Fungus gnats
Sciaridae
also called Sciarid flies, Compost flies
Small dark flies drifting over damp compost; their larvae nibble seedling roots. Let the compost surface dry between waterings and use yellow sticky traps.
How to know it
Small grayish-black flies 2–4 mm long that run over the compost surface and drift up in a slow cloud when a pot is moved. The larvae are translucent white with shiny black heads, up to 6 mm, and are visible on the wet compost surface if you look closely.
What you'll see
- Small dark flies rising from pots and seed trays whenever they are watered or moved.
- Thin white larvae with black heads on the surface of wet compost.
- Seedlings failing to thrive, stalling, or keeling over at the base.
- Fine root hairs and stem bases grazed away on cuttings and plugs.
- Worst in propagators, on cuttings and in freshly potted plants kept very wet.
Where it comes from
Eggs are laid on damp compost and hatch in a few days; the larvae feed mostly on algae, fungi and dead organic matter but turn to living roots when numbers rise. A generation takes about three weeks at room temperature, so an indoor population compounds continuously and never has a winter.
Overwatered pots, peat-based composts kept permanently wet, propagators, and warm indoor rooms. Established plants are essentially unharmed; it is seedlings and cuttings that suffer.
Dealing with it
- Let the top two centimeters of compost dry between waterings — this alone ends most outbreaks.
- Water from below so the surface stays dry, particularly for seed trays and cuttings.
- Hang or lay yellow sticky traps to catch the adults and track the numbers.
- Top-dress pots with a centimeter of grit, vermiculite or sand so the flies cannot lay.
- Water in Steinernema feltiae nematodes or Hypoaspis predatory mites for a persistent infestation.
Keeping it out
- Use fresh, clean compost and store bags sealed and dry.
- Do not leave standing water in saucers or trays.
- Clear dead leaves and algae from the surface of pots and the greenhouse bench.
- Pot on promptly so roots fill the compost rather than leaving wet dead zones.
- Keep the propagator ventilated and avoid permanently saturated seed compost.
What it goes for
Seedlings, cuttings and houseplants of any kind — the compost matters more than the plant.
Trouble it brings with it
Who hunts it for you
- Beneficial nematodesSteinernema & Heterorhabditis spp. Microscopic worms watered onto damp soil that hunt down soil-dwelling grubs and release a bacterium that kills them within days. Timing is the whole thing: warm, damp soil and a small target grub, which means late summer for most of what they are used against.
- CentipedesChilopoda Flat, fast, one pair of legs to a segment — and a predator, unlike the slow round millipede it is constantly confused with. It hunts slugs, cutworms, springtails and woodlice by night under mulch and stones. Learn the difference and leave this one alone.
- Dragonflies & damselfliesOdonata Aerial hunters that take mosquitoes, midges and small flies on the wing all day, and whose larvae spend a year or more clearing the same insects from the water. A pond with planted margins and no fish is the way to have them. Grow to recruit it: Bulrush,Iris,Nymphaea,Pickerel Weed
- Predatory mitesPhytoseiulus & Amblyseius Fast orange mites that hunt spider mites egg-by-egg, and cousins that clear thrips and fungus-gnat larvae. Sold as a biological control for greenhouses; outdoors they follow humidity — mist and mulch, don’t dust with sulfur.
- Rove beetleStaphylinidae Slim, quick beetles with short wing cases that hunt in soil and litter, taking root-fly eggs and larvae, fungus-gnat larvae, and mites. Mulch and undisturbed ground keep them resident year-round. Grow to recruit it: Mint,Thyme
See also
The Cottage Garden Companion — the gardener's bestiary at cottagegardencompanion.com