Sources of Drain Flies
As with most gnats, the larval drain flies need moisture. They can live most anywhere that water accumulates for a week or more. Common indoor sites include the fine slime layer that develops along the water surface in infrequently used toilet bowls and tanks, in sink or floor drains in basements or garages, or drain pans under refrigerators. Sometimes the gray, wriggling larvae can be seen swimming in the water. These areas need to be cleaned thoroughly with attention to removing surface films.
It can be hard to recognize breeding sites because the larvae are small and easy to overlook. Emergence of flies may be detected by the use of a simple trap. Clear plastic cups with a very light coating of vegetable oil or petroleum jelly can be inverted over drains for several days to catch emerging adults and to identify breeding sites. If these are not productive, then expand the search.
Drain flies can breed outdoors during the summer with adults entering homes through open doors or windows. Low, wet areas where air conditioning units drain, or clogged guttering are excellent places for these insects to develop. These should be addressed if adults do not appear to be emerging from within the house.
Control of drain flies should be aimed at elimination of breeding sites. The most effective control method is to clean pipes and traps thoroughly to remove accumulated slime. Pouring hot water down the drain may provide short-term control. Use Draingel (Pour 4 oz of drain gel per drain and let sit in drains overnight) Drain fly larvae are difficult to drown because they are able to trap air bubbles and remain submerged for a day or more.
Do not pour insecticides down drains to kill drain flies. The drain fly life cycle takes from about 10 to 15 days at about 70° F. Groups of eggs are laid on gelatinous films of organic matter. The larvae can develop in water or thin surface films. Actual time varies with temperature, development is slower at lower temperatures but can continue through the year indoors.