by Lee Townsend, Extension Entomologist
University of Kentucky College of Agriculture
Drain flies, or moth flies, are small, darkwinged, non-biting gnats. Their wings are covered with scales so they disappear in a cloud of fine dust when swatted or mashed. These nuisance gnats can be found resting on walls or ceilings, and make short hopping flights if disturbed.
Drain flies often are a temporary problem. They develop in standing water so most commonly they are seen after returning home from a vacation or period of extended travel. Usually, they disappear soon after normal household activity resumes and water starts to move again through toilets and drain traps. The few adults resulting from these small infestations can be killed easily with a swatter or flying insect spray. However, finding many flies over several weeks usually means a relatively permanent breeding site that must be found and eliminated. Ending a chronic infestation can be challenging.
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. 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.
CAUTION! Pesticide recommendations in this publication are registered for use in Kentucky, USA ONLY! The use of some products may not be legal in your state or country. Please check with your local county agent or regulatory official before using any pesticide mentioned in this publication.
Of course, ALWAYS READ AND FOLLOW LABEL DIRECTIONS FOR SAFE USE OF ANY PESTICIDE!