Fungus Gnats

Gnats In Winter? Check Potted Plants!

by Jody Green, Extension Educator

Fungus gnats are nuisance pests that occur indoors throughout the winter months. They breed and develop in overwatered potted plants. Many people do not realize plants take up less water in the winter, continue to water regularly, thereby creating a suitable habitat in the soil for the fungus gnat to complete its life cycle. Adults are grayish-black, about 1/8-inch long and have one pair of wings. At first glance, they may appear similar to fruit flies, but they complete their development in top layers of soil. Females lay 100–150 eggs in moist potting soil and the larvae feed and develop on the fungi and organic matter. Fungus gnat larvae are white, slender, legless maggots with translucent bodies and dark heads. Larval feeding sometimes includes gnawing on the roots and stems of plants. In warm conditions, overlapping generations may occur, producing large populations which can cause spotting, curling, yellowing or plant death. After pupating in the soil, they emerge as winged adults, bothering people by flying around faces, lights, windows and food items. In order to eliminate a fungus gnat infestation, the life cycle must be broken. This can be done by removing the fungus in which they breed, while simultaneously reducing the number of breeding and egg-laying adults. A non-chemical approach is to reduce the topsoil moisture by less frequent watering, drying out the soil and changing the plant medium to provide better drainage. To catch flying adults, yellow sticky card traps are available at garden stores and placed at the soil surface. In addition, there are biological control products such as the microorganism, Bacillus thuringiensis (Bt), which is applied to the soil to kill larvae after ingestion. Bt is selective to insect larvae, non-toxic to humans, pets and contains no harmful residues.

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