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. Bti (subspecies israelensis) is selective to insect fly larvae, non-toxic to humans, pets and contains no harmful residues. A product labeled to control fungus gnats in plant beds or pots include Mosquito Bits® by Summit Chemical Company.
Check out our guide to help identify other common pesky flies that may be bugging you in your home. For help with vinegar flies or fruit flies, check out this blog post.
Confessions of an Extension Entomologist: I have two plants in my home that have to get dry and droopy before I water them. I do not have a green thumb.
Keep calm and respect the critters,