How to Get Rid of Fungus Gnats
Got Fungus Gnats?
If tiny black flies are constantly appearing around your houseplants, landing on your desk, or making a beeline for your face, you've probably got Fungus Gnats.
They're one of the most irritating houseplant pests because they don't usually cause much harm. They just seem to be everywhere. You water a plant, a few take off from the soil, and suddenly you're wondering where they all came from.
The good news is they're very beatable. You don't need to spray every leaf, repot everything, or throw plants away. The adult flies, with their subtle buzzing and tendency to hover, are the annoying part, but it's the larvae living in the potting mix that keep the problem going.
The most reliable approach is to use yellow sticky traps to catch the adults while making the soil less welcoming for larvae. For a bigger or stubborn infestation, adding a treatment aimed at the larval stage can break the cycle properly.
The small flies darting around are adults. They have poor eyesight and are drawn to light, warmth, humidity and the carbon dioxide in our breath. This is why they can suddenly appear near our faces. But the real source of the problem is normally hidden in the damp potting mix below.
Did you know?
They also seem oddly interested in our faces. That's partly because we breathe out carbon dioxide, but warmth, humidity and light can all help draw them away from the pot and into our personal space.
Fungus Gnats are attracted to damp soil rich in organic material. That's why they often turn up after a spell of frequent watering, or after bringing home a new plant or bag of potting compost.
They're rarely a serious threat to established houseplants, even when there are lots of them. But they can quickly make having plants indoors feel less enjoyable, so let's get them under control.
Contents
Hi, I'm Tom!
If you're like me and enjoy the challenge of growing houseplants and getting them to thrive, then OurHouseplants can help. This website shares my knowledge and years of growing plants and provides my advice on properly caring for your indoor plant friends.
Understanding the Fungus Gnat Lifecycle
Fungus Gnats go through four stages: egg, larva, pupa and adult. The whole cycle can be completed in around three to four weeks when conditions are warm and the potting mix stays damp.
That quick turnaround is why they can feel impossible to get rid of. While you're catching the adult flies buzzing around the room, there may already be eggs, larvae and pupae hidden in the soil, all waiting for their turn.
Why does all this matter?
Yellow sticky traps (and other recommended solutions) catch the flying adults, which is great for reducing the annoyance. But they don't reach the eggs, larvae or pupae in the potting mix. That's why the most successful treatments tackle both the adults and what is going on in the soil.
Stage 1: Eggs
Adult females lay their eggs in the top inch or two of damp potting mix. A single female can lay a surprising number, so it doesn't take long for a small problem to become an irritating one.
Stage 2: Larvae
After around four to six days, the eggs hatch into tiny, almost transparent larvae with shiny black heads. You probably won't spot them unless you look closely in the soil, but this is the stage that matters most.
The larvae feed on fungi, algae and decaying organic material in the potting mix. They're usually more of a nuisance than a threat to healthy established plants, although a large population can sometimes bother delicate roots on seedlings, cuttings or very young plants.
Stage 3: Pupae
After roughly two weeks of feeding and growing, the larvae pupate in the soil. They don't feed at this point, but they are still protected below the surface while they change into adult flies.
Stage 4: Adults
A few days later, the adult Fungus Gnats emerge. These are the small, long-legged flies you see walking over pots, running across the soil, or lazily flying around your plants. Adults don't live especially long, but they only need a few days to mate and lay more eggs.
Did you know?
The name “Fungus Gnat” is used for several similar tiny flies, but the ones bothering houseplant owners are usually from the Sciaridae family. They aren't especially strong fliers, which is why you'll often see them walking across the soil, pot rim or nearby surfaces instead.
How to Get Rid of Fungus Gnats
The most effective way to get rid of Fungus Gnats is to deal with both parts of the problem at the same time: catch the adults that are flying around, then target the larvae in the potting mix.
If you only use sticky traps, you'll catch plenty of flies but the larvae in the soil can still grow into new adults. If you only treat the soil, adult females may still be flying around and laying more eggs. Using two approaches together is what usually gets you properly back in control.
For a small infestation, improving your watering routine and putting up yellow sticky traps may be enough. If you've got lots of flies, or they keep returning, add a treatment that works on the larvae as well.
Don't expect everything to disappear overnight. You'll normally notice fewer adults within days, but it can take several weeks to catch the remaining adults and interrupt the full lifecycle. Stick with it for a little while after things look better.
Here are the five methods I've used over the years, starting with the simplest.
1
Watering Skills
Fungus Gnat larvae need consistently damp potting mix, especially near the surface where adult females lay their eggs. Letting the top inch or two dry out between waterings makes the pot far less welcoming and can reduce the number of larvae that survive.
I don't usually bottom water my houseplants, but bottom watering can be very effective in helping to reduce Gnat numbers when performed for a few weeks.
Why does this work?
When you water from below, moisture is pulled upwards through the growing medium by capillary action. The lower part of the potting mix becomes damp first, while the surface usually stays much drier than it would with frequent top watering.
The downside is that bottom watering on its own isn't always a complete fix. A pot can still contain eggs, larvae or pupae that were already there before you changed your routine. It can also be very easy to bring new hitchhikers home in a bag of potting compost and not realise it until you start using it.
Once that compost is rewetted and kept moist, the conditions are right for the lifecycle to continue. A few weeks later, adult Gnats can suddenly start appearing and it feels as though they have come from nowhere.
So, bottom watering can be very helpful for reducing numbers, but I'd still combine it with one or more of the following ideas if you want to get properly on top of the problem.
2
Sticky Traps
Sticky traps deal with the adult flies. They're simply small pieces of yellow card covered in glue. Fungus Gnats are attracted to yellow, land on the surface and get stuck.
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Put one or two traps into the soil of each affected plant, close to the surface where adults tend to walk, emerge and lay eggs. If you have lots of plants, start with the worst-affected pots and any sitting close together.
Sticky traps won't stop the eggs, larvae or pupae already hidden in the potting mix, so they aren't a complete solution on their own. But they're brilliant for quickly reducing the number of adults flying around your home and (they will also help restore your sanity by drastically reducing the number of flies you see and hear) for showing whether your treatment is working.
If the traps are still catching large numbers after a couple of weeks, the larvae are probably still active in the soil. That's your sign to add one of the larval treatments below.
3
Soil Drench
A soil drench means applying a treatment through the potting mix rather than spraying the leaves. This can be useful when you have a stubborn infestation and want to target the larvae living below the surface.
Neem oil is one option. When mixed correctly and used as a drench, it may help reduce larvae and can be particularly handy if you're dealing with other houseplant pests at the same time.
How do I carry out a Soil Drench?
Don't worry, it's pretty easy to do. Here is my step-by-step guide to making and using a Neem Oil soil drench.
Always follow the dilution instructions for the product you're using. Don't apply concentrated Neem Oil straight into a plant pot, and don't keep repeating drenches unnecessarily if your plants are already stressed or struggling with overwatering.
You'll also see hydrogen peroxide recommended online. Some people report good results, but I'm more cautious with it. It can be quite harsh if mixed too strongly or used repeatedly, and it isn't my first choice for houseplants.
For most infestations, I'd try better watering, sticky traps and one of the biological treatments below before reaching for a soil drench.
4
Nematodes
Beneficial nematodes are a very active way to deal with a large or persistent Fungus Gnat infestation. They're microscopic worms that live in damp potting mix and seek out the larvae.
The species most commonly sold for Fungus Gnats is Steinernema feltiae. Once they find a larva, they infect it with naturally occurring bacteria. It sounds gruesome, but it is an effective form of biological control and is harmless to people, pets and plants when used as directed.
You may also see predatory mites sold for the same job, usually Stratiolaelaps scimitus. These are not nematodes. They are tiny soil-dwelling mites that hunt eggs, larvae and other small pests near the surface of the potting mix.
Both can work well, especially if you have a lot of plants, a major infestation, or recurring problems with soil-based pests. They can also be useful if you've got Thrips, as predatory mites may feed on Thrips pupae in the soil too.
The drawbacks are cost, shelf life and timing. They're living organisms, so you need to use them soon after they arrive and follow the supplier's instructions carefully. For a couple of mildly affected plants, sticky traps and BTi (see the next option below) are usually simpler and cheaper.
There is good evidence that Steinernema feltiae can control Fungus Gnat larvae, including in greenhouse settings. This study is a useful example.
5
Mosquito Bits
Mosquito Bits and Mosquito Dunks contain Bacillus thuringiensis var. israelensis, usually shortened to BTi. If I had a large Fungus Gnat outbreak and none of the other steps worked, this is the treatment I'd reach for.
BTi is a naturally occurring bacterium used as a biological larvicide. It targets the larvae of certain flies, including mosquitoes, blackflies and Fungus Gnats. It doesn't catch or kill the adults already flying around, which is why I always pair it with yellow sticky traps.
The larvae eat the BTi while feeding in the potting mix. This is what interrupts the lifecycle and stops a new generation of adult Gnats appearing.
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Usage is straightforward, but follow the instructions on the packet you buy, as products can vary. A popular method is to soak a Dunk or a measured amount of Bits in your watering can, then use that water on the affected plants.
You want the solution to reach the top few inches of potting mix, because this is where most Fungus Gnat larvae live. Water evenly around the surface rather than pouring everything into one spot.
Don't expect the adults to vanish immediately. The traps should begin catching fewer flies as the remaining adults die naturally and fewer larvae make it through to adulthood. For a bad infestation, repeat the treatment over several waterings according to the product label.
Use BTi and sticky traps together.
BTi deals with the larvae in the potting mix. Sticky traps reduce the adult population and help prevent females laying more eggs. Used together, they target both parts of the lifecycle.
You can also sprinkle Bits onto the soil surface or leave part of a Dunk in the watering can if the product label allows it. Personally, I prefer the “tea” approach because it gets the BTi into the area where the larvae are feeding, without leaving damp organic material sitting on top of the pot.
It isn't a permanent forcefield against future infestations, especially if a new plant or fresh potting compost brings more Gnats indoors. But it is one of the most effective, low-fuss ways to get an existing outbreak under control.
In my experience, BTi combined with sticky traps is the most reliable way to deal with Fungus Gnats. It is easy to use, doesn't require repotting everything, and deals with the part of the problem you can't see: the larvae in the soil.
What I don't recommend and why
There are many ideas out there that people have used to repel Gnats. Here are some I keep coming across but don't recommend, and I'll explain why.
- Electric Bug Zapper.
Zappers will often have a type of light that attracts flies, and then when they get too close, they fry. However, they can be expensive to buy and run.
Although the bigger issue for me is that it's usually over the top, once you've got rid of your Gnats, these zappers don't have a purpose anymore, so they don't work any better than good old-fashioned Sticky Traps. - Pest Sprays.
Although some bug sprays will be effective against the adults, the larvae are often protected from the spray by hiding in the potting medium and will just keep growing and turning into flies. You'll just be spraying every week getting more annoyed in the process. Frustrating and time consuming. - Apple Cider Vinegar.
I've seen this recommended as a way to attract Fungus Gnats. You put a little in a small bowl or upside down lid mixed with three parts water which then lures the adult fungus gnat in. Others use it mixed into a DIY bug spray. Apple Cider Vinegar gets the thumbs down from me. I still think Yellow Sticky Traps are less messy (and smelly!) and I've covered my issue with bug sprays already. - Grit Surface Toppers.
This one is controversial. A very dry, fine surface layer can make it harder for adult Gnats to reach the compost, but I don't like thick layers of grit or pebbles on most houseplants. They make it harder to judge soil moisture properly, and that can lead to overwatering or underwatering.
Fine sharp sand or diatomaceous earth may have a small role, but neither would be my first choice when sticky traps and BTi are available. - Carnivorous Plants.
Many carnivorous plants like the Venus Fly trap and Trumpet Pitcher will happily eat your Fungus Gnats. However, all they will be doing is helping to control the adult flying population. You need to break the life cycle to stop them entirely and a carnivorous plant isn't going to do this. Buy them because you like carnivorous plants, not as pest control.
How to Prevent Flies from Indoor Plants
How to avoid Fungus Gnats in indoor plants? You can't usually prevent Gnats completely, as some things are beyond your control. But two things really increase your chances of an infestation starting. Get a handle on the following two points, and you'll avoid having to deal with them on a regular basis.
Your Watering Skills
Gnats need a food source; and as their name suggests, fungus is one of their favorites. This stuff will grow easily in damp low light conditions, it's not often visible to the human eye, but Gnats will feed on it happily.
This is why overwatering can be a real problem as it can provide good growing conditions for fungi to thrive in.
READ MORE - Is it better to bottom water or top water plants?
You can use bottom watering when you have a Gnat problem happening right now, especially if it helps the surface of the potting mix dry out. However, I'm not a huge fan of relying on it for every plant in everyday care, for the reasons I explore in my post linked above.
The longer-term answer is simply to improve your watering routine. Most houseplants don't need constantly moist soil. Letting the top inch or so dry before watering again is good for many plants, makes it harder for Fungus Gnats to breed and gives you a better chance of spotting watering problems before they turn into root rot.
Potting Compost
This is a tricky one to solve. In my experience, a large number of my Fungus Gnat problems have started with bagged potting compost bought from a store.
Large bags of potting compost can be excellent value, but they can also provide ideal conditions for Fungus Gnats if they have been stored damp.
Big bags are often stored for a while before we buy them, sometimes in sheds, warehouses or outdoor sales areas where they may be exposed to damp conditions. The mix is full of organic material, so if Gnats gain access, it can become an ideal place for eggs, larvae or pupae to be present unnoticed.
Optional Tip
If you've had repeat problems from new compost, consider treating freshly opened bags with a BTi tea when you first use them. This can help deal with any larvae before they become a bigger problem indoors.
Often, everything seems fine until you use the compost, water it in and then notice Gnats a few weeks later. That doesn't necessarily mean eggs have been “activated” after months of dormancy. More likely, there were already eggs, larvae or pupae in the mix, and rewetting it gave them the conditions they needed to continue their lifecycle.
If you suspect a bag is the source of a major infestation, use what is left outdoors if you can, rather than bringing more of it into your house. Otherwise, keep opened bags sealed and as dry as practical between uses.
Let's Sum Up
Fungus Gnats are common houseplant pests. They're usually more irritating than dangerous, but a large infestation can be a genuine nuisance and may trouble seedlings, cuttings or plants with already-stressed roots.
The key is to treat both parts of the problem. Use yellow sticky traps to catch the adults, then tackle the larvae in the potting mix with better watering, BTi, nematodes or another suitable soil treatment.
Fungus Gnat adults are poor fliers and often prefer to walk over plant pots, soil and nearby surfaces.
Don't be discouraged if you still see a few adults during the first week or two. The ones already flying around won't vanish instantly. Keep the traps in place, continue the larval treatment as directed and give the lifecycle time to break.
In my experience, the most reliable combination is sticky traps plus BTi. It is simple, targeted and doesn't require you to repot your entire plant collection. Once things are back under control, sensible watering and a little caution with fresh potting compost should make repeat outbreaks much less likely.
Fungus Gnats are common, but they are very manageable. If you've found a method that worked particularly well for you, let me know in the comments.
About the Author
Over the last 20 years, Tom has successfully owned hundreds of houseplants and is always happy to share knowledge and lend his horticulture skills to those in need. He is the leading content writer for the OurHouseplants Team.
