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How to tackle house plant flies


It’s not uncommon to see tiny brown or black flies hovering around your house plants. Take a closer look and you’ll see them scuttling over the compost too.

These are fungus gnats, also known as sciarid flies. 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 seriously harm seedlings or weak plants.

Getting rid of fungus gnats is easier than you might think - here are four of the best ways:

How to do it

Watering an African violet

Reduce watering


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.

Adding gravel mulch

Mulch the compost

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.

Yellow sticky trap

Hang yellow sticky traps

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. 

Carnivorous sundew plant

Use a 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.

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Talkback: How to tackle house plant flies
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flowering rose 13/09/2013 at 15:34

sounds good advice about watering less and covering with gravel.I had noticed it was better when I did this so good to confirm.

indigo6ix 18/01/2014 at 11:39

I wanted to find kut how to deal with sciarid fly and read the artice above. I am not sure when this was written but I would like to draw attention to this webpage by the RHS that I read today.

maggie-1 28/12/2014 at 21:28

We've been driven mad by these pests, always flying around. Not sure how I found out, but have tried only watering plants from the bottom, not onto the compost. It seems that they lay their eggs on the top of the compost when it is nice and damp. This has made a big difference, far fewer of the little blighters now.