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Drain Flies & Fungus Gnats

drain fly

Some folks call the extension office about "gnats," but to an entomologist, a gnat is a small fly. All flies and gnats (very small flies) have two wings, which places them in the order Diptera. Several types of small flies infest damp or wet locations inside the home, but drain flies and fungus gnats are often found during the fall and winter.

Drain Flies. These grayish-brown flies often hover over the sink or bathtub drain. Under magnification, they look similar to tiny moths, so these flies are sometimes called moth flies. Drain fly larvae develop by feeding on fungi and organic matter living in the gelatinous gunk that gradually builds up in drain pipes. Adult flies emerge from the drain and mate. Then, the female fly goes back down the drain to lay eggs. The drain is the most common breeding site, but infestations also can begin when there are broken or leaking pipes, a sink overflow, or from a grungy garbage disposal. The photo of the drain fly (left) is by Jim Kalisch, University of Nebraska with the Department of Entomology.

Control:  Drain flies are usually eliminated by removing the gelatinous slime on the inside of the pipes. Drain cleaners are good for clogs, but might not do a good job of cleaning pipes and removing the slimy gunk. Pouring boiling water down the drain may loosen the gelatinous slime, but the surest way to clean the drain pipe is to clean it manually with a plumber's snake or wire brush. In addition, check pipes to make sure they are not leaking. If the flies seem to be above the garbage disposal, clean it according to the manufacturer's directions.

Fungus gnats. A Fungus gnat is a small, dark, delicate-looking fly with slender legs and a long body. They'll be near potted plants and may hop across the soil surface. Fungus gnat larvae feed on fungi growing in the top 2 to 3 inches of soil. The life cycle from is 3 to 4 weeks at room temperature. To the right is a dark winged fungus gnat, photographed by Jim Kalisch, UNL Entomology.

Fungus gnats are more common in the fall and winter because plant growth slows, but many people continue their frequent summer watering schedule. Wet soil increases soil fungi and fungus gnat development. House plants taken outdoors during the summer are often infested with fungus gnats when they are brought indoors.

Control:The best management practice for controlling fungus gnats is to allow the top two-inches of soil to dry thoroughly between watering. Remove any dried plant materials on or in the soil. Don't allow water to set in saucers under plants. Purchase and use only sterile potting soil. According to Whitney Cranshaw, Colorado State University, you can use ¼-inch potato slices to detect fungus gnat larvae in the soil of potted plants. Insert potato slices into the soil surface. Within a couple days, the larvae will move into the potato slices. Check the potato slices to see the tiny larvae.

Insecticides can supplement the cultural control of reduced watering. To control emerging adults, apply pyrethroid insecticides to the soil surface at 2 to 3-day intervals for three to four weeks. Appropriate insecticidal products can be purchased at garden centers. Whenever using pesticides read the label and follow the directions carefully.

To make sure appropriate management tactics are used, "gnats" (i.e., small flies) should be identified by an expert. In addition to drain flies and fungus gnats, some other types of small flies are sometimes found indoors. Contact your local extension office or a pest management professional for help with identification of these tiny flies.

Barb Ogg
Former Extension Educator, Entomology
Barb Ogg shared her love of entomology with clientele throughout Nebraska for many years through Nebraska Extension. Barb retired in 2015.

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