Fungus gnats are small black flies that like to colonise the soil in the pots of our plants. Find out how to get rid of fungus gnats and their larvae naturally and how to prevent them from infesting your plants in the first place.
Plant lovers know and fear them. Those small flies that you suddenly find hovering around the potting soil and that multiply at breakneck speed. Fungus gnats are particularly common in the pots of houseplants. Read on to find out how an infestation can occur and how to combat fungus gnats and prevent further infestations.
- What are fungus gnats?
- How to recognise fungus gnats larvae
- How to get rid of fungus gnats
- Natural fungus gnat control methods
- How to get rid of fungus gnats using nematodes
- How to prevent fungus gnats in potting soil
What are fungus gnats?
Fungus gnats belong to the sciarid fly family known as Sciaridae and are found in nature in damp habitats such as forests, moors, and wet meadows. Adult fungus gnats can grow up to seven millimetres long and lay their eggs in moist soil. Outdoors, this happens in May to June, but indoors or in greenhouses they can lay their eggs all year round. In just a few days, a single female fungus gnat can lay up to 200 transparent eggs. The fungus gnat larvae hatch about a week after the gnat eggs are laid. The small flies live in the soil and feed off decomposed organic material such as leaves, fungal mycelium, and roots. It is precisely these larvae that become problematic in the soil of houseplants. In the absence of sufficient dead plant material and fungal mycelium, the hungry larvae start attacking the living plant roots and thus damage our beloved plants. These small flies on houseplants can be a nuisance and are unpleasant to have in your house, especially when they appear in large numbers.
Tip: hobby mushroom gardeners need to be aware that the fungus gnats larvae are drawn to the mycelium of mushrooms as a food source.
How to recognise fungus gnats larvae
Fungus gnats are easy to spot in potting soil. They often stay close to the soil surface, lingering around the soil and pot. When you water or move a plant infested with fungus gnats, the small flies will fly up from the base of the plant.
To detect an infestation of fungus gnats before it becomes a huge issue, we recommend using some of our Plantura Yellow Sticky Traps, which we deliver straight to your door. Yellow sticky traps work by attracting the flies which then stick to the glue on the traps. To catch and detect fungus gnats as effectively as possible, place the sticky traps directly into the soil the plant is in, using small wooden skewers, for instance. If you discover fungus gnats, you will want to act quickly to combat them.
- For detecting infestations early on and monitoring pest populations
- 20 double-sided yellow sticky traps (or 40 smaller traps) to hang from plants or stick straight into soil
- Odourless & insecticide-free
Fungus gnat larvae stay in the soil after hatching and so you may only notice them while repotting. The fungus gnat larvae are worm-like in shape, white in colour and up to five millimetres long. Especially in young plants, an infestation can quickly cause the plant to wilt or die despite having plenty of water. These signs may be indicative of a fungus gnat infestation.
Tip: people often confuse fungus gnats with fruit flies. However, the two are easy to distinguish as fruit flies need fruits or vegetables as a food source and therefore do not stay in or around houseplants.
How to get rid of fungus gnats
If you have fungus gnats in your house, you may be wondering how to combat them, or whether it is worth fighting them at all. In gardens and on balconies, these small black flies never multiply into large populations, which is why it is usually not necessary to combat them. When fungus gnats take up residence in the soil of your houseplants, however, it’s not good news. Our warm homes and the wonderfully water-retaining potting soil create the perfect conditions for these pesky flies to multiply – and the problem can quickly get out of hand if you do not intervene.
What to do against light to moderate infestations
If you only notice fungus gnats on the pot and in the immediate vicinity of the plant, the infestation is probably not too severe yet. To prevent a severe infestation, start combatting the fungus gnat as soon as possible with biological plant protection products. At this stage, it is still possible to use home remedies against fungus gnats.
What to do against a severe infestation
At what point is it a severe infestation? Coming across several gnats in other areas of your home indicates that the existing population has outgrown its original breeding site and many of the gnats are looking for a new home – a clear indication of a severe infestation. One way of monitoring an infestation is by using our Plantura Yellow Sticky Traps. They can be used in the house and in greenhouses. If the sticky traps are overcrowded with gnats after just two to three days, you are dealing with a heavy infestation. A severe infestation requires the use of biological plant protection products on top of some other measures, as the preparations alone will not do.
How to tackle a severe fungus gnat infestation:
- Repot infested plants, removing as much soil as possible from the roots, if necessary, under running, lukewarm water.
- Repot plants in the immediate vicinity of the heavily infested plants or treat them with fungus gnat repellent.
- Cover the surface of the potting soil of every single plant in your home with a layer of dry material at least 0.5cm thick, like expanded clay balls, lava chippings or ornamental gravel, for instance.
- Provide each plant pot with a Plantura Yellow Sticky Trap, positioning the traps close to the soil surface to directly intercept flies hatching from the soil.
- If you find fungus gnats on the sticky traps, use a pesticide again.
Natural fungus gnat control methods
If home remedies against fungus gnats and preventive measures have failed or you want to play it safe, you can use alternative plant protection products. However, we recommend steering clear of chemical pesticides, especially in your home or in the garden, to avoid exposing yourself, your children and your pets to any unnecessary risk. We suggest using biological products against fungus gnats instead.
A highly effective and natural method of combating fungus gnats is the use of neem-based products. The seed oil of the neem tree contains the active ingredient azadirachtin, which acts on the fungus gnat larvae in the soil.
How to get rid of fungus gnats using nematodes
Beneficial nematodes are an effective, safe, and simple pest control method and pose no danger to other plants, insects, or pets. Once applied, these predatory nematodes of the species Steinernema feltiae attack fungus gnat larvae and reproduce inside of them, thereby killing the larvae and disrupting the gnats’ life cycle. The first larvae die after about three days and since the nematodes continue to multiply inside the larvae, the treatment gets more and more successful until there are no new hosts left for the nematodes to infect. The tiny nematodes are extremely easy to apply via the water you use for watering. They are highly-specialised to only target the larvae of a select few pests and have been used successfully in organic farming for many years. They are also safe for humans and warm-blooded animals – so pose no danger to you or your furry friends!
Tip: when using Steinernema feltiae nematodes, the soil needs to be at least 12 °C. It is also important to avoid applying the nematodes in direct sunlight, as they are very sensitive to UV light.
How to prevent fungus gnats in potting soil
Fungus gnats only become a problem when they find soil that is permanently wet. Therefore, the best way to prevent fungus gnats is a good watering regime. Always allow the soil surface to dry slightly before watering again. Fungus gnats lay their eggs directly onto the soil surface and the fungus gnat larvae need moist soil to live. So, if the substrate is too dry, the larvae die immediately after hatching.
As a preventive measure, you can also apply a 0.5cm layer of sand, lava chippings or crushed expanded clay to the soil. This ensures that the surface always stays dry and thus unattractive to fungus gnats. To use this method effectively, water houseplants from below via the saucer or planter.
You might also be bringing fungus gnat eggs into your home through the potting soil. If the conditions are favourable for reproduction, fungus gnats can enter soil bags through the small ventilation holes and lay their eggs while the bags are in storage. This often happens with the unprotected soil stored at DIY stores and garden centres. Unfortunately, the fungus gnats can also make their way into potting soil bags during transport and while being stored at your home. It is therefore impossible to entirely rule out the possibility of an infestation, even with high-quality soils such as our Plantura potting soils. When you buy new plants, we recommend repotting them directly into new soil that is not infested. This will also ensure that your new plant grows well.
Tip: if your potting soil is infested with fungus gnats, you can sterilise it. To do this, slightly moisten it and put it in the microwave at the highest setting for seven minutes to sterilise it, or in the oven at 200 °C for 20 minutes for smaller quantities.
Summary: how to prevent a fungus gnat infestation
- Always allow soil to dry before watering again
- Spread a layer of sand, lava chippings or crushed expanded clay on top of the potting soil
- Water the plants from underneath using the planter or saucer
- Check potting soil for larvae and sterilise in a microwave or oven if necessary
- Always repot potted plants after purchase
For helpful advice and tips on repotting, see our article on how to repot houseplants.