Fungus Gnats: How to Get Rid of Annoying Insects in the Substrate

Are tiny black flies buzzing around your pots and do translucent larvae with black heads sometimes appear in the soil? Then you are dealing with fungus gnats. These inconspicuous pests can trouble plants more than they seem at first glance. Adults do not cause direct damage, but the larvae eat fine roots, transmit pathogens and slow growth. The good news? They can be successfully controlled — using natural and biological methods. We'll show you how.

What are fungus gnats and why are they dangerous?

Fungus gnats (Sciaridae) look like small flies resembling mosquitoes and reach 1–4 millimetres in size. Female fungus gnats lay eggs in the substrate, from which translucent larvae with black heads hatch. It is the larvae that cause the most problems. They feed on organic matter, algae and young roots, disrupting the plant's uptake of water and nutrients. The result is slowed growth, yellowing leaves and general decline of the plants. Fungus gnats also transmit molds and pathogens, which together with a humid environment can have a devastating effect on your plants.

How do you know you are dealing with fungus gnats?

An infestation of fungus gnats is not hard to notice. The main sign is adult gnats hovering above the substrate, especially when watering or handling the plants. Fungus gnat larvae are found just below the surface of the substrate. If you don't intervene in time, you may observe these symptoms on your plants:

• wilting even when fully watered
• slow growth
• yellowing leaves

Natural methods for controlling fungus gnats

When your garden is infested with fungus gnats, try natural methods first before resorting to chemical sprays. These proven approaches work against fungus gnats:

• Yellow sticky cards: Adult fungus gnats are attracted to the color yellow. Place yellow sticky traps around your plants and catch as many adults as possible. Replace the sticky cards regularly.
• Let the substrate dry out: Fungus gnat larvae need moisture and thrive in waterlogged substrate. Allow the soil to dry out thoroughly between waterings to reduce the larval population around the roots.
• Watering with a weak hydrogen peroxide solution: A weak solution (e.g. 3% hydrogen peroxide diluted 1:4 with water) helps kill larvae in the substrate. Apply once every few days, ideally only as a one-off treatment.

Products against fungus gnats

Chemical sprays are, in the case of fungus gnats, less effective than with some other insect pests, and it is better to eliminate them by natural means. Biological control and protective bio-sprays based on natural oils are effective.

• Biological control against fungus gnats: Parasitic nematodes (Steinernema feltiae), bacteria (Bacillus thuringiensis var. israelensis) and predatory mites that actively hunt larvae are effective against fungus gnats.
• Sprays against fungus gnats: Biocont Na smutnice v substrátu, Agro Natura Smutnice
• Preventive products to protect plants from fungus gnats: Biocont PREV-GARD, Neem Azal, Rock Effect, CannaCure, BioBizz Leaf Coat, BioNova NovaFoliar, Botano Health

You might be interested in: How to deal with spider mites, How to deal with whiteflies, How to get rid of slugs

For more advice on fighting pests and other tips and tricks for indoor and outdoor growers visit our Higarden blog.