University of Maryland Extension

Fungus Gnats

 pinned dark wing fungus gnat
Dark winged fungus gnat
Photo: Paul Langlois, USDA APHIS ITP, Bugwood.org

Key Points

  • Fungus gnats are tiny, black flies that are commonly seen around lamps and windows.
  • They are annoying but harmless.
  • The larvae breed in moist soil, primarily in potted plants containing soil rich in organic matter. Larvae feed on fungi in the moist soil. Overwatering plants contributes to the growth of the fungi on which the larvae feed.
  • They can also breed in drains. 

Management

  • Determine if the gnats are coming from a particular plant or drain.
  • Cover the plant or drain with screen or cheesecloth and check the next day for evidence of the flies.
  • If the fungus gnats are breeding in potting soil, dry it out. When the soil dries, the larvae will die. Change the watering cycle for your houseplants to prevent the soil from remaining wet. Another option is using Bacillus thuringiensis var. Israelensis (Bti), also called H-14 strain, it is a naturally occurring bacterium found in soils. Bti is the active ingredient in soil drench products labeled for houseplants to control fungus gnat larvae in the soil. Look in garden centers or do an online search for sources.
  • If they are breeding in a drain, use a commercial drain cleaner and or clean out the trap to eliminate the breeding site.


Picture of Larva of a fungus gnat.
Larva of a fungus gnat

Dark Winged Fungus Gnats

A very unusual insect occurrence may be noticed during wet years, near wooded areas or even crossing a driveway. These insects are a species of dark winged fungus gnat larvae. These and related species have this interesting habit of moving in snake-like masses. The larvae are harmless and live in decomposing organic matter in wooded areas.

Dark Winged Fungus Gnats Picture
Photo: S. Klick, UME-IPMnet


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