Fungus gnats, or sciarids, often appear in the compost of indoor pots, especially in cases of excess moisture. Initially discreet, they proliferate quickly, and their larvae damage young roots. A treatment with nematodes allows for natural eradication without chemicals.

Why use nematodes against fungus gnats?

Adult sciarids lay eggs in moist, organic-rich compost. Their larvae feed on roots and plant debris.

Nematodes are tiny microscopic worms that parasitise the larvae of gnats in the compost. Harmless to plants, humans, and pets, they have been used for biological control for a long time. Once introduced into the substrate, they actively seek out the larvae to infect them. The result: no more reproduction, hence the disappearance of adult gnats.

Adult sciarid and larva
Adult sciarid on the left and larva on the right

When to treat compost with nematodes?

As soon as you spot small black flies flying on the surface of the pot or walking on the compost, it’s time to act. Don’t wait for the population to explode. The treatment works year-round indoors, as long as the compost temperature remains above 12 °C.

Note: adult gnats are not always visible in time. Seedlings that are leggy or stagnant, or abnormal growth may indicate the presence of larvae.

Nematodes are tiny microscopic worms that parasitise the larvae of gnats
Close-up of nematodes

How to apply nematodes?

1. Order nematodes suitable for sciarids, such as Steinernema feltiae. They are usually delivered in powder form to be diluted.

2. Lightly water the compost before application to ensure good moisture: nematodes move better in a moist substrate.

3. Dilute the dose in water according to the supplier's instructions, adhering to the volumes. The water should not be too cold.

4. Water your pots with this solution, evenly. Pay special attention to the edges of the containers.

5. Keep the compost moist for 7 to 10 days after treatment to allow the nematodes to survive and act fully.

Good to know: nematodes can be stored in the refrigerator for a few weeks, away from light.

Nematodes are usually delivered in powder form to be diluted.

What to avoid after applying nematodes

Do not use fertilisers or chemicals just before or after treatment. Do not let the compost dry out too much: nematodes die without moisture. One treatment may be sufficient, but in case of heavy infestation, repeat after 2 to 3 weeks.

To prevent the return of fungus gnats, allow the surface of the compost to dry between waterings and place a layer of sand or gravel as mulch.