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. 

They are actually several species of bradysia flies. The most common is Bradysia difformis, which is black and around 3-4mm long. They aren’t strong fliers and usually flit about close to the surface of the compost, or crawl around the edge of pots. 

Their ideal habitat is damp compost. Each female lays around 100 eggs just below the surface of compost – these eggs aren’t visible to the naked eye. 

When the eggs hatch, the four larval stages feed on organic matter, including compost, decaying plant material and young plant roots and stems. The final larval stage is white with green or brown gut content and measures 5mm long

Find out how to control fruit flies

What damage do they do?

The adult flies are annoying and can help to spread fungal diseases, such as pythium (also known as damping off), but they generally cause little harm. 

The larvae are more destructive as they bite through roots and the base of stems, and can even tunnel through young stems, weakening the plant and reducing its likelihood of surviving. While they’re unlikely to devour entire seedlings or cuttings, they can cause significant damage if numbers build up. 

Slow-rooting cuttings, such as some shrubs, and seedlings are particularly at risk. Poinsettia and cyclamen are also favourites of fungus gnats, so keep an eye on these and other houseplants. 

As plants mature and have a more extensive root system, they’re less likely to be affected.

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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. 

When are fungus gnats a problem?

At 15°C, the whole life cycle of a fungus gnat takes around 34 days – the larvae feed for 16 days before they pupate, emerge as adults, mate and lay eggs. At 25°C, the life cycle speeds up to around two weeks and the larval stage is only five days. This means the feeding time is reduced, but far more eggs could potentially be laid. 

Due to this, infestations can increase rapidly as the weather warms or if you keep plants in a heated greenhouse. Warmer weather is precisely when you’re growing many seedlings and cuttings, so it’s worth trying to prevent a build-up of the flies early in the spring. 

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. We switched to using solely peat-free compost at our Capel Manor trial grounds in north London six years ago. Although we’ve had regular infestations of fungus gnats in that time, we’ve always been able to control the pest and it’s never impeded our trials.

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. 

Discover the best composts for your garden

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. 

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?

Nematodes eat larvae and pupae once the compost temperature is high enough. There are several brands to buy online and all contain the natural predator of fungus gnats – Steinernema feltiae. Remember, the compost temperature needs to be above 10°C and you’ll need to use the control promptly when it arrives – generally within 14 days. Mix it with water, according to the packet instructions, and apply using a watering can. 

Alternatively, NemaKnights Nematodes Biological Gnat Control is a new product that encases the nematodes in ‘pearls’, which slowly release them onto damp compost. It can be stored out of the fridge for up to six months. 

Rove beetles (Atheta coriaria) and hypoasis mites can also be used. These feed on fungus gnat eggs and larvae. Hypoasis mites need a moist compost and can last for up to four weeks without food, so are useful for early infestations.

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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