Florida Division of Plant Industry , Florida Department of Agriculture and Consumer Services, Bugwood.org
Updated: April 11, 2022
Signs of fungus gnats around indoor plants
Fungus gnats are tiny, black flies that are commonly seen around lamps and windows.
They are annoying, but harmless and do not harm indoor plants.
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.
Larvae of dark-winged fungus gnats (Bradysia sp.) on the root of a greenhouse plant.
Photo: David Cappaert, Bugwood.org
Determine if the gnats are coming from a particular plant or drain. To do this, cover the plant or drain with screen or cheesecloth and check the next day for evidence of the flies.
Monitor for fungus gnats and other flying indoor plant pests by placing yellow paper sticky traps in the containers.
If the fungus gnats are breeding in potting soil, dry it out. When the soil dries, the larvae will die.
Change the watering cycle of the indoor plants 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 indoor plants 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.
Dark winged fungus gnat larvae found outdoors
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. Management is not necessary.