Backyard Gardening

Fungus Gnats Are Pesky Pests that Can Damage Plants!

Record EditorJune 20, 2025Last Updated: June 20, 2025

4 minutes read

By Kathryn Dorman

Recently, fungus gnats have become the bane of my existence! My miniature roses are currently infested with a serious fungus gnat problem. Since these gnats are attracted to light, they’re rather irritating when I try to work on my computer. They can also be found fluttering around my reading light and landing on my laptop. I finally realized that I needed to research fungus gnats so that I could eventually be free of this pesky problem.

Fungus gnats (Bradysia and Orfelia species) are not gnats at all, but are technically flies. According to North Carolina State University Extension, the life cycle of the fungus gnat includes four stages: egg, larva (with four larval stages), pupa, and adult. Most of a fungus gnat’s life is spent as larva and pupa.

A yellow sticky fly ribbon with dark-winged fungus gnats.
(Photo Credit: Duane Dorman)

Adult fungus gnats are grayish or black, delicate-bodied, mosquito-like insects, but they lack the long mouthparts characteristic of mosquitoes. They don’t bite, and they won’t suck your blood! The adults actually don’t eat anything. On average, adult fungus gnats are an eighth of an inch in length. They have slender legs with segmented antennae. The light gray or transparent wings of the common Bradysia species have a Y-shaped wing vein. Fungus gnats are weak fliers with erratic flight patterns, but can run fast on the soil surface.

Adult fungus gnats live for about 7 to 10 days. Females can lay several hundred eggs in their brief lifetimes. The tiny eggs are oval, smooth, shiny, and semi-transparent. Eggs hatch into larvae in 4 to 6 days. The translucent, worm-like larvae, or maggots, have shiny black heads and are a quarter of an inch long.

Fungus gnat larvae feed for 12 to 14 days. The larvae primarily feed on fungi and decaying organic matter in the soil, but may also consume plant parts. The larvae start by feeding on root hairs and roots, usually in the upper two inches of soil. They can then work their way up the plant inside the stem. The larvae spin silk-like cocoons in which they pupate (become adults). The pupal stage lasts about 5 to 6 days.

Warmer and wetter conditions promote faster development, resulting in more generations of fungus gnats within a given period. With indoor plant infestations, there is typically continuous reproduction, which often results in overlapping generations. All developmental stages will likely be found simultaneously.

Fungus gnats thrive in moist conditions, especially in decaying vegetation and fungi. For this reason, avoid overwatering your plants. It is a good idea to amend the potting mix (which should be pasteurized) with perlite or sand to improve its permeability. Every plant pot needs at least one drainage hole. Drain any standing water after each and every watering. Allow the soil surface to dry between waterings. Stick your finger into the soil. If it’s damp, wait a day or two and check again.

Check your plants often for fungus gnats. It should be easy to see the adults when they are flying. Looking closely, you can see the larvae wiggling around in the soil. New plants should be kept separate (quarantined) from other plants until you have determined that they are free of fungus gnats. Yellow sticky traps can trap adults and disrupt the life cycle. I used a fly ribbon I bought at a local grocery store (See Photo). Other types of sticky traps can be purchased online.

Outdoors, favorite breeding spots for fungus gnats include raw compost, grass clippings, organic fertilizers (such as manure and blood meal), and mulch. Growing media containing high amounts of peat moss is particularly attractive to adult females for laying eggs. Woodchip-type mulch often promotes the growth of fungal layers. According to Ohio State University Extension, it is better to use bark-based mulch. To minimize fungal growth, limit the layer thickness to less than three inches in depth.

A few common-sense tips: Make it difficult for fungus gnats to get inside to your houseplants—use screens on windows and doors. If leaves fall off your plants, dispose of them properly. Dead leaves on the soil surface can become a source of food for fungus gnat larvae. Cultivate the top layer of soil to expose eggs and larvae to drying air.

According to the University of California Department of Agricultural and Natural Resources, several biological control agents can safely combat fungus gnats in houseplants. These include beneficial Steinernema feltiae (Sf) nematodes and the biological insecticide Bacillus thuringiensis subspecies israelensis (Bti). Bti is a naturally occurring, spore-forming bacterium. Bti is only toxic to larvae such as black flies, fungus gnats, and mosquitoes. Please note that this Bt is a different subspecies from that applied to control caterpillars, and it is NOT effective against fly larvae, including fungus gnats. Both Sf and Bti can be obtained online. Chemical insecticides are generally NOT recommended for controlling fungus gnats in and around the home.

To summarize, fungus gnats can be annoying. The keys to controlling them are prevention and treatment. Prevent fungus gnat infestations by keeping the soil dry between waterings. Treat fungus gnat infestations with sticky traps, BTi, and/or beneficial nematodes (Sf). You can enjoy your houseplants and keep them healthy, without the constant fluttering of unwelcome fungus gnats.

Kathryn Dorman has been a Penn State Extension-certified Master Gardener of Clinton County since 2017. She can be reached at clintonmg@psu.edu or through the Clinton County Master Gardener Hotline at (570) 858-0198.

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