How do I treat fungus gnats?

Let the top inch or two of compost dry out fully between waterings. This alone ends most infestations. Yellow sticky traps catch adults and break the cycle.

Will fungus gnats spread to my other plants?

Yes. Fungus Gnats spreads, so deal with affected material promptly and keep it out of the compost.

Fungus Gnats

Q: What causes fungus gnats?

Fungus Gnats is pest. Fungus gnats are almost entirely a nuisance rather than a threat — the adults do no damage at all and the larvae eat fungus and decaying matter, only occasionally nibbling fine roots. What they do reliably indicate is soil that is being kept too wet, which is a real problem in its own right.

Mostly cosmetic

Pest · spreads to other plants · also called soil gnats, sciarid flies

Fungus gnats are almost entirely a nuisance rather than a threat — the adults do no damage at all and the larvae eat fungus and decaying matter, only occasionally nibbling fine roots. What they do reliably indicate is soil that is being kept too wet, which is a real problem in its own right.

What it looks like

Small dark flies, a little smaller than a fruit fly, that run across the soil surface and fly in short erratic hops when the pot is disturbed. Larvae are translucent with black heads and live in the top inch of damp compost.

How to confirm it

Adults run on the soil surface rather than flying around fruit like fruit flies.
They appear from the pot, not from the plant.
Always associated with consistently damp compost.
Larvae are visible as translucent threads with black heads in the top inch of soil.

What to do

1Let the top inch or two of compost dry out fully between waterings. This alone ends most infestations.
2Yellow sticky traps catch adults and break the cycle.
3A layer of coarse grit or sand on the surface stops adults reaching the compost to lay.
4Bacillus thuringiensis israelensis, sold as mosquito bits, kills larvae when soaked into the water.

What it gets confused with

One observation settles each of these.

Whiteflies →

Whiteflies are white, cluster on leaf undersides and rise from the foliage. Fungus gnats are dark and come off the soil.

Preventing it next time

Water less often and more thoroughly, rather than little and often.
Use fresh, well-drained potting mix and avoid pots without drainage.

Plants affected

Houseplants Orchids Succulents & Cacti

Check these two things first

Push a finger into the soil, two knuckles deep. Wet soil plus a wilting plant means too much water, not too little — and it is the commonest reason a plant is dying. Dry soil plus crisp leaves means the opposite. The leaves look nearly identical either way, so the soil is what tells you.
Turn a damaged leaf over. Half of all diagnoses live on the underside: fuzz means downy mildew, fine webbing means spider mites, clusters of insects mean aphids. Nothing on the top of a leaf is as informative.

Most garden problems are cosmetic and resolve without treatment. Reach for a spray last, not first.

Not sure it is fungus gnats? Run the diagnostic key →