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Fungus gnats: how to recognise and treat it

Pest

Fungus gnats — Photo: Jean-Raphaël Guillaumin from Lausanne, Suisse, CC BY-SA 2.0, Wikimedia Commons

Symptoms

Small dark grey-brown flies, 3–4 mm long, running over the compost surface or flying slowly around pot plants and seed trays.
Slender whitish, see-through larvae up to 6 mm long with shiny black heads in the top layer of damp compost.
Seedlings and soft cuttings wilt or are stunted because larvae eat their roots or tunnel into the base; a wilting plant in wet compost may be a sign.

Organic treatment

Let the surface of the compost dry between waterings, as far as the plant tolerates it.
Hang yellow sticky traps near the plants to catch adults (they also catch beneficial insects).
Drench the compost with Bacillus thuringiensis israelensis (Bti), repeating about every five days, or with Steinernema feltiae nematodes or predatory mites (Hypoaspis), following the label.

Chemical treatment

Only if Bti or nematodes are not available and numbers are intolerable — pyrethrins sprayed on the compost surface and where adults rest give short-term control; use a product labelled for house plants and follow the label.

Reference information, not a guarantee. Read and follow the label of any product: dose, safety and the days to wait before harvest.

Prevention

Avoid overwatering and give pots good drainage; empty standing water.
Use fresh, pasteurised potting compost made for house plants; repot into new compost and do not reuse old compost.
Do not use half-rotted organic matter in potting mixes or heavy doses of manure or blood meal.

Sources

Figures checked against these sources.