You water your houseplants and suddenly a small cloud of black flies rises from the pot? Then you most likely have fungus gnats. The good news: fungus gnats are harmless to people and pets – and with the right measures you can get them under control within a few weeks. In this guide, you'll learn how to recognise fungus gnats, why they feel so at home in potting soil and how to tackle fungus gnats and get rid of them for good.
Contents
- Identifying fungus gnats
- Where do fungus gnats come from?
- Are fungus gnats harmful?
- Getting rid of fungus gnats: 7 measures
- Home remedies for fungus gnats put to the test
- Preventing fungus gnats
Identifying fungus gnats: what the gnats and their larvae look like
Fungus gnats (Sciaridae) are small, slender gnats with a dark body, long legs and smoky grey wings. The species that turn up in homes are usually only about 3 to 5 millimetres long. Their tumbling, somewhat clumsy flight close to the soil is typical. When you water the plant or move the pot, they fly up and quickly settle again.
The actual damage isn't done by the gnats but by their larvae. They are glassy to greyish-white, about 5 to 7 millimetres long and have a black head capsule. You'll find them in the top few centimetres of moist potting soil.
Tip – the potato test: Place a thin slice of potato cut side down on the soil. If there are small white larvae with black heads underneath after one or two days, the soil is infested.
Fungus gnat or fruit fly?
| Fungus gnat | Fruit fly | |
|---|---|---|
| Appearance | black-grey, slender, long legs | light to dark brown, red eyes |
| Size | approx. 3–5 mm | approx. 2–4 mm |
| Where to find them? | above potting soil and houseplants | on fruit, food waste and glasses |
| What helps? | keep the soil dry, yellow sticky traps | remove breeding sites, fruit fly trap |
If the flies are buzzing around the fruit bowl rather than your plants, read on in our guide Getting rid of fruit flies.
Where do fungus gnats come from?
In nature, fungus gnats live in moist, humus-rich soils. They usually get into your home with new plants or bought potting soil that already contains eggs or larvae. Individual gnats can also fly in through open windows.
They become a problem when the soil is constantly moist. The females lay their eggs in the top layer of soil. After about a week, the larvae hatch and feed there on dead plant material, fungi and algae – and, in heavy infestations, on fine roots too. At room temperature, development from egg to adult gnat takes about a month. The adult gnats only live for around a week, but lay new eggs during this time.
Why in autumn and winter in particular? During the heating season, most houseplants grow more slowly and need less water. If you keep watering as you did in summer, the soil stays wet for a long time – ideal conditions for fungus gnats.
Are fungus gnats harmful?
Fungus gnats are not dangerous to people and pets: they don't sting or bite and, according to the German Environment Agency (Umweltbundesamt), pose no health risk to humans or animals. They're still a nuisance, though – especially when large numbers of them are buzzing around your home.
For your plants, it's a different story: in heavy infestations, the larvae feed on the fine roots. Strong houseplants can usually cope with this, but a mass infestation can be dangerous for seedlings, cuttings and young plants. The first signs are wilting leaves despite moist soil and stunted growth.
Getting rid of fungus gnats: 7 measures that really help
A combination works best: you catch the adult gnats so that no new eggs are laid, and you make life difficult for the larvae in the soil. As one generation develops in around four weeks, you should keep up the measures for at least three to four weeks.
1. Water less and let the soil dry out
This is the most important measure of all – the German Environment Agency also lists it first. Let the top two to three centimetres of soil dry out well between waterings, as far as your plant can tolerate this. The larvae can't continue to develop in dry soil. It's best to water from below via the saucer and pour away excess water after about 15 minutes. This keeps the surface dry, and the gnats find nowhere to lay their eggs.
2. Put yellow sticky traps in the pot
Fungus gnats are attracted to bright yellow. If you put yellow sticky traps for fungus gnats directly into the soil, the gnats get stuck to the adhesive surface and can no longer lay new eggs. At the same time, the traps show you how severe the infestation is and whether your measures are working.
- Place the trap in the pot close to the plant and as near to the soil as possible.
- For large pots or heavy infestations, two traps per pot help.
- Replace the trap when the surface is full or no longer sticky.
- Use yellow sticky traps indoors. Outdoors, beneficial insects get stuck to them too.
The Ollie Yellow Sticky Traps work purely physically, without insecticides. Their waterproof adhesive holds even when you water and mist the plants, and they can be used for up to three months. They are available with 30 or 60 traps – enough to equip all your plants at once.
Important: yellow sticky traps only catch the adult gnats. In heavy infestations, they're usually not enough on their own – so combine them with the following steps.
3. Add a layer of sand to the soil
A layer of quartz or play sand about one centimetre thick dries quickly. The gnats can't find a suitable place on it to lay their eggs, and newly hatched gnats find it harder to get out of the soil. The German Environment Agency also recommends this method. Afterwards, it's best to water from below.
4. Use nematodes against the larvae
Nematodes of the species Steinernema feltiae have proven effective against the larvae in the soil. The tiny roundworms are applied with the watering water, penetrate the larvae and kill them. They are considered harmless to people, pets and plants. You can get them from specialist garden retailers; follow the instructions on temperature and shelf life on the pack. Nematodes and yellow sticky traps complement each other well: the nematodes take care of the larvae, the traps take care of the gnats.
5. Repot heavily infested plants
If the soil is teeming with larvae, a fresh start is worthwhile: carefully shake off the old soil, briefly rinse the roots and put the plant in fresh, high-quality soil. Keep opened bags of soil tightly closed and dry – open bags are a popular breeding ground.
6. Keep saucers, outer pots and the soil surface clean
Standing water in the outer pot, rotting leaves on the soil or a layer of algae provide the larvae with extra food. Empty saucers regularly, remove dead plant parts and loosen the top layer of soil now and then so that it dries out faster.
7. Keep at it and keep checking
After just a few days, you'll see on the yellow sticky traps whether fewer new gnats are appearing. Even so, stay consistent for at least one complete development cycle – that is, three to four weeks. Leave the yellow sticky traps in place for a while afterwards: they'll show you early on if a new infestation is on the way.
Home remedies for fungus gnats: do they work?
Lots of home remedies are doing the rounds online. However, there is hardly any evidence that most of them work:
- Matches in the soil: The sulphur in the match head is supposed to kill the larvae. There is no reliable evidence for this.
- Baking powder, cinnamon or coffee grounds: The effect is uncertain. Baking powder and coffee grounds can also alter the soil and harm the plant.
- Garlic: Has a strong smell – whether it reliably kills larvae has not been proven.
- Nylon stocking over the pot: Can actually make egg-laying more difficult, but is impractical in everyday life.
Our advice: rely on the proven measures – correct watering, yellow sticky traps, sand and, if necessary, nematodes. They'll get an infestation under control much more reliably.
Preventing fungus gnats: your checklist
- Water as needed rather than to a schedule – usually much less often in winter.
- Check the soil with your finger: if it's still moist two to three centimetres down, wait before watering.
- Pour away excess water from saucers and outer pots.
- Use high-quality soil and store opened bags closed.
- Check new plants and put a yellow sticky trap in the pot as a precaution.
- A layer of sand protects particularly sensitive pots such as herbs and seed trays.
Conclusion: getting rid of fungus gnats takes a little patience – but it works
Fungus gnats are annoying, but no reason to panic. The key lever is moisture: if you keep the soil surface dry, you deprive the larvae of what they need to live. At the same time, yellow sticky traps catch the adult gnats and show you how the infestation is developing. With this combination, you can usually get fungus gnats under control within a few weeks.
View Ollie Yellow Sticky Traps for Fungus Gnats · All products against fungus gnats
Source: German Environment Agency (Umweltbundesamt) – fungus gnats