Drain flies and fungus gnats are another couple of household nuisance pests. They can be found any time of the year but may be more noticeable in winter when we are inside a lot more.
Finding drain flies and fungus gnats
As their names imply, both like higher levels of moisture in the home. In the wintertime, that would mean floor drains and areas like bathrooms and basements too. Drain flies are most commonly associated with floor drains and basements. Fungus gnats likely will be found near potting soil (organic matter) in houseplants that are overly moist or kept on the moist side. If potted plants are not part of the home décor, then other locations may be those floor drains, a crawl space with open dirt, or even very slow plumbing leaks that have yet to be found. If you happen to still own a second refrigerator with a drain pan that is another possible place.
Drain flies, while small, have a distinct wing shape that extends away from the body. An easy identifying feature is if you mash one on the wall, you are going to see a dark mark left behind as the wing scales come off the wing. Another possible characteristic, as gross as it sounds, is if the surface of the water in the trap seems to be alive, you have drain flies.
Fungus gnats are much much smaller than a drain fly and have wings that are narrower and follow the body line. They also will walk across the surface of the potting soil in a pot and be found on the foliage.
Remedy for Drain flies
Floor drains naturally collect dirt and debris suitable for the fly to live and breed. Several times a year – you decide the frequency, but quarterly is a good interval – the drain should be cleaned and flushed with several gallons of water to thoroughly clear the trap. Also clean any debris by scrubbing the walls above and just below the surface of the standing water in the trap. A long- handled scrub brush works well for this task. Once the drain is clean, a routine flush with hot water will continue to prevent the flies.
Remedy for Fungus gnats
If the gnats are associated with your houseplant collection, allow the soil surface to dry down one or more inches between watering. (This is easier to do with larger pots.) Keep the water collection trays clean, removing plant debris and organic matter. As a matter of good watering practices, remove any standing water that remains after an hour. If you have a strong outbreak, there are “yellow sticky traps” to attract the adults to assist your other efforts. When repotting a houseplant, be sure to use new potting soil. Add the old potting soil to the compost pile. The same cleaning process used for drain flies will help take care of fungus gnats too.
About the author: Richard Hentschel’s expertise extends across several subject areas with specialties in lawn care, fruit tree production, woody ornamentals, and home and community gardening. During his 45-year career in horticulture and agriculture, Hentschel became a well-known and respected expert for commercial and homeowner audiences, industry organizations, and media. He retired from University of Illinois Extension in April 2022 with nearly 30 years of service as a Horticulture Specialist and Educator in northern Illinois.