Steinernema feltiae
Entomopathogenic nematode- Rate
- 250k–500k /m²
- Temp
- 10–30 °C
- Best for
- Larvae present now; fast drench
Pest
Fungus gnat
Bradysia spp.
The short answer
The small dark flies drifting up from houseplant compost are fungus gnats. The adults do not bite and do no harm; the damage is done by their larvae, which live in the top few centimetres of wet compost and feed on fungi and roots. To get rid of them, let the compost surface dry between waterings, catch the adults on yellow sticky cards, and treat the larvae with a nematode or Bti drench.
hero macro · larva on compost, adult above
Bradysia larva with its shiny black head capsule, and adult fungus gnat. Photo: pending.
Identification
Adults are delicate grey-black flies about 3 mm (1/8 in) long, with long legs, long segmented antennae and clear to smoky wings; the common Bradysia species have a Y-shaped vein near the wing tip. They are weak fliers, usually found on the compost surface or near light sources. RHS puts most at 3–4 mm; UC IPM gives 1/16–1/8 inch, with a few species larger.
The larvae are the stage that matters: slender, legless, whitish to translucent worms up to about 6 mm, with a shiny black head. They sit mostly in the top 2.5–7.5 cm (1–3 in) of compost, though some go deeper. Shore flies share the same wet benches but are stouter, with very short antennae and pale spots on dark wings, and their tan larvae have no dark head capsule.
Easily confused with
- Shore fliesAlso a pestStouter, faster fliers with very short antennae and about five pale spots on each dark wing; fungus gnats are slender, weak fliers with long antennae.
- Fruit fliesAlso a pestUsually red-eyed, tan in front and black behind, and bred in fermenting food, drains and bins; fungus gnats are delicate black flies with long legs and antennae.
- SpringtailsHarmlessSpringtails have legs and jump by flicking a tail-like lever under the abdomen; fungus gnat larvae are legless, translucent, black-headed maggots in the compost.
Damage symptoms
An established plant in a large pot rarely shows anything beyond the flies. The larvae prefer fungus and decaying matter and turn to living tissue when that runs short, so the plants that suffer are the ones with small root systems: seedlings, plugs and freshly stuck cuttings, where feeding on the callus and root initials can slow or stop rooting. The wounds also let root-rot fungi in, and the adults carry their spores from pot to pot.
- Small dark flies rising from the compost when the pot is moved or watered
- Wilting, yellowing or stunting in seedlings and young plants
- Cuttings slow to callus or root, or rotting at the base
- Root hairs stripped; tunnels in soft stems at compost level
- Root rot following the feeding — Pythium, Fusarium or Thielaviopsis
Lifecycle & degree days
Everything except the adult happens inside the compost, which is why sprays aimed at the flies do so little. The cycle is short and speeds up with warmth, and indoors and under glass it runs all year with generations overlapping, so eggs, larvae and pupae are all present at once. Any single treatment misses whatever is in the egg or pupa at the time, and that is why every control is repeated.
- Egg to adult
- 17–18 days at 24–25 °C; ~40 days at 16 °C
- Eggs per female
- Up to 200 (UMass: up to 300)
- Adult life
- About 7–10 days
- Larvae found
- Mostly top 2.5–7.5 cm (1–3 in) of compost, some deeper
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Host crops
Fungus gnats are not tied to a plant; they are tied to wet, organic compost. Any crop raised from seed or cuttings under glass can host them, and the risk is highest in propagation, seedling flats and anything overwatered. Established plants are usually unharmed. Each crop hub below carries the setting-specific program.
Monitoring & action thresholds
Action threshold
3–5 larvae per potato slice
Colorado State's figure for propagation pots; 15–20 per slice in production pots. Clemson Extension says no threshold is established for propagation and advises starting biological control at first detection.
Watch both stages. For adults, yellow sticky cards laid face up on the compost or held just above the canopy — in a greenhouse, one to four cards per 1,000 sq ft (about 90 m²); on a windowsill, a small square of card on a skewer in the pot. For larvae, press slices of raw potato about 2.5 cm across into the compost surface and lift them after 48 hours: the larvae gather underneath. Replace the slices each time and record the counts weekly.
Beneficial
Biological control
Every biological option works on the larvae in the compost; none touches the adults, so fly counts fall only after the flies already in the pots have died off. The nematode is the fast curative drench. The predatory mite and the rove beetle live in the compost; the mite works best put in at planting, before numbers build. Bti is a drench that has to be repeated as needed.
Stratiolaelaps scimitus
Hypoaspis miles, soil predatory mite- Rate
- 100–500 /m² per release (Koppert); Biobest 50–500, up to 1,000 at high pressure
- Temp
- Media above 15 °C (Biobest: active from 10 °C); compost moist, not wet
- Best for
- Preventive, at potting or sticking
Bacillus thuringiensis israelensis
Bti (mosquito dunks, Gnatrol)- Rate
- Label rate — Mosquito Dunks 1 per 100 sq ft of water surface; Gnatrol home label 1–1.5 tsp per gallon as a soil drench
- Best for
- Drench, repeated as needed
Cultural controls
Water is the lever. Larvae survive best in compost that never dries, and letting the surface dry out is often enough on its own for houseplants, as long as the plant can take it. In a greenhouse the same logic runs through the whole range: dry benches, no algae, clean compost.
- Let the top 2.5–5 cm (1–2 in) of compost dry between waterings
- Tip out water standing in saucers and under benches
- Repot badly infested houseplants into fresh compost
- Keep compost bags closed until use; pasteurise reused media
- Clear algae, weeds and spilled compost from benches and floors
Chemical, last resort
Sprays aimed at the flies miss the larvae doing the damageCheck any product before it goes on. Open the compatibility matrix →
Rating: the effect on this pest's natural enemies if a spray hits them or they feed on treated plants. REI is the re-entry interval on farm and greenhouse labels; home labels usually give none, so follow the label.
Sources
UC IPM Pest Notes: Fungus Gnats, Pub. 7448 (Bethke & Dreistadt, rev. 2013)
UC IPM Floriculture & Ornamental Nurseries Guidelines — Fungus Gnats (updated 2022)
Colorado State Univ. Extension fact sheet 5.584, Fungus Gnats (reviewed 2025)
Colorado State Univ. Agricultural Biology — Fungus gnats (IPM pests page)
UMass Extension — Fungus Gnats and Shore Flies (T. Smith, updated 2026)
Clemson Land-Grant Press LGP 1131 — IPM for Fungus Gnats in Propagation (2021)
Purdue Extension E-111 — Fungus Gnats and Shore Flies (Sadof, 2021)
RHS — Fungus gnats (sciarid flies)
BASF — Nemasys fungus gnat technical bulletin (2013)
Page status
Drafted from the sources below, then checked claim by claim against those sources on 23 September 2026.
Suggest a correction →Fungus gnat by crop
The same pest behaves differently on different hosts. Each page below gives the host-specific control, rate and threshold.