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 on houseplants, seedlings and cuttings.  

They aren’t strong fliers and usually flit about close to the surface of the compost, or crawl around the edge of pots and their ideal habitat is pots of damp compost. 

What damage do they do?

Adult fungus gnats don't do much damage, but can help to spread diseases such as damping off. However their larvae eat the delicate roots of seedlings, cuttings and house plants and bite young stems as plants grow. 

A female adult fly can lay up to around 100 eggs at a time and in a heated greenhouse or in the house the gnats can lay eggs year round. As temperatures rise the lifecyle of the fungus gnats accelerates, so it takes less time for the eggs to hatch, mature and for the adults to emerge and lay the next generation of eggs. It's important to take action as soon as you spot the adult fungus gnats to prevent the numbers building up. The more larvae are in the compost, the more damage they will do.  

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

Houseplants are often at risk, particularly pointsettia and cyclamen. Otherwise, pots of seedlings are vulnerable as well as cuttings, especially those that are slow to grow new roots. Grow-your-own mushroom kits can become infested. 

Peat-free composts and peat-reduced composts are thought to be more attractive to fungus gnats as they decompose more rapidly than peat. However, with careful management, it’s possible to avoid problems. 

How can I deter fungus gnats?

Allow the surface of the compost 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, pebbles or bark can help.

Fungus gnats are also very fond of stored compost. Keep all your spare compost bags tightly closed, or better still, buy new compost as you need it and avoid storing it altogether. 

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

Yellow sticky traps are useful both to catch the adults and to monitor the number of fungus gnats round your plants. The traps are plastic sheets covered in a sticky substance which attract the gnats. Place them close to the surface of the compost near to susceptible or infested plants. The traps also work for aphids, but can trap any beneficial insects such as ladybirds, so use them carefully. 

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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 feeds on the maggots. These can live for up to four weeks without food, so are useful for early infestations. Rove beetles also eat fungus gnat eggs and larvae. 

Nematodes eat the larvae and pupae once the compost temperature is above 10^oC. Order them online and apply them as soon as possible after they arrive. Follow the instructions on the pack and water them onto your pots. 

Where can I get the predatory mite  and nematodes for fungus gnats?

Hypoaspis miles, rove beetles and nematodes 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.

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