how to get rid of black flies

Do you have small black flies living in your house plant compost? Or hovering around your house plants? These are fungus gnats, also known as house plant flies and sciarid flies. They're mostly harmless – adult gnats cause little or no harm to plants, but they can become a nuisance in the home. Their tiny worm-like larvae live in the top 5-8cm of compost, where they feed on algae, fungi and plant roots. Healthy house plants usually tolerate this minor root damage, but the larvae can harm seedlings or weak plants.

How to identify fungus gnats

Fungus gnats are small black flies that fly around house plants and live in house plant compost. You may mistake them for fruit flies – but if they're in and around your house plants,

How to get rid of fungus gnats

Getting rid of fungus gnats is easier than you might think. Simply by watering less often and using a gravel mulch you can break the fungus gnat lifecycle and stop them breeding in your house plant compost. But there are other ways, too. We list four ways to get rid of fungus gnats, below.

You Will Need

House plants
Yellow sticky traps
Grit mulch
Biological control for fungus gnats

Step 1

Water less often

Fungus gnat larvae need damp compost to live, as this is where algae and fungi thrive, on which the larvae feed. Simply by allowing the compost dry out between waterings, you'll greatly reduce the fungus gnat population.

Step 2

Use a gravel mulch

Most commercially available composts have been sterilised, so they don't contain fungus gnat larvae. If you cover the surface of the compost with a 1cm-thick mulch of gravel, grit or ornamental glass pebbles, this will stop house plant flies from being able to lay their eggs. Avoid using home-made garden compost indoors, as this can be a source of fungus gnats.

Covering the compost with gravel

Step 3

Use sticky traps

Yellow sticky traps work by trapping the adult fungus gnats and breaking their lifecycle. Simply hang up a trap near affected plants, or attach it to a bamboo cane inserted into the compost. Keep the trap near soil level, as gnats rarely fly far from the compost. Avoid hanging the traps outside as you'll also trap butterflies and hoverflies.

Step 4

Use biological control

If you have lots of house plants, it may be worth applying a biological control. To tackle fungus gnats use the nematode Steinernema feltiae, predatory mites or rove beetle larvae, and apply according to the pack instructions. These are available from online suppliers. While nematodes can be used in the home, the mites and beetle larvae are best used only in the contained environment of a greenhouse or sealed conservatory. If you've only got a few house plants, try growing a sundew (Drosera) nearby, as these sticky carnivorous plants are very good at trapping fungus gnats.