The common name "drain fly" is applied to many species of tiny flies from several genera in the Psychodida family of insects. The most common species is Clogmia albipunctata, often called the common drain fly or the moth fly (due to its fuzzy, mothlike appearance). The adult flies are 1/10 to 1/5 of an inch in size, with wings that are slightly lighter in color than the dark gray to black body. The antennae or long and curved. Also known as sink flies, filter flies, and sewer gnats, this small fly is often mistaken for the fruit fly. It generally causes no real damage, but it is an undeniable nuisance when it shows up in high numbers.
C. albipunctata flies are common worldwide and are frequently found around household drains, where they feed on decaying organic material found in the pipes. Other indoor areas where there is moist, decaying organic material can also be a breeding site for flies. Drain flies can also be found in dark, swampy outdoor locations where there is plenty of decaying organic material and standing water.
Identifying the Source
The key to controlling drain flies is finding and eliminating their water and food source. You can do this using simple traps made with a plastic cup or an insect glue board.
For the plastic cup method, lightly coat the inside of the cup with vegetable oil or petroleum jelly then invert the cup over a drain where you suspect flies may live. 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.
Another method is to create a cardboard frame for elevating a glue board above a drain where you suspect drain flies may live. Place an insect glue board upside down on the frame. 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 to another suspected spot. If you cannot find a source inside the house, expand your search to the outdoors, looking for any areas with moisture and decaying material.
5 Ways to Get 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.
Clean and Repair Drains
Thoroughly clean in and around your home's drains, sinks, and drain pipes. Clean down into drains with a long, stiff brush, then pour boiling water through the drain to clean out any remaining sludge.
After scrubbing, a good natural rinse made with baking soda and vinegar will sterilize your drains. Mix 1/2 cup salt and 1/2 cup baking soda and pour the mixture into the drain. Pour 1 cup of liquid white vinegar into the drain, where it will begin a foaming action with the baking soda. Allow the mixture to work overnight to kill any remaining larvae. In the morning, pour some very hot water down into the drain to rinse it and any remaining fly larvae away.
There are also a variety of biodegradable drain gels and liquids that will thoroughly clean the pipes, eliminating the breeding ground for drain flies. Try to avoid using chemical cleaners, however.
If you find any leaky drain traps or other drain pipes, replace these, as the dripping water can provide a source of water and food for flies on the floor of cabinets or in wall cavities.
Clean Indoor Garbage and Recycling Areas
Clear any organic material from floors and walls around garbage and recycling containers, then clean the containers themselves, using hot soapy water. Thoroughly dry these containers before putting them back. Clean, dry containers will not lure drain flies.
Clean Floor Drains and Dry the Floors
Floor drains often collect standing water in their drain traps, and the stagnant water and sludge in these traps are prime areas for drain flies to lay eggs. Clean floor drains with a stiff brush and hot water, then rinse with a baking soda and vinegar mixture (see above).
Also check sump pump pits, as these can be locations where drain flies breed. Make sure the sump pits are being drained completely by the pump.
Eliminate Standing Water
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. Any areas where moisture or water is allowed to grow stagnant is an attractive location for drain flies, especially if there is organic material nearby.
Look for Outdoor Sources
Dirty birdbaths, rain barrels, garbage bins, soggy compost bins, dog kennels—and any other area where there is stagnant water and organic material—should be thoroughly cleaned and dried if drain flies are a problem outdoors. Wherever possible, eliminate areas where water can puddle and grow stagnant, as these are areas where drain flies will lay eggs.
What Causes Drain Flies?
Drain flies can become a problem wherever moisture and decaying organic material are found together. Fixture drains and floor drains—especially those that aren't flushed out often—are prime locations for these flies. But drain flies can appear anywhere that conditions are to their liking. Mature breeding drain flies are naturally drawn to moist areas where there is a source of decaying organic material for the larvae to feed upon after they hatch from eggs.
How to Prevent Drain Flies
The best way to prevent drain flies is to keep all areas of your home dry and clean. Be especially watchful for dark areas where decaying, moist organic material can collect.
Drain Flies vs. Fruit Flies
Drain flies are often mistaken for fruit flies (Drosophila melanogaster) since both are very small flies with distinguishing features that are only evident under magnification. Most fruit flies, however, have bright red eyes and hairless bodies and wings. Fruit flies are found only in and around fruit and vegetables that are overly ripe and beginning to decay. They fly about openly in the daylight, while drain flies prefer the dark.
Fruit flies often are found flying about alone, while drain flies are usually found in small swarms, nearly always around drains or places where there is standing, fetid water. Drain flies are rather poor flyers and are often found resting, while fruit flies are almost always in flight.
Do Drain Flies Have Any Useful Functions?
In small numbers, the larvae of the common drain fly can help dissolve sludge in drain pipes and sewer mains, speeding the process by which wastes are broken down in the municipal sewer networks. An occasional sighting of a few drain flies is not a cause for alarm.
Do Drain Flies Carry Disease?
The common drain fly (Clogmia albipunctata) does not itself carry any pathogens that cause human disease. However, its larvae have been linked to some cases of myiasis—a parasitic infection of the body that can occur when the larvae enter the body through open wounds. This is a fairly rare occurrence, however; for the most part, this is a nuisance insect, not a dangerous one. It's also possible for harmful bacteria from dirty drain to be transferred by drain flies, though the danger of this is minimal. Heavy infestations of the common drain fly may exaggerate asthma symptoms in some people—probably through reaction to the dried parts of dead flies floating through the air.
Do Drain Flies Bite?
Drain flies do not bite or sting, as their mouthparts are much too small to pierce the skin. These are not blood-feeding insects, and they have no interest in biting.
How Long Do Drain Flies Live?
The common drain fly lives about 12 days. It mates and lays eggs once during this time. The larvae hatching from the egg feed on organic material for about 18 days before transforming into pupae, which will develop into adults after roughly five days.
Can I Use Pesticides?
Many spray pesticides will kill drain flies, but unless you eliminate the moisture and organic material that provide breeding areas, more drain flies will quickly appear. And using chemical pesticides in and around drains is a questionable strategy, as these toxic chemicals will quickly be flushed into the sewer system, where there is the potential for contaminating groundwater. Toxic chemicals in a sewer's system may also destroy helpful organisms that help break down solid waste.
Drain flies are much more effectively handled by thoroughly cleaning drains and other areas where moist organic material is found.