How to Manage Pests
Pests in Gardens and Landscapes
Moth flies, often called drain flies, are small, about 1/8 inch in length and often dark-colored. Their wings are covered with fine hairs, which give them a moth-like appearance. These flies rest on surfaces with their wings held over their back in a roof-like manner, and they have wing veins that extend in a parallel arrangement from the base to the tip of the wing, a pattern unique to the Psychodidae family of flies. They are weak flyers and exhibit a characteristic flying behavior of short hopping flights. The combination of characteristics described above can be used to distinguish moth flies from other flies in and around homes and other buildings.
Like all flies, moth flies undergo complete metamorphosis with egg, larval, pupal, and adult stages. Female moth flies lay eggs in moist to nearly saturated organic matter. In an urban environment, moth fly development often occurs in the slimy organic matter coating sink or shower drains, giving these flies an alternate common name “drain flies” used by many pest management professionals. However, moth flies may also be found developing in wet animal manure, sewage or even compost. Very large numbers of these flies in one area probably indicate a development site bigger than a few indoor drains. Once the eggs have been laid, they hatch in about 48 hours and continue to develop in the wet organic matter as larvae. Moth fly larvae in the final (third) larval stage are approximately 1/4 inch in length, have a distinct head, and a siphon on one end, which allows them to breathe in the wet environment. Immature flies pupate at their developmental site before emerging as adult flies. The life cycle of moth flies can be completed in as little as 8 days but can take as long as 24 days depending on temperature.
Moth flies do not bite people or animals, and they cause no damage to structures or plants. However, because these flies develop in the decaying organic matter found in sink and sewer drains or even wet manure and raw sewage, they have the potential to carry pathogens acquired at these development sites to areas where sterility is important, such as health care facilities and food preparation areas. Moth flies may also affect human health when present in high numbers, because the bodies of dead flies may disintegrate to form potential allergens. In addition to these health risks, moth flies are also nuisance pests, especially when present in large numbers or when they land on people or fall into food. Where large development sites are present, flies can be so numerous that it can be difficult to talk without swallowing them!
Inspection and Prevention
The key to managing moth flies is the elimination of breeding sites. In residential homes, the most common developmental sites are bathroom drains. In commercial facilities and restaurants common developmental sites include sink and floor drains in food preparation areas, grease traps, mop drying tubs, and evaporation pans placed beneath appliances. When adults are found within a room, first examine all the drains within the room for the presence of moth fly larvae. Because moth flies are weak flyers, adults can often be found very near to their development site. The presence of adults within a drain or resting on walls near a drain is a clear sign that this drain is a development site. Flies may be developing in more than one drain, so all drains in the room where adults are found should be inspected for moth fly larvae. Check drains by removing the drain plate and scraping the slime from the sides of the drain using a dull knife or similar device that will reach several inches into the open drain. Look for larvae within the collected slime. If larvae are not found in the slime that can be scraped from exposed drainpipes, it may be that moth flies are developing in organic material found much deeper within the pipes. A more thorough assessment of moth fly development within a drainpipe can be made using a drain fly trap. Coat the inside of a cup or jar with Vaseline or vegetable oil and then invert the cup to cover the drain opening. Any moth flies emerging from within the pipe will stick to the inside of the trap. Traps should be placed over all drains within an infested room and left in place for 24 hours. Following this exposure period, examination of the traps will indicate which drains are development sites.
While moth flies are most commonly found developing in organic buildup within drains, they will develop in a wide range of wet organic detritus that may be found in a home or commercial facility. If all drains have been inspected and flies were not found to be developing in them, additional locations that might contain wet organic material should be inspected. Moth flies have been found developing in elevator pits, around sump pumps, and in material beneath leaking drain and sewage pipes. Pipes in crawlspaces, wall voids, and the underside of concrete slabs may need to be inspected for breaks or leaks if developing moth flies are not found in drains or other exposed locations.
When inspecting for moth fly development sites, keep in mind that these flies can sometimes disperse from off-site sources. Moth flies can often be produced in very large numbers at sewage treatment facilities and dispersed via wind to nearby homes and buildings where they can accumulate in large numbers. However, in these instances, the flies are typically present in greater numbers outside the structure than within the structure.
Physical control is an integral part of eliminating moth flies. Physical control methods will vary, depending on where the flies have originated. If flies are developing within the home or other structure, removal of larval habitat will provide suitable control. A common misconception is that pouring boiling water and/or bleach down a drain will control these flies. However, these treatments do not remove the organic film within the drain; and they provide at best short-term control of flies. To remove the organic film within the drain, use a hard bristle brush with an industrial drain cleaner. Drains may need to be snaked first to clear any blockage causing slow draining and also to prevent further buildup of organic material within the drain. For sinks equipped with a J trap underneath, the trap should be removed and cleared of organic debris. After cleaning the drain, run water through the drain to ensure that water flows freely. In addition, regular use of an enzymatic drain cleaner may help to prevent the organic buildup that provides a good development site for moth flies.
Similar removal of wet organic material followed by cleaning will eliminate development sites in exposed locations. If development sites are not exposed (e.g., within wall void or beneath a structural foundation), a professional pest control service should be contacted for advice.
Flies produced at an off-site source (e.g., a sewage treatment facility) where favorable larval habitat cannot be removed are best managed through exclusion practices. Flies can be excluded from a structure using mesh screens placed over building openings, including air vents. All windows should be screened, and torn or damaged screens should be replaced. Air curtains placed above doors will help to reduce flies that might enter as the doors open. Lastly, it is important to consider the role of light attraction to moth flies. Flies are often attracted to structures during the evening hours by external building lights. Lights should be placed away from doors or other building openings to reduce fly access to the interior of the building, and lights near the structure should use sodium vapor lamps or insect lights, which are less attractive to moth flies.
Chemical control is generally considered unnecessary for moth fly management, as control will only be temporary unless larval development sites are located and removed. The use of insecticides to control these flies in residential homes is particularly discouraged because usually only one or a few drains are involved and cleaning them can provide an effective long-term solution. In nonresidential urban structures, the application of nonresidual chemicals such as pyrethrins formulated as ultra low volume (ULV) fogs or space sprays may provide immediate control of adult flies where sterility is an issue but should be combined with the removal of larval development sites. Insecticides may also be utilized when the flies are produced off-site or larval development sites cannot be eliminated. In these cases, treatment with a residual pesticide applied to resting surfaces may provide some relief.
Pesticide applications should be made only by those licensed to do so. A professional pest control company should be consulted in cases where pesticide applications are considered. When applying pesticides, be sure to read the product label and follow instructions for application closely.
Gulmahamad, H., 2011. Flies, Gnats & Midges. In A. Mallis, S. A. Hedges, and D. Moreland, eds. Handbook of Pest Control. The Behavior, Life History, and Control of Household Pests, 10th ed. Cleveland: GIE Media Inc. pp. 733-835.
Hedges, S.A., 1998. Field Guide for The Management of Structure Infesting Flies. Cleveland: GIE Media Inc.
Pest Notes: Moth or Drain Flies
Authors: David Theuret, Entomology, UC Riverside and Alec Gerry, Entomology, UC Riverside.
Produced by University of California Statewide IPM Program
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