The easy way to get rid of Fungus Gnats on indoor plants (once and for all)
Oh. My. Goodness. I HATE fungus gnats. And that's from someone who dislikes using the word hate. And I've put it in capitals. Yep. That much...
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When I first started collecting plants I was quickly over-run by Fungus Gnats. And because they love carbon dioxide, the only thing more appealing to them than the moist soil of my plants, was the CO2 in my breath, so you always know when you have Fungus Gnats.
Even one or two adults and they'll quite literally be all up in your face. Plus I couldn't leave a cup of tea uncovered without finding little unwanted floaters, and I really love my tea. They had to go.
I tried almost every remedy out there, starting with what was in the pantry (cinnamon, apple cider vinegar, potatoes...), you name it, I tried it. Everything helped a little, some more than others (and the cinnamon smelt lovely when I watered), but nothing totally eliminated them, they only reduced the population a little. Simply watering less helped also, although I did take it a bit far and ended up almost losing some plants, so had to find a solution that didn't compromise my plant's health.
What DID get rid of Fungus Gnats entirely for me in the end, was this simple two step method below. That's not to say this is the one-and-only method. Just the one that finally worked for me once and for all.
Do Fungus Gnats harm your plants?
If you get on top of them while the infestation is small, the good news is they are more annoying to you than harmful to your plants. Even in large numbers, it's not the adults you need to worry about, it's the larvae. It's rare, but in large numbers when larvae run out of yummy organic matter to eat in your soil, there's a risk they may chomp on your plant's roots.
Fungus Gnats love warm, moist conditions, so if you tend to water too often or keep the soil moist, and it's a warmer time of year, that's a big 'yes please' from fungus gnats. A bad infestation can continue right through winter, so just waiting it out often won't work. When Fungus Gnat larvae do feed on your plant's roots, that can stunt plant growth, particularly in seedlings, newly rooted cuttings and younger plants.
The potato test
(skip this section if you already know you have Fungus Gnats)
If you don't know if you have Fungus Gnats or some other flying pest, look for something the size of a fruit fly, but that looks like an itty bitty mosquito. Since they love CO2, you're way more attractive to the flying adults than your plants are, so you don't usually have to search, they'll come find you.
But instead of little tubby, round fruit flies, fungus gnats are a similar size, but skinny and leggy, like a mini mozzie, but they don't bite or make a sound. If you're still unsure what you're dealing with, or which plant they have set up home in, you can try the potato trick.
Slice up a raw potato and place a few slices laying flat on the surface of the soil of any plants you think might be infested. Check the potato slices a few hours later and you may be unlucky enough to find a few - or lots - of little larvae happily chomping on the potato.
Throw out those potato slices, larvae and all, and get treatment started now. The method I use below won't harm your plants, so you can treat every plant in your jungle if you're not sure which ones are infested, or just to be safe since fungus gnats can spread so easily.
The easy way to kill fungus gnats for indoor plants
The secret to this method is to break the cycle by targeting both larvae and adults at the same time
First, you need only these 2 things for this simple method:
1. Which sticky traps to get
I personally use uBloomd green sticky traps now for my indoor plants as having tested them side-by-side with the yellow ones, I found the uBloomd ones trapped more gnats, but the old yellow sticky traps do a great job too. The uBloomd green ones are made for indoor plants and are easier to use, no prep required, but if you get the yellow ones I have a couple of tricks to make them work better when used indoors (prep tips shared below).
In New Zealand, the yellow ones you can easily get from hardware stores and garden centres are called Mini-Stickies by Easy Trap (or for my international plant buddies they're also available here on Amazon).
2. Which mosquito bits to get
The brand I use is Mozzie Bits, but mosquito bits, mozzie bits or BTI granules are all the same thing, although the dose I use, below, might be different depending on which brand you buy.
The key here is to make yourself some 'Mozzie Tea' using BTI granules. In NZ I use these Mozzie Bits or, if sold out, you can also use Mozzie Dunks but the preparation is different (directions for both coming up). If you're overseas, you can also get Mosquito Bits from Amazon.
How to use sticky traps to kill fungus gnats
My sticky trap method for indoor plants
For the green traps, there's no extra steps required before use, just follow the directions included with the pack. For the yellow traps, remove the paper on one side of the trap only to expose the sticky surface, instead of removing both sides. Then lay the trap down flat, sticky side up, on the soil surface. I also cut the yellow ones in half or even in quarters, whereas I find the green uBloomd ones are already the ideal size. When trimming the yellow ones, cut them before removing the paper, otherwise you end up with sticky snips!
Yellow traps: Lay the trap flat on the soil, after watering. I find the trap stays sticky a good two weeks or more (although in dustier environments they can lose their stickiness faster). In a couple of weeks, flip the trap over, peel off the paper on the other side to expose a fresh sticky layer, and you get double the use compared to how they recommend using them.
Green traps: The uBloomd traps ones are water-proof and so far I'm finding they stay stickier longer than the yellow ones (about a month for me, so roughly twice as long). For the green ones, you just stick the trap to the wooden support stick (included in the pack with the traps), and place the stick into the soil so the trap sits close to the soil surface.
How do you use mosquito bits to kill fungus gnats?
The brand I use available locally is called Mozzie Bits made by EasyTrap, but these directions are the same if you've got Mosquito Bits from Amazon. The box gives you the directions for larger areas of water based on surface area. That's because - like the name suggests - you can use them for mosquito control in areas like ponds and water tanks. But used for fungus gnats for indoor plants, I use the same method plant YouTubers have nicknamed 'Mozzie Tea'.
How do you make 'mozzie tea' for fungus gnats using mozzie bits?
For indoor plants, you can use a mixture of 4 tablespoons of Mozzie Bits (NZ) or Mosquito Bits (USA), per 4.5 litres of warm water. The warmer the water the shorter the time it needs to soak. In very warm water (not boiling) soak for at least 30 minutes so the BTI infuses into the water, then allow time for the water to cool before watering.
Once soaked, scoop out the used granules. They will float to the top but can be a pain to sieve or scoop out, so another tip is to use a mesh bag (NZ) or media bag (US) for no mess. Once those granules are removed, give your 'mozzie tea' a bit of a stir and simply top water as usual. If using mozzie dunks the dilution rate and process is different (coming up below).
You can bottom water if you prefer, but the larvae tend to hang out near the top, so it won't be as effective. If you do bottom water you'll need to leave the plant to soak long enough for the top surface of the soil to be wet. You can also bottom water and just keep a little of the mozzie tea aside to spray the top layer of the soil with so the top of saturated.
One 200gram Mozzie Bits works out to just over 33 tablespoons, (a tablespoon averages 6gms of granules), so one pack makes over 37 litres of 'Mozzie Tea'.
The cooler the water, the longer the mixture needs to infuse. I've heard of some leaving it in cold water to soak overnight but the bacteria do become inactive over time, so I prefer the warm water + short soak method myself. You do have to wait for the water to cool before you water. I normally do a warm water soak for at least one hour by which time it's soaked and room temperature, ready for watering. Once mixed, use fresh.
How do mosquito bits work?
Mosquito bits target the larval stage of the Fungus Gnat lifecycle. The larvae are killed by a microbial insecticide called BTI. A naturally occurring bacterium that is deadly to mosquito larvae and fungus gnat larvae. BTI is short for Bacillus thuringiensis israelensis. BTI is what's in Mozzie Bits (also called Mosquito Bits overseas), and Mozzie Dunks. Although deadly to fungus gnat larvae, it's harmless to other living things. BTI is nature’s way of keeping mosquito populations under control and works a treat on fungus gnat larvae too.
Is BTI safe for plants?
Yep! This is pest control using nature. Mozzie Bits won't harm animals, birds, bees, livestock, fish, vegetation or us. It can be safely used in bird baths, indoor and outdoor potted plants, rain barrels and roof gutters, water troughs, vases, ponds, fountains and more. Just don't put it in your treated drinking water supply.
Can you water with 'mozzie tea' and with fertiliser?
Yes, you can use mozzie bits and fertiliser together. I make my mozzie tea first, and once it's cooled, I add my fertiliser, and water with both at the same time. I use liquid fertiliser but if you're using a granulated or powder fertiliser you may need to give everything an extra good-mix so both the BTI and fertiliser are evenly throughout the water.
How to make 'mozzie tea' for fungus gnats using mozzie dunks?
I prefer mozzie bits as they are faster acting and faster to make into 'mozzie tea' (prep takes around one hour), but if you can't get Mozzie Bits, you can use Mozzie Dunks instead.
Dunks are made for larger bodies of water outdoors, and designed to slowly release BTI over time, so the directions on the pack won't be much help for indoor plants. One dunk is like a flat donut. Here's the indoor plant directions.
For indoor plants: Break up one dunk into quarters. Soak one quarter of a dunk in 4 litres of water overnight. What doesn't dissolve you can put aside to dry and use next time (dunks are reusable). Using warm to hot water (not boiling), helps the dunk break up and 'brew' better. Remove the dunk after about 12 hours (I normally leave it to soak overnight), give the 'mozzie tea' a good mix, and water as usual. Top watering is more effective as the larvae often concentrate near the top few inches of the potting mix.
How long does it take to get rid of Fungus Gnats?
Firstly, don't give up. This method may be easy, but will take a little patience. One adult fungus gnat can lay up to 200 eggs, so every one you get rid of makes a huge dent in the future population.
But if you only get rid of the flying adults using sticky traps, it's still doable to eradicate them eventually, but will take a lot longer than the mozzie tea + sticky traps method, that tackles fungus gnats at both the larvae and flying stage to break the cycle.
Fungus gnats can go from egg to larvae, pupa to flying, egg-laying adult, in as little as 17 days when conditions are just right, although the average is typically 3 weeks in summer. In cooler weather it can take 4 to 5 weeks or more to complete the life cycle, so winter treatment can take longer.
That's why I always recommend treating for 2 months (8 weeks), but at least 4 weeks minimum if you've caught it early and only have a tiny infestation. A longer treatment is completely safe for your plants, so treating for longer to make sure you've got them all won't harm your plants and is definitely a better way to go.
How often do you need to repeat the treatment to kill Fungus Gnats?
You'll need to give them a good top water drench with Mozzie Tea at least every fortnight (ideally more often), or every time you water, until eliminated. You can also bottom water if you prefer. The tricky thing is the directions say to drench weekly, but because fungus gnats love moist soil, you want to avoid over-watering or you may end up with root rot instead. Unless the plant needs watering weekly, I don't follow that advice.
My advice is to water normally, and use a mozzie tea drench every water, but that's why you need to use this method with adult gnat control also using Sticky Traps to reduce the adult population at the same time (so they don't lay more eggs and start the cycle all over again!).
How do you stop Fungus Gnats coming back after treatment?
The combo of Mozzie Bits and Sticky Traps deals with the current infestation. It doesn't prevent them coming back forever, but now the hard work's done, there are some easy ways to prevent Fungus Gnats taking over again.
The best method I've tried is to sprinkle Fungus Gnat Barrier over the surface of plants most likely to get them (such as Peace Lilies and plants that prefer to stay moist). Gnat Barrier creates an abrasive, physical layer that prevents fungus gnats from landing and laying eggs. This is the brand of gnat barrier I use by Yates, which is natural and non-toxic, made from abrasive pumice granules, making it a safer and more effective alternative to using diatomaceous earth or DE (I don't recommend using DE personally as it doesn't work when wet, and can be a risk to pets and kids).
Here are the most common places fungus gnats come from and how to avoid them taking over again...
Where do fungus gnats come from?
The most common source of Fungus Gnats is new plants. They won't usually arrive with swarms of gnats buzzing around, as the adults normally buzz off when the plant's picked up and moved around, but once your new arrival has settled in, new adults hatch from the eggs and larvae hidden in the soil, and they're back!
Always isolate a new plant for at least 2 weeks (although ideally 4 weeks if you have the patience, since that covers the lifecycle of a lot of other pests too), and if you do find any flyers, you can much more easily treat just that one plant.
Soil stored outside is also Fungus Gnat heaven. It's not the soil itself as such, but the combo of being moist and stored outside that does it. Moist soil left outside is an irresistible combo. If you're unsure how your soil's been stored before you bought it, you can give the new soil a good drench with mozzie tea (made from Mozzie Bits) before or after potting up, or also use hydrogen peroxide to sterilise the soil.
Don't water outside: The other place these annoying insects commonly set up home in our indoor plants, is when you put your plants outside - especially if you've just watered! You're basically putting a 'Vacancy' sign up and your plant is Fungus Gnat hotel central. Bring your plants back inside, and you've bought some uninvited guests along with it. It doesn't take long for them to find a freshly watered plant left outdoors to drain.
So that's what fungus gnats are, what they look like, what they like to munch on, and what worked for me to finally get rid of them (plus how to avoid them coming back).
A few years ago my fungus gnat situation was so out of control it was causing me to question even having plants at all if that meant I had to put up with fungus gnats all up in my face, and picking them out of my freshly made cups of tea before I even got a sip.
Then I became determined to get rid of them. When home remedies didn't do the job, I didn't give in. I spent a little bit on this two step method (I don't know why, but I was really reluctant at the time to spend money on getting rid of them even though they were so annoying). I'm so pleased I did as I've been fungus gnat free ever since - although I still keep some leftover mozzie bits and sticky traps on hand just in case!
I really hope this method helps you like it did me. Please do share this with other plant buddies to help them too - Anna :)