Fungus gnats

What you are looking at

Small dark flies, two to four millimetres long, that drift up out of the compost when you move a pot and settle again a foot away. They have long legs, long beaded antennae and a single pair of clear wings, and they fly badly, wandering rather than darting.

Most of them are on the compost surface or walking up the inside of the pot rather than on the plant. That is the clue that separates them from anything that matters. A pest of leaves is found on leaves.

The stage doing anything at all is underground. Scrape back the top two centimetres of compost and look for translucent white legless larvae up to about six millimetres long, each with a hard shiny black head. They are unmistakable once seen and invisible until looked for.

What they are eating

The clue is in the name. These larvae are fungus eaters, living on the fungi, algae and decaying organic matter in permanently damp compost. A pot that stays wet at the surface, with a layer of fallen leaves on it and compost that has begun to break down, is what they came for.

They also graze root hairs and fine roots while they are there. On an established houseplant that costs the plant almost nothing, which is why this page rates the problem as cosmetic when so much written about it does not.

The exception is worth taking seriously. In a propagator, a seed tray or a pot of soft cuttings, the roots are the whole plant, and a heavy population will kill seedlings outright and open wounds that let Pythium in behind them. If you are sowing, deal with fungus gnats. If you have a mature monstera, deal with the watering.

Why they keep appearing

At room temperature an egg becomes an adult in three to four weeks, and a female lays a hundred or more eggs in the top layer of compost. Indoors there is no winter, so generations run continuously and overlap.

That is why killing the adults changes nothing. Every fly you swat has already laid, and the pot holds the next fortnight’s worth. Anything that only addresses the flying stage is a sedative rather than a treatment.

Their arrival is generally traceable. They come in on a new plant, in a bag of compost left open in a shed, or from a pot that has been overwatered all winter and is now the source for the whole windowsill.

What to do about them

Stop watering so often. Let the top two centimetres of compost dry properly between waterings, and the eggs and young larvae in that layer die without anything being applied to them. For most houseplants this is what they wanted anyway, and the population collapses within a few weeks.

Where a plant cannot be allowed to dry at the surface, cover it instead. A centimetre or two of horticultural grit or perlite over the compost dries within hours of watering, gives the female nowhere to lay and stops the adults getting out. It works on ferns and calatheas, which are the plants where drying out is not on offer.

Take the dead leaves off the surface while you are there, and repot anything sitting in dense broken-down compost.

For a serious population, or any seed tray, water in Steinernema feltiae nematodes. They hunt the larvae in the compost, they are sold for exactly this, and they do not touch the plant. Keep the compost moist for a fortnight afterwards so they can move, then let the surface dry again.

Yellow sticky traps are worth putting up not because they control anything but because they measure. A trap filling more slowly each week tells you the watering change has worked.

What it gets mistaken for

Whitefly are white, they sit on the undersides of leaves rather than on the compost, and disturbing the plant sends up a cloud that resettles on the foliage. Fungus gnats are dark and stay near the pot.

Fruit flies are tan or yellowish with red eyes, they hover around fruit, wine and the kitchen bin, and they fly quickly and precisely. Fungus gnats are blackish and drift.

Drain flies are furry, moth-like and triangular at rest, and they come out of sink and shower drains rather than from compost.

Thrips are the ones worth ruling out properly, because they are also small, dark and associated with plants. They are on the leaves, they are slender rather than fly-shaped, and they leave silvered patches with tiny black specks on the foliage. No leaf damage means no thrips.