Fungus gnats in seedlings: how to spot and stop them

The first seed trays are barely on the windowsill when small dark flies start dancing over the compost. Fungus gnats are the most common nuisance in propagation — and one of the few pests you bring on yourself, by keeping the surface permanently wet.

The adults are harmless. It is their larvae that do the damage: they live in the compost and feed on fine roots and the root collar. A sturdy tomato shrugs that off, a freshly germinated seedling does not — a bad infestation can cost you a whole tray.

In short

Fungus gnats are 2–4 mm dark flies, weak fliers, often seen running across the compost.
The damage is done by the glassy white larvae with a black head capsule, not by the adults.
The cause is almost always a permanently moist compost surface.
Yellow sticky traps catch the adults; nematodes or Bacillus thuringiensis israelensis deal with the larvae.
Prevention beats treatment: water from below, let the surface dry, top with sand or vermiculite.

What are fungus gnats?

Fungus gnats (Sciaridae) are a large family of small flies. Adults reach 2 to 4 mm, are greyish black with noticeably long antennae, and fly clumsily — the giveaway is the jerky take-off when you lift a pot. They live about a week, and they neither bite nor suck.

In that week a female lays 100 to 200 eggs in the top few millimetres of moist compost. The larvae hatch within days: up to 7 mm long, translucent white, with a clearly visible black head capsule. At room temperature a generation takes three to four weeks. That is why a few flies become a plague within a month.

How do I recognise an infestation?

Adults: small dark flies running over the compost or sitting on the window. They take off when you water.
Larvae: scratch the surface with a finger — the white larvae with black heads are visible to the naked eye.
Potato test: place half a raw potato cut-side down on the compost. After two days the larvae gather underneath, which also shows how heavy the infestation really is.
Plants: seedlings stall, wilt despite moist compost, or topple over. That looks a lot like damping-off — and often occurs alongside it.

What damage do the larvae do?

The larvae mainly eat dead plant material, algae and fungal threads in the compost. When that runs short, they turn to living roots. In young plants this matters: the root system is still small, and every tunnel costs noticeable uptake area.

The bigger damage is indirect. Feeding wounds are open doors for fungi such as Pythium or Fusarium, the agents of damping-off. A gnat infestation is therefore often the groundwork for losses that people then blame on wet compost alone.

Prevention: a dry surface beats any product

Fungus gnats need moist compost at the surface to lay eggs. Take that away and the cycle collapses on its own. This works better than any treatment and costs nothing.

Water from below. Fill the saucer, pour off what is left after 20 minutes. The roots get enough, the surface stays dry.
Let it dry back. Only water again when the top centimetre or two is dry. Use your finger, not the calendar.
Cover it. One to two centimetres of quartz sand, vermiculite or fine expanded clay. The layer dries fast and holds no appeal for egg-laying females.
Check your compost. Peat-rich, fresh or damply stored compost often arrives with eggs already in it. Store seed compost dry and cool.
Sticky traps from day one. Not only once you have a problem — they warn you early and keep the number of egg-laying females down.

Treatment: what actually works

The key is to tackle both stages at once. Catch only the adults and the next generation arrives from the compost in a fortnight. Treat only the larvae and fresh eggs arrive from outside.

Yellow sticky traps placed low over the compost. They catch adults and interrupt egg laying. Replace every one to two weeks.
SF nematodes (Steinernema feltiae): microscopic roundworms stirred into the watering can. They enter the larvae and kill them. The compost should be above 12 °C and evenly moist — so here, deliberately, you do not keep it dry. Two applications two weeks apart.
Bacillus thuringiensis israelensis (Bti): a bacterium that targets gnat larvae specifically and is harmless to people, pets and bees. Sold as sticks or powder for the watering can, applied several times a few days apart.
Repotting for heavy infestations: shake the root ball off gently, move into fresh dry compost, then water sparingly from below.

Tip

Nematodes and Bti only reach the larvae in the compost. Always combine them with sticky traps — otherwise the females lay the next batch of eggs while you treat, and the problem drags on for months.

Beneficial predators

Besides nematodes there are predatory mites of the species Stratiolaelaps scimitus (formerly Hypoaspis miles). They live permanently in the upper compost layers and eat eggs and young larvae. For greenhouses and larger stocks they are the most convenient answer, because they sustain themselves as long as there is prey.

On a windowsill it rarely pays off — there, a dry surface plus a sticky trap and one dose of nematodes gets you there faster.

Caution

Chemical sprays aimed at the flying adults achieve almost nothing against this pest: the real population sits in the compost as larvae and is never reached. You burden your seedlings without touching the cause.

Frequently asked questions

Are fungus gnats dangerous to people or pets?

No. They do not bite, do not suck blood and carry no diseases. They are a nuisance, nothing more. The damage is limited to the roots of young plants.

Why do they keep coming back after treatment?

Usually a residual population sits in another pot in the same room, often in a houseplant nobody checks. Treat every pot in the room at the same time, not just the seed trays.

Do cinnamon, baking soda or matchsticks help?

There is no reliable evidence for these remedies. Cinnamon inhibits fungi at high concentrations but does not deal with larvae. Nematodes and Bti are just as natural and demonstrably do the job.

Can I still use infested compost?

Outdoors yes, it sorts itself out. For windowsill propagation, better not — use fresh compost that has been stored dry.

If you would rather skip the stage where things go wrong: our young plants are raised in the greenhouse under controlled conditions and arrive well rooted — the trickiest phase of propagation is already behind them.