Pest profile

Fungus Gnat

Bradysia and related species · Diptera: Sciaridae[3][1]

Fungus gnats are small flies whose larvae live in moist potting mix and organic matter; indoors the adults emerge from houseplants and become a nuisance.[1]

Adult fungus gnat, a slender dark fly with long legs and antennae
Adult fungus gnat, a slender dark fly with long legs and antennae. (c) aispinsects, some rights reserved (CC BY) — via iNaturalist, CC BY 4.0

How to identify it

  • Adults are dark, delicate, mosquito-like flies, usually 1/16 to 1/8 inch long, with long legs and segmented antennae longer than the head.[1][2]
  • Wings are light gray to clear; common Bradysia species have a Y-shaped wing vein.[1]
  • Larvae have a shiny black head and a whitish to clear, legless body.[1]
  • Adults are weak fliers that stay near pots and run across the soil; they are drawn to light and windows.[1][2]

Look-alikes

Look-alikeHow to tell them apart
Shore flyMore robust, with short, bristlelike antennae; associated with algae.[1]
Fruit flyTan with red eyes and found around fermenting food rather than plant soil.[2]

Where it lives

Lives in moist potting mix, compost, mulch and other decaying organic matter; indoors, houseplants are the usual source, often brought in from outdoors.[1][2]

Biology and life cycle

At 75°F eggs hatch in about 3 days, larvae feed for about 10 days, and adults emerge about 4 days after pupating; a generation can take about 17 days.[1]

Adults live only about 7 to 10 days, but generations overlap all year indoors.[2][1]

Damage and signs

  • Adults don't bite or damage plants; large numbers of larvae can damage roots and stunt seedlings and young plants, though overwatering and root rot are more common causes of wilting.[1]
  • In very moist pots, larvae can leave shiny slime trails on the soil surface.[1]

Health risk

Fungus gnats don't bite people and are a nuisance only.[1][2]

Control

Prevention

Avoid overwatering, let the soil surface dry between waterings, improve drainage, and use pasteurized potting mix; don't use incompletely composted material in pots.[1][2]

Traps and physical control

Yellow sticky trap squares on skewers catch adults; raw potato chunks, cut side down on the soil, attract larvae and can be checked and replaced every few days.[1]

Biological control

Bti (Bacillus thuringiensis subsp. israelensis) products for fungus gnats, Steinernema feltiae nematodes and Hypoaspis predatory mites target larvae in the soil; Bti doesn't persist, so repeat about every five days.[1]

Pesticides

Insecticides generally aren't recommended for fungus gnats around the home.[1]

Common questions

Why do I have gnats around my houseplants?
They breed in moist potting soil and organic debris; letting the soil dry between waterings is the key fix.[1][2]
Does Bti kill fungus gnats?
Yes, it targets larvae in the soil, but it doesn't persist indoors, so reapply about every five days.[1]

Sources

  1. University of California Statewide IPM Program, “Pest Notes: Fungus Gnats, Publication 7448 (August 2013).” ipm.ucanr.edu/legacy_assets/PDF/PESTNOTES/pnfungusgnats.pdf (accessed 2026-10-06)
  2. University of Kentucky Entomology, “ENTFACT-657: Small Flies.” entomology.mgcafe.uky.edu/ef657 (accessed 2026-10-06)
  3. iNaturalist, “Bradysia taxon page (order and family).” www.inaturalist.org/taxa/248001 (accessed 2026-10-06)

Updated 2026-10-06. Built from sourced records. Spot an error? Tell us.