Fungus gnats: the small dark flies around houseplant soil

Check your plant with the questions

If small dark flies drift up from a pot when you water it, or walk over the surface of the potting mix, they are almost certainly fungus gnats. They are a sign that the top of the mix stays wet, far more often than a sign that the plant is in danger. The flies themselves do nothing to a plant; the damage, when there is any, comes from their larvae in the mix, and it matters mostly to seedlings, fresh cuttings and plants whose roots are already failing.

What they look like

The adults are slim, gray-black flies with long legs and long, jointed antennae, about an eighth of an inch long or less. They fly weakly and not far: you will see them resting on the rim of the pot, on the mix, on nearby leaves, or collected against the nearest window, since light draws them. The larvae live in the top inch or so of the mix. They are thin, legless and nearly clear, with a glossy black head, and in a very wet pot they can leave faint trails of slime on the surface. In British guides you will also see them called sciarid flies.

What they are often mistaken for

• Whitefly rise from the undersides of leaves, not from the soil, and they are white rather than dark.
• Fruit flies gather around ripe fruit, compost bins and drains. They are stockier, often tan with red eyes, and have no interest in a pot of damp mix.
• Shore flies also breed in wet mixes and algae, but they are heavier-bodied and stronger in flight, and their antennae are stubby bristles where a fungus gnat's are long and jointed.

Do they harm the plant?

On an established plant in a pot that dries out properly, rarely. The larvae feed mainly on fungi and decaying organic matter in the mix. Where the mix stays wet and they build up in large numbers, they also graze fine roots, and that is when harm follows: seedlings and cuttings, with few roots to spare, stall or collapse. A houseplant that wilts while the gnats multiply is more likely to be suffering from the wet itself, through root rot, than from the gnats; the flies and the decline share a cause.

Why they turn up

Constant moisture near the surface is the common thread. Watering on a schedule rather than when the mix has dried, saucers left standing full, pots without drainage holes and mixes rich in peat, bark or partly rotted compost all give the larvae what they need. A new bag of potting mix can bring them in, especially one stored open or on a damp floor.

What to try first

1. Let the top of the mix dry out. Water only once the upper inch or two feels dry, and empty the saucer after watering. This is the step that ends most infestations, because the larvae cannot finish developing in dry mix.
2. Trap the adults. Yellow sticky cards cut into small squares and pushed into the mix on a stick catch the flying adults and show whether numbers are falling.
3. Check for larvae. Press a few slices of raw potato, cut side down, onto the surface. Two or three days later, lift them: larvae gather underneath. Throw those slices out and replace them.
4. Treat the larvae if drying is not enough. Products based on Bacillus thuringiensis subspecies israelensis (Bti), sold for mosquito control, kill fly larvae and little else. Watered into the mix, they need repeating about every five days, since they do not persist. The nematode Steinernema feltiae, bought by mail order, hunts the larvae in moist mix.
5. Repot badly affected plants into fresh, packaged mix, removing as much of the old mix as the roots allow.

Sprays aimed at the flying adults do little: most of each generation is out of sight in the mix, and new adults keep emerging. Indoors, insecticides are seldom worth using against them.

How to tell it is working

In a warm room a new generation can appear in under three weeks, so give any change that long before judging it. If the sticky cards catch fewer flies each week and the potato slices come up clean, the cycle is broken. If flies keep appearing after the mix is allowed to dry, look for a second source: another pot, or an opened bag of mix or bark stored nearby.

Plants whose sources name fungus gnats

The plants below are the ones whose own RHS or NC State Extension entry lists fungus gnats among their pests. Any plant grown in mix that stays wet can host them.

• Dumb cane (NC State Extension)
• Gloxinia (NC State Extension)
• Hoya (NC State Extension)
• Zonal geranium (RHS)

If the mix is growing a white, green or crusty coating as well, see mold or crust on the soil; the two often share a cause. To work out what else is going on with a plant, start the diagnosis.

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