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Fungus gnat larvae prefer damp compost, as this is where algae and fungi thrive, on which the larvae feed. Let the compost dry out between waterings and you’ll greatly reduce the gnat population. Not only that, but plant roots exposed to too much water are likely to die off, so cutting back will benefit the plants too.
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 flies from laying their eggs. Avoid using home-made garden compost indoors, as this can be a source of fungus gnats.
Sometimes the traditional methods are still the most effective. Yellow sticky traps are organic and pesticide free, and they work because the colour is very attractive to fungus gnats. 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. The traps will also capture whitefly, aphids and bluebottles.
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.