How to Get Rid of Drain Flies Indoors and Outside

A drain fly at close range

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"Drain fly" is one of the common names for the moth fly (Psychoda sp.). True to this nickname, drain flies can be a problem anywhere there is standing water, particularly around drains. Also known as sink flies, filter flies, and sewer gnats, this small fly, often misidentified as a fruit fly, causes no real damage. However, when it shows up in high numbers it can become a nuisance pest in or around the home.

Drain Fly Identification

You need a magnifying glass to see all the specific characteristics of the drain fly, but some features you can make out with just your eyes (or a pair of reading glasses):

  • 1/5 to 1/10 inches long
  • Body and wings covered with long hairs that make it appear fuzzy and moth-like
  • Dark gray to black body and lighter-colored wings
  • Long, curved antennae

Drain Fly Behavior

Due to it being a poor flyer, able to fly only a few feet at a time, you can identify the drain fly by its habit of making short, hopping flights when disturbed, rather than flying a distance. It also is usually found near the source of standing water or moisture. Its small size and light weight can enable it to be carried on the wind to a home from a sewage plant up to a mile away. Once at a home, the fly is small enough to get in through standard screening.

The drain fly feeds on decaying organic material in mud, moss, and standing water, as well as flower nectar. It can live and breed almost anywhere that standing water or organic material accumulates for a week or more. It is most active in the evening.

Drain flies are most likely to be found:

  • In the home: in, on, and around sink drains, floor drains in basements and garages, unused sinks, the sides of showers, windows and walls near drains, unwashed garbage disposals, infrequently used toilets, and loose tiles where moisture accumulates
  • Outside, around the home: in or around compost piles, garbage areas, damaged septic lines, and standing water (rain barrels, tree holes, shallow and polluted ponds)
  • In nature: in and around shallow, polluted water or high-moisture organic material or compost

Drain Flies Are Nuisance Pests

Drain flies cause little damage, as they do not bite and are not known to transmit disease. However, because they breed in filth, such transmission is possible. Additionally, drain flies can become a severe nuisance pest, as they can lay eggs in masses of 10 to 200 at a time, and they hatch within two days. The flies then mature within two weeks, and adults live for about two weeks. As a result, their populations can grow quite large in a short time and often seem to appear suddenly.

Finding Drain Fly Sources With Traps

The key to controlling drain flies is finding and eliminating their water and food source. You can do this using simple traps can be made with a plastic cup or an insect glue board:

Using a clear plastic cup:

  1. Lightly coat the inside of the cup with vegetable oil or petroleum jelly.
  2. Invert the cup over a drain where you suspect the flies may live.
  3. Leave the cup in place for several days, but check it daily. If there are flies living in the drain, they will begin to accumulate on the interior surface of the cup.

Using a glue board:

  1. Create a cardboard frame for elevating a glue board above a drain where you suspect drain flies may live.
  2. Place an insect glue board upside down on the frame.
  3. Leave the board in place for several days, but check it daily. Flies exiting the drain will likely stick to the glue board.

Whichever type of trap you use, be sure to check it regularly. If no flies are detected after several days, move the trap (or recoat another cup) and place it over another suspected drain or other areas. If you cannot find a source inside the house, expand your search to the outdoors.

Getting Rid of Drain Flies

Once the flies' source is found, you can eliminate the pests through the removal or cleaning of the source. However, it can take persistent, ongoing work to eliminate a drain fly problem completely.

Indoors:

  • Clean in and around drains, sinks, or drain pipes. Clean down into drains with a long, stiff brush.
  • Fix leaky pipes.
  • Clear any organic material, cleaning in and around all garbage containers.
  • Eliminate any other standing water or moist areas, such as wet lint in the laundry area, standing water in a drain pan beneath a refrigerator, or soggy areas around house plants.

Outdoors:

  • Eliminate standing water wherever possible.
  • Clean garbage bins, air conditioners, and bird baths.

Once you have eliminated the population, prevent future problems by keeping the area clean and free of standing water.