Fungus gnats: where they come from and how to protect your plants

Small black flies around your houseplants are almost always fungus gnats. The adults are merely a nuisance; the real problem is the larvae in moist potting compost. Let the compost dry out and catch the gnats with yellow sticky traps, and you break the cycle.

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What is the difference between fungus gnats and fruit flies?

Fungus gnats are black to dark grey, slender and long-legged; they mostly sit and run on the compost. Fruit flies are light brown with red eyes and circle around fruit and food waste. Sciarid flies is simply another name for fungus gnats.

What attracts fungus gnats and where do they come from?

Moist, humus-rich potting compost. That is where the females lay their eggs, and the larvae feed on fungi, dead plant material and fine roots. The eggs or larvae often arrive with new compost or a new plant.

How fast do fungus gnats multiply?

A female lays up to 200 eggs in the top layer of the compost. After four to six days the larvae hatch; they feed in the compost for two to three weeks, pupate and then emerge as new gnats. At room temperature a full cycle takes about three to four weeks. The adults live for only about a week.

When are fungus gnats most common?

Indoors all year round, because houseplant compost is always warm enough. You see the most in winter, when plants use less water and the compost stays wet for longer, and in spring after repotting with fresh compost.

Are fungus gnats harmful or dangerous?

Not to people or pets: fungus gnats do not bite and do not transmit diseases. They can be harmful to plants, though. In a heavy infestation the larvae feed on the fine roots, and cuttings, seedlings and young plants in particular can then struggle or die.

How do you get rid of fungus gnats?

With two steps at the same time. Let the top two to three centimetres of compost dry out between waterings, because the larvae need moisture. And put yellow sticky traps in the pots: the yellow colour attracts the adult gnats, which stick to the glue and lay no more eggs. After a few weeks the cycle is usually broken.

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How do you prevent fungus gnats?

1. Check the compost of new plants. Keep new plants apart for a few days and watch for gnats.
2. Water the right way. Less often but thoroughly, and let the top layer dry out.
3. Do not leave water in the saucer. Standing water keeps the compost permanently wet.
4. Remove dead plant material. Wilted leaves and flowers are food for the larvae.
5. Repot infested plants. Replace the top layer of compost or cover it with a layer of sand so the females can no longer lay eggs.