Plant Problem Diagnostic
Fungus Gnats: How to Identify and Manage Them
The short answer
Fungus gnats are delicate, dark flies about 1/8 inch long that hover around houseplants, while their translucent, black-headed larvae live in the top layer of moist potting soil and feed on decaying matter. Controlling soil moisture is the key: let the top 1 to 2 inches of soil dry completely before watering again, since larvae cannot survive in dry soil. Bottom-watering keeps the surface dry and discourages egg-laying.
Yellow sticky traps set near the soil catch adults and help you monitor, and a Bacillus thuringiensis subspecies israelensis (Bti) soil drench targets larvae without harming the plant. If gnats persist after the soil is allowed to dry, check for overwatering, poor drainage, or decaying debris, and contact your local Extension office if the problem continues or the identification is uncertain.
How to identify fungus gnats
Adults are weak-flying dark gnats that stay near the pot; you may see them scatter when you water. Larvae are tiny, clear worms with black heads in the moist surface soil.
- Small dark flies hovering around houseplant soil
- Worst in consistently damp, overwatered pots
- A potato slice pressed into the soil will draw larvae to confirm them
Least-toxic management, in order
- 1Dry the soil surface — Let the top 1-2 inches of soil dry between waterings so larvae cannot survive.
- 2Bottom-water — Water from below so the surface stays dry and less attractive for egg-laying.
- 3Yellow sticky traps — Set traps near the soil to catch and monitor adults.
- 4Bti soil drench — Bacillus thuringiensis israelensis targets the larvae in the growing medium and is safe for plants.
When to escalate
Persistent gnats usually mean the soil is staying too wet or holds decaying material; repot into fresh, well-draining medium if needed. If the problem continues after drying and Bti, or you are unsure the insect is a fungus gnat, contact your regional Extension office.
✔ Avoid letting pots sit in saucers of standing water, which keeps the soil perpetually damp and feeds the larvae.
Sources
Reviewed 2026-07-18. We cite university Extension, government, and established horticultural sources — and never invent pesticide rates or dangerous treatments.
- University of Minnesota Extension — How to treat pesky fungus gnats in houseplants — Soil moisture control, bottom-watering, sticky traps, and Bti for larvae
- Colorado State University Extension — Fungus Gnats as Houseplant and Indoor Pests — Adult and larval identification, larvae in growing medium, and the potato-slice detection method