Skip to main content

Fungus Gnats

Garden pest · Sciaridae

Description​

Fungus gnats are small, dark flies commonly found around houseplants, seed trays, and damp potting mix. The plant-associated pests are usually dark-winged fungus gnats in the family Sciaridae, rather than one species. Adults are weak fliers that often hover near pots or run across the soil surface. They are mainly a nuisance and do not feed on plants. Their slender, pale larvae live in moist growing media, where they eat fungi and decaying organic matter. When numerous, larvae may also feed on tender roots and seedlings. Established, healthy plants usually tolerate them, but heavy infestations can slow growth or cause young seedlings to wilt and collapse. Persistent moisture and organic-rich media favor their development, so gnats often become noticeable after frequent watering. Seeing adults alone does not prove that they are causing a plant’s decline; check the soil for larvae and consider other causes of poor growth.

Symptoms​

  • Insects visible on the plant
  • Yellowing leaves
  • Wilting
  • Stunted growth
  • Seedlings collapse at soil level

How to Identify​

Look for tiny, dark, long-legged flies hovering close to damp soil or resting on pot rims. A yellow sticky trap placed beside a pot can help confirm that adults are present. To check for larvae, inspect the upper layer of moist mix or lay a thin slice of raw potato on it for a day or two; larvae may gather underneath. They are translucent, slender, and have distinct black heads. Fruit flies are usually tan and gather around ripening or fermenting food, while shore flies are stockier and often have pale spots on their wings. Springtails jump from soil rather than fly.

Prevention​

Let the upper layer of potting mix dry between waterings when the plant can tolerate it; do not dry out plants that require consistently moist soil. Empty saucers after watering and check that containers drain freely. Start seeds in clean, well-draining growing mix, and avoid leaving trays continuously waterlogged. Remove fallen leaves and other decaying material from pots, and discard badly decomposed potting mix. Inspect new plants before placing them near existing ones, especially if flies rise from their pots when disturbed. Yellow sticky traps can reveal an infestation early, but they will not prevent larvae from developing. Good watering practices matter more than trying to keep every adult fly away.

Treatment​

First, correct excess moisture: allow the top of the mix to dry as much as the plant safely permits, empty standing water, and remove decaying debris. Use yellow sticky traps near affected pots to catch and monitor adults; traps alone will not control larvae. If gnats persist, apply a product containing Bacillus thuringiensis subsp. israelensis (Bti) to the growing medium according to its label, repeating applications as directed to reach newly hatched larvae. Beneficial nematodes, particularly Steinernema feltiae, are another soil treatment; apply them to moist media following the supplier’s temperature and storage instructions. For a heavily infested pot, repotting into fresh mix can help, especially if old mix has broken down and stays soggy. Protect delicate roots during repotting. Avoid routine broad-spectrum insecticides: adult flies are short-lived, and lasting control depends on reducing favorable conditions and targeting larvae.

Not sure this is your problem? Try the problem diagnoser.