Common pests: Fungus gnats
Fungus gnats are those tiny flies that suddenly seem to appear from nowhere and start hovering around your plants and windows. They can be frustrating, but with a bit of understanding and a simple routine, you can get them under control and keep your indoor jungle comfortable again.
In this guide
- What are fungus gnats?
- How to recognise an infestation
- Life cycle – and why it matters
- Natural treatment options (nematodes & BTI)
- Extra prevention tips
- When to ask for help
What are fungus gnats?
Often confused with fruit flies, fungus gnats are small, flying insects about the same size as their almost equally annoying cousins. Most people who live with houseplants will meet them at some point. Because adult fungus gnats are attracted to light, you might first notice these pests flying near your windows, making windows a great spot to place yellow sticky traps.
Fungus gnat larvae live in the top layer of potting mix. They mainly feed on fungi and organic matter in the soil, but they can also nibble on young roots. Over time this can weaken sensitive plants, especially seedlings or recently repotted plants.
How to recognise an infestation
Because they’re small and quick, fungus gnats are easy to overlook at first. These signs usually appear before things get out of hand:
- Small black flies hovering around the surface of the soil after watering.
- Tiny insects flying up when you gently tap the pot.
- More flies around windows, grow lights, or glass doors.
- In heavier cases: slower growth, more frequent wilting, or smaller new leaves on young plants.
If you are seeing flying insects but you’re not sure they’re fungus gnats, compare them with our other pest guides like spider mites or scale – those behave very differently and stay on the plant rather than flying around the pot.
Life cycle – and why it matters
Fungus gnats develop through four stages: egg, larva, pupa, and adult. The adults only live for about one to two weeks, and the full life cycle takes roughly 18–30 days.
On paper that doesn’t sound dramatic, but adult females can lay many eggs over their lifetime. That means a small problem can turn into a constant cloud of flies if you don’t break the cycle.
The most important points for plant parents:
- Adults = nuisance. They’re annoying but mostly harmless to the plant itself.
- Larvae = real issue. They live in the top few centimetres of the substrate and can damage fine roots, especially in young plants.
- Moist, peaty soil helps them thrive. Constantly wet pots are essentially a fungus gnat nursery.
Natural treatment options
There are many tips and tricks online, from cinnamon to sand on the soil surface. Some of these can slow gnats down a little, but they rarely solve a stronger infestation on their own. For most homes we recommend combining a targeted treatment for the larvae with traps for the adults.
Beneficial nematodes
Nematodes are predatory micro-organisms – essentially microscopic, worm-like creatures that attack and feed on the larvae of other insects. Products sold specifically against fungus gnats and similar pests usually come as a powder or gel that you keep refrigerated until use.
How they’re typically used:
- Check the packet for the right dose based on how many plants or litres of soil you want to treat.
- Mix the nematodes into water following the instructions.
- Water your plants thoroughly with this solution so it reaches the top layer of the substrate.
The numbers can sound wild – often around one million nematodes per ten pots – but they are microscopic, so the mix just looks like slightly cloudy water or a fine powder in your watering can.
Bacillus thuringiensis israelensis (BTI)
The second widely used option is a specific strain of bacteria, Bacillus thuringiensis israelensis (BTI). In some countries it’s sold as granules (often called “Mosquito Bits”), and in parts of Europe you’ll also find it in tablet or pill form.
BTI is mixed with water according to the instructions and then used to water your plants. The spores in the solution produce toxins that specifically target certain fly and mosquito larvae, including fungus gnats. Always follow the product label for mixing rates, frequency, and any safety notes relevant to your region.
Compared to nematodes, BTI products often have a longer shelf life and don’t always need refrigeration, which can make them easier to keep on hand.
Sticky traps and supporting steps
Because only the larvae live in the soil, we like to combine a larva treatment (nematodes or BTI) with simple tools to reduce the number of new eggs being laid.
- Yellow sticky traps placed near the soil surface or windows help catch adult gnats before they reproduce.
- Letting soil dry slightly between waterings makes conditions less attractive to larvae.
- Repotting with fresh, well‑draining substrate can be helpful if a single pot is particularly badly infested, or the soil is very compact and stays wet for too long.
If you are experimenting with new potting mixes anyway, you can explore airy, fast-draining substrates from our substrates collection so pots don’t stay wet longer than your plants need.
Extra prevention tips
Once you’ve reduced the number of gnats, a few small habits make it easier to keep them away over time:
- Water with intention. Try to water thoroughly but less often, instead of small daily sips. Most houseplants prefer a cycle of moist, then slightly dry.
- Check new plants and soil. When you bring home a new plant, watch the soil closely for a couple of weeks. If needed, isolate and treat before adding it to the rest of your collection.
- Improve airflow. Light air movement in crowded plant corners helps the topsoil dry more evenly and can make conditions less inviting.
- Consider a carnivorous plant. Species like Pinguicula (butterworts) or Drosera capensis can catch some of the adult gnats with their sticky leaves. Think of them as a helpful, decorative assistant rather than a complete solution.
Curious about bringing a carnivorous plant into your collection? Have a look at our current selection in the plants collection for characterful additions to your windowsill.
When to ask for help
If you’ve tried drying the soil, using sticky traps, and at least one round of nematodes or BTI but the number of gnats still doesn’t really change, it can be helpful to step back and review your setup:
- Are some pots staying wet for many days because they have no drainage hole?
- Is old substrate breaking down and staying soggy?
- Are there hidden water trays where moisture collects?
Sometimes repotting a few key plants into fresh, well‑draining substrate and adjusting your watering routine makes all the difference. If you’re unsure what to change, you can compare your situation with other posts in our pest series, like spider mites, to better understand how plant stress and environment influence pests.
With a bit of patience and a clear plan, most homes manage to bring fungus gnats back down to a tolerable level so you can focus on enjoying your plants again.
Explore more
Helpful plants, care products, and related guides:
- Bio Traps for Potting Soil Flies – Natural Pest Control (15 Pieces)
- Spider Mites on Houseplants: How to Spot, Treat, and Prevent Them
- Mealybugs on Houseplants: How to Identify, Treat, and Prevent These Fuzzy Little Nightmares
- Plant care: Predator bugs
- Thrips on Houseplants: Complete Guide to Identification & Treatment (2026)