How to get rid of fungus gnats

What are fungus gnats?

Fungus gnats are small black flies, also known as sciarid flies, mushroom flies and mycetophilids. They are a very common pest, especially when plants are grown in pots of soil-free compost. In fact, the less soil there is and the more artificial compost additives (perlite and vermiculite, for example) the more sciarid flies there appear to be.

Although most kinds do no damage, a few may be a menace at times. They can be hard to clear up, but you can avoid or at least minimise their numbers with careful watering and other measures.

25% off Which? Gardening

Pay £36.75 for the first year and get the garden you want for less

Get this offer

Which plants are most at risk from damage from fungus gnats?

Houseplants, carnations, chrysanthemums, cyclamen, freesias, orchids, pelargoniums, primulas, saxifrage, sempervivum and sweet peas are susceptible.

Other likely targets are cucumbers and cultivated mushrooms. When conditions are favourable – in houses or warm greenhouses – breeding can take place all year round.

Fungus gnats are attracted to composts that are high in organic matter, such as peat-based composts, decaying plant tissue and compost that is kept constantly damp, such as pots containing insectivorous plants. They feed mainly on plants, but are also partial to any form of decaying organic matter.

Discover our Best Buy pop-up greenhouses

How can I deter fungus gnats?

Allow the surface of peat-based composts to dry out a little before watering the plant again. Remove dying leaves and dead flowers regularly, so that they do not begin to decay and so attract fungus gnats. Adults are less likely to lay eggs on dry surfaces with no decaying plant tissue for the newly hatched maggots to feed on. Covering the compost with grit or pebbles can help.

Fungus gnats are also very fond of stored compost, especially if it is freshly sterilised or contains animal-derived fertilisers. Keep all your spare compost bags tightly closed. Even better, buy new compost as you need it and avoid storing it altogether. Loam-based John Innes composts tend to be less attractive to fungus gnats than soil-less composts.

Pot-bound plants that have filled the pot with roots are also thought to be more susceptible, so make sure you move them to larger pots as needed.

Discover our Best Buy composts

Are there any methods of attracting fungus gnats away from plants?

Sticky yellow plastic sheets can be suspended over susceptible plants, attracting the gnats and trapping them in glue. Brushing the plants regularly will disturb the gnats and encourage them to fly towards the yellow sheets. 

Breathe new life into your outside spaces. Sign up for our Gardening newsletter, it's free monthly

Where can I get sticky yellow traps?

They are available from garden centres or many mail order biological control suppliers. 

Sticky trap suppliers include: Ladybird Plantcare; Amazon

Is there a biological control method for fungus gnats?

There is a predatory mite, hypoaspis miles, that lives in the surface layers of compost and will feed on the maggots if the temperature is kept at around 21˚C. 

Where can I get the predatory mite for fungus gnats?

Hypoaspis miles can be obtained by mail order from biological-control suppliers, such as Dragonfli; Ladybird Plantcare; Green Gardener

Are there any suitable chemicals I could use to control fungus gnats?

Chemicals for controlling this pest are not especially effective and need to be reapplied regularly. You will have better results with cultural or biological controls.

Discover the best plants for patio pots and hanging baskets