Fungus gnats

Identification

Small, dark gnats (2–4 mm) flying around pots and scurrying across the surface of the substrate. The adults are a nuisance, but the real damage is done by their larvae in the soil, feeding on the roots and the base of the stems of young plants. A constantly moist, humus-rich substrate favours their development — the classic result of overwatering houseplants. Adult fungus gnats are delicate, mosquito-like flies with a slender, dark grey body, long, beaded antennae and a single pair of smoky wings; they fly clumsily in short bursts, readily settle on the surface of the substrate and on window panes, and when disturbed rise up in a little cloud. They themselves do not damage plants — they neither pierce nor suck sap — but their presence is a reliable sign that larvae are developing in the substrate. The larvae are translucent, whitish, legless and elongated (up to 5–6 mm), with a distinct, glossy black head capsule — it is this black head that distinguishes them from other soil organisms. They feed mainly on dead organic matter, fungal mycelium and algae, but at high densities they gnaw on young roots, root hairs and the base of stems. The signs of their feeding are wilting and stunted growth of seedlings and young potted plants, yellowing, poor rooting and, in extreme cases, seedlings toppling over and dying off where they have been chewed at the base. The wounds inflicted by the larvae are also entry points for infection, and the larvae and adults themselves can carry the spores of soil pathogens (including Pythium, Fusarium, Botrytis), linking the pest problem with root-rot diseases. The development cycle (egg → larva → pupa → adult) takes about 3–4 weeks in warmth and shortens at high temperatures; a female lays several dozen to over a hundred eggs on the moist surface of the substrate, and in a heated home or greenhouse the generations overlap throughout the year, so that with constant overwatering the population builds up quickly.

Treatment

Above all, reducing watering and letting the top layer of substrate dry out (the larvae die in dry soil). Yellow sticky traps for the adults, covering the surface with sand or perlite, and, if needed, insect-parasitic nematodes or preparations targeting the larvae. The foundation is cutting off the larvae's moist environment: water less often and only once the top 2–3 cm of substrate has dried out, preferably from below (from the saucer), so that the surface layer stays dry between waterings; ensure good drainage and remove dead leaves and organic debris from the surface, which are food for the larvae. Covering the surface with a 1–2 cm layer of dry sand, perlite, fine gravel or expanded clay makes it harder for females to lay eggs and for emerging insects to get out of the substrate. Yellow sticky traps catch the adults and serve mainly for monitoring and reducing egg-laying — they do not themselves destroy the larvae, so treat them as a supporting method, not a standalone solution. Real control of the larvae comes from biology: insect-parasitic nematodes Steinernema feltiae destroy the larvae in the soil, and the predatory soil mite Stratiolaelaps (Hypoaspis) eats the eggs and youngest larvae — both are introduced into the substrate by watering or scattering; also effective is a bacterial biopreparation based on Bacillus thuringiensis subsp. israelensis (Bti), applied as a soil drench and selectively affecting the larvae of flies. Quarantining newly bought plants and using fresh, well-decomposed substrate limit the introduction of the pest. Chemical products are rarely necessary in amateur cultivation; when the population gets out of control, you can reach for a contact insecticide from the pyrethroid group to reduce the number of adults — pyrethroids, however, bind strongly to the organic matter of the substrate and, applied as a drench, reach the larvae poorly, so against the larvae biology (Bti, nematodes, mites) and drying out remain more effective; without simultaneously drying out the substrate and removing the source of moisture the problem quickly returns.

How to tell it apart from look-alikes

Feature	Fungus gnats	Fruit flies (Drosophila)
Where they appear	Around pots, on the surface of the substrate, settling on the soil	Around ripe and rotting fruit, the waste bin, the compost
What attracts them	Moist, humus-rich substrate; mycelium and algae in the soil	Fermenting fruit, vinegar, wine, kitchen scraps
Appearance	Dark grey, slender, mosquito-like, long antennae, weak flight	Stocky, yellow-brown, often red eyes, agile flight
Harm to the plant	Larvae damage roots and seedlings in the substrate	They do not harm plants — they develop in fruit and waste
Larvae	In the soil, whitish with a glossy black head	In rotting fruit and scraps, never in the pot's substrate

Control — organic and chemical methods

Organic and biological methods

Method	How it works	When to use
Drying out the substrate and watering from below	The larvae need a moist surface — letting the top 2–3 cm dry out and watering from the saucer interrupts their development.	Continuously, as the foundation of prevention and control
Mineral barrier on the surface	A 1–2 cm layer of dry sand, perlite or gravel makes egg-laying and the emergence of adults from the substrate more difficult.	After repotting and at the first gnats
Insect-parasitic nematodes and predatory soil mites	Steinernema feltiae destroy the larvae in the soil; the predatory mite Stratiolaelaps (Hypoaspis) eats the eggs and youngest larvae.	When a larval population is confirmed
Bacillus thuringiensis israelensis (Bti) biopreparation	A bacterial preparation applied as a soil drench, selectively affecting fly larvae while sparing other organisms.	Repeated drenches every ~7 days
Yellow sticky traps	They catch the adults and reduce egg-laying; they serve mainly for monitoring — they do not destroy the larvae, so they are a supporting measure.	On an ongoing basis, for monitoring and reducing adults

Chemical methods

Method	How it works	When to use
Pyrethroid (contact insecticide)	Reduces the adults (contact); as a drench it reaches the larvae poorly — rarely necessary in home cultivation, only at high intensity.	A last resort, always together with drying out the substrate

We give active-ingredient groups and modes of action, not brand names — specific products and their approval vary by country and are sometimes withdrawn. Always check and follow the current label.