

Chris Sansone
Extension Entomologist
and Rick Minzenmayer
Extension Agent
Texas Cooperative Extension

Photo Credits:
Bastiaan M. Drees
Texas Cooperative Extension

Bastiaan M. Drees was co-author of the previous printing of this publication.

Educational programs of Texas Cooperative Extension are open to all people without regard to race, color, sex, disability, religion, age or national origin.

Issued in furtherance of Cooperative Extension Work in Agriculture and Home Economics, Acts of Congress of May 8, 1914, as amended, and June 30, 1914, in cooperation with the United States Department of Agriculture. Chester P. Fehlis, Director, Texas Cooperative Extension, The Texas A&M University System.