The basics
What are fungus gnats?
Written by Mustafa Ayan Fact-checked against university extension/IPM sources Last reviewed: October 14, 2026
Direct answer
Fungus gnats are small, dark, weak-flying flies (family Sciaridae, usually Bradysia indoors) whose larvae live in moist potting mix and feed on fungi, decaying matter, and sometimes fine roots. The adults hovering around plants are harmless to people: they don't bite or sting. The soil-dwelling larvae are the real issue for plants, especially seedlings.
Fungus gnats in one minute
Adult fungus gnats are roughly 1/8 inch long, with slender bodies, long legs, and long antennae, so they're often mistaken for tiny mosquitoes. They're poor fliers and tend to hop, flutter, and drift low over the soil rather than fly in a straight line.
They're an indoor problem because houseplant mix gives them what they need year-round: constant moisture, organic material, and stable warmth. Outdoors the same group of flies lives in leaf litter and damp soil without anyone noticing.
How they live
Females lay eggs in the top layer of damp mix. The eggs hatch into legless, translucent larvae with shiny black heads, which feed for roughly two weeks, then pupate near the surface and emerge as adults. Depending on temperature, a full cycle takes about three to four weeks, so populations can climb quickly when the soil stays wet.
Adults live only about a week, but long enough to lay eggs. That's why killing the flying adults alone never ends an infestation.
Are fungus gnats harmful?
To people, no. They don't bite, sting, or transmit disease to humans. To plants, it depends on the plant. Established, healthy plants usually tolerate a modest number of larvae. Seedlings, cuttings, and stressed plants with small root systems can be set back, because larvae feed on fine roots and can provide an entry point for soil-borne disease.
What to do if you have them
Control comes down to three things: let the mix dry appropriately between waterings, trap adults with yellow sticky cards, and use a Bti larvicide if moisture control isn't enough. The full step-by-step guide covers all of it.
Common questions
What are fungus gnats?
Small, dark, mosquito-like flies whose larvae live in moist potting mix. The adults are a nuisance but harmless to people; the larvae feed on fungi, decaying organic matter, and sometimes fine roots.
Are fungus gnats the same as fruit flies?
No. Fruit flies are tan with red eyes and breed in fermenting produce or drains. Fungus gnats are dark, slender, and breed in damp soil. The two need different treatments.
Can fungus gnats kill a plant?
A healthy mature plant rarely dies from them. Heavy larval feeding can seriously damage seedlings, cuttings, and plants that are already stressed or have small root systems.