Sewer Fly

These flies get the common name of moth fly from their fuzzy appearance, their bodies and wings being very hairy. The drain/filter/sewage fly common names are from places or situations which represent typical breeding and developmental sites. Although usually thought of as nuisance pests, there have been cases where inhalation of their body parts caused bronchial asthma. Moth flies are found throughout the United States and most of the world.


Adults about 1/16-1/4" (1.55 mm) long, delicate and fuzzy. Color pale yellowish to brownish gray to blackish, depending on the species. Antennae 12- to 16- segmented, each segment bulbous and with a whorl of long setae (hairs). Wings broadly oval, pointed apically, veins and margins hairy, and held roof-like over body at rest. Non-biting. In addition, ocelli absent and cross veins restricted to basal 3rd of wing. Mature larvae about 1/8-3/8" (4-10 mm) long, eyeless and legless. Subcylindrical in form, with head narrower than body and terminal rear (apical) segment narrow, forming a short hardened (sclerotized) breathing tube. All or some body segments with narrow, transverse, sclerotized, strap-like bands (usually 2 or 3) on their dorsum. Color pale with head, dorsal bands, and apical breathing tube dark.

Similar Groups

  1. Sand flies (Psychodidae, subfamily Phlebotominae) with 2nd longitudinal vein of wings branching distant from wing base (moth flies with it branching near wing base), legs long and slender, wings held together above body at rest, females bite.
  2. Mosquitoes (Culicidae) with wings long and narrow, veins and wing margin covered with scales.
  3. Other flies lack dense hair on body and on wing veins and margin.

Representative Species

  1. Pacific drain fly. Psychoda phalenoides (Linnaeus). Adult about 1/8" (2-2.3 mm) long; wings brownish gray; antenna 15-segmented, with segments 13 or 14 each half the size of 12th segment; found along the Pacific Coast from southern California to Alaska.
  2. Psychoda alternata Say. Adult about 1/16" (2mm) long; body light tan, wings lighter but faintly mottled with black and white, wings with brown spots at ends/tips of veins; antenna 15-segmented but appearing 14-segmented (segments 13 and 14 fused), segment 15 small, buttonlike; ranges from Florida to Massachusetts and westward to Washington and California.
  3. Psycholda satchelli Quate is pale yellowish and antenna 14-segmented with segments 13 and 14 subequal in size; ranges from Georgia to Quebec and westward to Alaska and California.
  4. Psychoda cinerea Banks is pale yellowish with 16-segmented antenna having terminal 3 segments equal in size; eastern United States.
  5. Telmatoscopus albipunctatus (Williston) is brown or blackish, white hairs on thorax, and white spots at tips/ends of wing veins; antenna 14-segmented; found throughout the United States.


Adult females of Psychoda alternata lay their 30-100 eggs in irregular masses on the surface of the gelatinous film which covers filter stones of sewage treatment plants or which lines the water-free portions of drain pipes. Both the larvae and pupae live in this gelatinous film with their breathing tube(s) projecting through the film. The larvae feed on the algae, bacteria, fungi, microscopic animals, and sludge of this film. At 70F (21C) eggs hatch in 32-48 hours, the larval stage lasts 8-24 days, and the pupal stage lasts 20-40 hours. The developmental time (egg to adult) is 7-28 days, depending on conditions. Adults typically live about 2 weeks. Although they may breed in sewage, apparently they do not transmit human diseases. However, in South Africa, there have been cases of bronchial asthma where the inhalant allergen consisted of dust composed of dead moth fly body parts.


Because of their small size, moth flies are able to penetrate ordinary screens. Moth flies are weak fliers, so indoors they are usually seen crawling on walls or other surfaces. When they do fly, it is only for short distances of a few feet and their flight is in characteristic short, jerky lines. During the day, they typically rest on vertical surfaces near drain openings indoors and in shaded areas outside. Their greatest activity is in the evening when they can be seen flying or hovering above drain openings indoors or sewage filter beds, etc. outside. Typically, only a few adults are seen at a time in structures because the adult flies live only about 2 weeks but are continually being replaced with newly emerging flies as they die. However, large numbers of adults usually means that an outside source such as a sewage treatment plant is involved. Even though they are weak flies, they can be carried distances of 300 feet (91 m) or more by the wind.


After proper identification, a thorough inspection is required to find the breeding site. Places to check include: slimy drains, sewer leaks or backup, dirty garbage cans, saucers under potted plants, bird baths or feeders, clogged roof gutters, clogged storm drains, air conditioners, cooling towers, moist compost, rain barrels, and septic tanks. If large numbers of flies are seen, be sure to check for nearby sewage treatment plants, especially upwind from the structure. In relatively new structures, drains can often be cleaned out with over-the-counter drain cleaners followed by very hot water. If this is not successful, mechanical cleaning of the drain with a stiff brush is required to remove the gelatinous film lining, and this should be followed with a caustic drain cleaner. Pyrethrin or pyrethroid aerosols can be used to kill large numbers of adult flies, but only the elimination of the breeding site(s) will provide long-term control. When large populations of these flies are breeding in sewage filter beds, control usually consists of the periodic flooding for a minimum of 24 hours to kill larvae and pupae; eggs are unaffected. In addition, weed control should be practiced to remove adult roosting sites and any adjacent vertical surfaces should be treated with an appropriately labeled pesticide. Microencapsulated and wettable powder formulations are particularly effective.