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.
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 to some extent, some more than others (and the cinnamon smelt lovely when I watered), but they only reduced the population rather than eliminated them all together. Simply watering less helped a lot also, although I did take it a bit far and ended up almost losing some plants.
What DID get rid of Fungus Gnats entirely for me in the end, was this simple one two method below. That's not to say this is the one-and-only method. Just the one that finally worked for me.
Do Fungus Gnats harm your plants?
*Skip this section if you don't want to learn about these annoying bugs and just want to find out the two steps*
If you get on top of them while the infestation is small, the good news is they are usually annoying rather than harmful. Even in large numbers, it's not the adults you need to worry about, it's the larvae.
Fungus Gnats eat all sorts of yummy organic goodies. Just some of what they love includes the organic bits in your potting mix, grass clippings, compost, fungi, leaf mould, rotting roots and more. They 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.
However, if their preferred food choice runs out, the Fungus Gnat larvae may feed on your roots, which can stunt plant growth, particularly in seedlings, newly rooted cuttings and younger plants. I imagine it would have to be left to get really bad for them to cause so much damage that fungus gnats could kill a plant, so that's very unlikely to happen.
Use the potato test to find which plants are infested with fungus gnats
*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.
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 of your 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 underway. 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
First, get yourself these 2 things: Sticky Traps & Mozzie Bits
The first thing you want to get is some yellow insect sticky traps.
I use these differently than the label says, which makes them last a good 2 to 4 times longer. The brand I use are called Mini-Stickies by Easy Trap. I sell them but you should also be able to find them at hardware stores and even your local supermarket.
Look for indoor pot plant size ones (that's the mini size I sell), but if you can only find the big outdoor grower ones just get snipping. The ones I sell come in a pack of 5 and my method makes one trap last up to 4 weeks. More about that below.
The second thing you want is mosquito bits, or BTI granules.
The brand I use is also by Easy Trap and they call theirs Mozzie Bits, but it's 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.
These are the two products I use for this method (both are available in NZ):
How to use yellow sticky traps to kill fungus gnats
If you get the Mini-Stickies, one pack of traps comes with 5 double-sided sticky traps included. The directions on the back of the pack will tell you to peel off the paper from both sides, then stick the pointy end in the soil. Nuh uh. Great for other insects (as these traps also work for thrips, aphids, whitefly and more), but for Fungus Gnats, I've had much better results using them this way...
My sticky trap method for indoor plants
Remove the paper on one side of the trap only, exposing the sticky surface. Lay the trap down flat, sticky side up, on the soil surface. And yes, you can cut them in half or smaller for smaller pots (each trap measures 16cm long x 6cm wide) but if you want to do that, cut before removing the protective paper. It's best to cover at least one quarter of the soil surface, ideally one half, with the trap.
Lay the trap flat on the soil after watering with Mozzie Bits. I find the trap stays sticky a good two weeks or more (the dustier the air is in your house, the faster they lose their stickiness). Every 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.
Once both sides are used up, get a new trap. Used this way, you'll get up to 4 weeks Fungus Gnat trapping from one sticky trap, instead of 2 weeks each, and catch many more gnats than using them upright. Using one trap at a time, a pack of 5 will last you up to 20 weeks. That's the adult population fast declining, which of course reduces the eggs, but for this to work you want to interrupt the larvae cycle too. That's where a little 'Mozzie Tea' comes in...
How do you use mosquito bits to kill fungus gnats?
The brand I use is called Mozzie Bits by Easy Trap. The box gives you the directions for larger areas of water based on surface area (as like the name suggests, you can use them for mosquito control in areas like ponds and water tanks), but used for treating indoor plants, I use the same method plant YouTubers have nicknamed 'Mozzie Tea'.
How do you make 'mozzie tea'?
For indoor plants, you can use a mixture of 4 tablespoons of Mozzie Bits per 4.5 litres of water. I use hot water (warm to hot but not boiling), and let the granules soak for 30 minutes so the BTI infuses into the water. I scoop out the used granules, give the 'tea' a bit of a shake up as the BTI can settle, and top water as usual.
I've also seen people put the granules in a mesh bag or similar to soak, which makes it easier to remove the used granules. One 200gram Mozzie Bits works out to just over 33 tablespoons, (a tablespoon averages 6gms of granules), so one pack should make 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 a hot water / short soak method. You do have to wait for the water to cool before you water, so another option is a warm water soak for one hour instead of hot for 30 minutes. Once mixed, use fresh as the microbial activity breaks down in 1 to 2 days.
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. That's what's in Mozzie Bits. 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?
Sure can! I make my mozzie tea first, and once it's cooled, I add my fertiliser and water 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 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, it's still doable to eradicate them, but will take a lot longer than this method below, that tackles fungus gnats at both the larvae and flying stage.
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 cooler weather it can take 4 to 5 weeks or more to complete the life cycle. That's why I recommend treating for 2 months (8 weeks), but at least 4 weeks minimum if you've caught it early. A longer treatment is completely safe - no poisons or chemicals are needed with this method - so treating for longer to make sure you've got them all won't harm your plants.
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), 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 don't want to over-water or may end up with root rot instead. My advice is to water normally, and use a mozzie tea drench every water, but in conjunction with adult gnat control using Sticky Traps to reduce the adult population so they don't lay more eggs and start the cycle all over again.
Will the Fungus Gnats come back after treatment?
Yes they can. The combo of Mozzie Bits and Sticky Traps deals with the current infestation. It doesn't prevent them coming back forever, but YOU can make sure that doesn't happen. Here's how...
The main 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, 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, you can give the new soil a good drench with mozzie tea (made from Mozzie Bits) before or after potting up.
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.
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).
Two 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 and pay it forward.