Fungus Gnats
Fungus gnats are small dark flies whose larvae live in the top layer of damp, organic potting soil, feeding on fungi, decaying matter, and occasionally…
Fungus Gnats at a glance
| Also known as | Sciarid flies, soil gnats |
|---|---|
| Affected parts | Soil / fine roots (larvae) |
| Most at risk | Indoor trees in damp, organic-rich soil |
| Main cause | Larvae breeding in persistently wet, organic potting mix |
| Severity | Mostly a nuisance; larvae can nibble fine roots of seedlings/weak trees |
| Reversible? | Yes — fix by drying the soil surface |
What to look for
- Soil: Small (~2–4 mm) dark flies running across or hovering just above the soil surface; tiny translucent larvae with black heads in the top centimetre of wet, peaty soil.
- Whole tree: Clouds of little flies disturbed when you move the tree or water it; on seedlings or weak trees, slight loss of vigor if larvae graze fine roots.
Overview
Fungus gnats are small dark flies whose larvae live in the top layer of damp, organic potting soil, feeding on fungi, decaying matter, and occasionally fine roots. The adults are mostly a nuisance, but a population usually signals that the soil is being kept too wet — so they double as a useful warning sign of overwatering.
Causes
Constantly wet soil; peaty/organic, water-retentive mixes; poor drainage; and warm indoor conditions.
Commonly confused with
Whiteflies fly around the foliage and are white; fruit flies hover around fruit. Fungus gnats are dark and tied to the soil surface — and their presence usually means the soil is too wet (link to Overwatering).
Which species are affected
Any tree in damp organic soil, but overwhelmingly an indoor-bonsai problem.
Treatment
Let the soil surface dry out between waterings — this alone breaks the breeding cycle. Use yellow sticky traps for adults; a layer of grit on the surface deters egg-laying; biological controls (Bacillus thuringiensis israelensis 'mosquito bits', or Steinernema nematodes) kill larvae. Switch to a grittier, less peaty mix.
Prevention
Water properly (let the surface dry); use free-draining inorganic-leaning mixes; ensure good drainage; quarantine new plants and bagged soil.