How I Finally Got Rid of Fungus Gnats in My Houseplants
If you’re reading this, you’ve likely grown tired of swatting at tiny fungus gnats flying around your nose. Perhaps you’ve even embarrassed yourself in front of guests as you try to expel one from your nostrils. Or maybe you’ve forewarned said guests as they crossed your front door threshold because the fungus gnat issue is THAT bad.
“Um, I know this is going to sound weird but if you experience any little bugs flying around your face this evening, don’t worry. They’re totally harmless. Hope you’re hungry!”
Yep, this scenario played out over the entire holiday season with lots of friends and family coming and going. I wonder what they thought as they left our home. It’s funny watching guests try to be polite as they sip their wine and cross their eyes to see the fungus gnat, attracted to the CO2 in your breath, that’s landed on the rim of their wineglass. Even better, when they grow frustrated with a persistent gnat, tighten their lips and exhale through the tiny opening (imagine if you were pushing air across the opening of a flute) they’ve created as they wave their hands. Not exactly the most elegant dinner party but it’s given us something to laugh at later.
I hope everyone comes back now that the problem is fixed! Perhaps I should mention that before our next gathering.
“By the way, if you’re wondering about the bug issue, they’re dead so come on over. Bring an appetite!”
What Are Fungus Gnats?
Often mistaken for a fruit fly, and they are similar in size, a fungus gnat has no interest in fruit. They prefer decaying plant matter found in soil. Adult gnats don’t bite or sting, they simply seek out moist soil to deposit their eggs.
Upon hatching, the larvae feed on the decaying plant matter, fungi and algae in the soil. Which is why constantly moist soil is such an attractant and why your first line of defense against these relatively harmless, but completely annoying creatures, is to starve your houseplants to the brink of dehydration before offering another drop.
For 20 years, I’ve been a serial underwaterer which is why I’ve been able to avoid fungus gnats until now. I am to houseplants what Joan Crawford was to her children in Mommy Dearest. Instead of “No wire hangers!” it’s “No water ever!”
When it comes to the health and longevity of your houseplants, water deprivation is the secret. Water only when the soil feels dry. The best way to know this is to, you guessed it, get your fingers in there! Poke around about two inches into the soil and if it feels dry, water it. If not, wait a few days. There’s no set watering schedule for houseplants as indoor humidity levels vary from home to home and season to season.
What Didn’t Work Against Fungus Gnats
It’s taken months to finally rid every last houseplant of fungus gnats, and I have a lot of houseplants! Here’s what I tried before finding what worked:
- Letting soil really dry out between waterings
- A soil drench solution of water and hydrogen peroxide
- Diatomaceous earth (the most expensive option)
- Various sprays
- Sticky traps (they work but not by themselves, read on)
Where Did the Fungus Gnats Come From?
I will not accept blame for this as they showed up only after I had repotted a string of pearls that had outgrown it’s container. My situation began in a new bag of potting soil, but there are other ways. Some infestations begin when a new plant comes home with you. Or maybe a fungus gnat rode in on your coat or your dog’s. However they arrive, it only takes one egg-laying female to get things started.
Back to the potting soil. Manufacturers don’t sterilize their mixes before shipping them to retailers. Doing so would kill all the beneficial organisms inside that help our plants grow. The bags may ship out fungus gnat-free, but they may not stay that way for long. During the shipping process and the time they spend on the retail shelf, there’s opportunity for fungus gnats to invade. All it takes is a tiny pin-sized hole in the bag and a determined fungus gnat and BOOM! Infested.
Every character, hero or villain, has an origin story. And that’s how the story of the fungus gnat got its start in my houseplants. Aside from being a nuisance, they don’t pose much threat. Things get a little trickier though if you have seedlings around. Without much of a root system to begin with, seedlings don’t stand a chance if fungus gnat larvae are hanging out in the soil. They’ll gobble up the few roots there are and you’ll have to start over. With seed starting right around the corner, I need to get the fungus gnat population under control.
Biological Larvicide to the Rescue
Fungus gnat control requires a two-pronged death manifesto – one to treat the larvae and one to treat the adults. Treating one without the other keeps you in perpetual swatting mode. Crushed mosquito dunks, used to control mosquito populations in ponds and water fountains, work but because it’s illegal to use a pesticide in any way other than what is recommended on the package, I won’t suggest that here. I also won’t report you to the cops if you use this method.
Instead, I suggest a bag of Mosquito Bits which you can buy here. At $15 for 30 ounces, it’s the most affordable and effective solution. It contains Bti, short for Bacillus thuringiensis subspecies israelensis. Try saying that three times fast! Bti it is, then.
There are several strains of Bt, which is why it’s important to use Bti for the control of fungus gnat larvae. I use another strain, Btk or Bacillus thuringiensis kurstaki, to control cabbage moth larvae on my brassicas in the vegetable garden. Bt is a bacteria whose spores, once ingested by the target larvae, produce crystal proteins that bind to the gut wall, causing cells to break apart. Within moments of ingestion, larvae stop feeding and starve to death.
Bti is effective only when larvae are present in the soil, so it may take up to a month of once weekly applications to eliminate the problem.
The Final Death Blow
You can’t have eggs and larvae without adults. Yellow sticky traps in every container help decrease their population. Thirty-six traps for $6 won’t break the bank either. You’ll notice that with every Bti application, the number of adults on your traps will be fewer until finally, there won’t be a single fungus gnat on any of your stickies.
And that’s pretty satisfying.
Have you dealt with fungus gnats in your houseplants?
Reminds me of the fly tape. I should buy again.
I loved it 😂
Thanks for your info, Heather!
You’re welcome, Gina!
My garden center recommended the mosquito product so I bought it. But I have been afraid to use it! Your info about it has been so helpful so now I feel more confident. Thank you!
That’s great! It really is easy, safe and effective.
If I throw out my plant will they go away?
The larvae in that plant’s soil will be gone but if you have other houseplants, it’s very possible that they have fungus gnat larvae as well. If your plant is otherwise healthy, I’d do the mosquito but treat for the next several watering along with sticky traps to catch the adults. Good luck!
Brilliant post!
Thanks Nina!
Are the mosquito bits harmful chemical wise whatsoever ?
I don’t like using poisons like roundup etc…
Hi Adam, no you should be fine. The bits contain Bt, Bacillus thuringiensis. It’s a naturally occurring bacteria that’s effective at interrupting the fungus gnat cycle.
Thanks! Been an absolute war for the past two months here
Finally got some BTI for plants and substrate
I can totally relate!
Thank you for your suggestion. I have been using those same yellow sticky traps you show, and have only had more insects stuck to them every week. Finally, today, I got so sick of it that I put my few houseplants outside, as the weather is warm enough to support them until the Fall. When Fall comes, I will try the mosquito bits before I bring in the plants to overwinter. I am saving this page!
Hi Debra! They’ll keep coming unless you address the larva too. Annoying little things, aren’t they?! Thanks for reading!
Do you put pebbles on top of soil? And how are BT bits applied?
Thanks
Kate
Hi Kate! I soak four tablespoons of the bits in a gallon of water for a few hours than strain out the bits. I believe you can do it sooner but I tend to forget about them for at least an hour before remembering I was soaking them. Then I water my houseplants. I don’t use pebbles on the soil, although I have used them on my succulents as I like the look.
Can I find the Mosquito Bits at any Garden Center?
Need to try this, very tired of fungus gnats in my houseplants which I have been battling since last November or perhaps earlier?!!
I would call first to be sure they have them. I had to order from Amazon because my local garden centers didn’t stock it.
I used beneficial nematodes and sticky traps. The combo had them all gone in 3 weeks. The nematodes are expensive due to shipping, but if you have a good garden supply house in town, some of them will stock nematodes. I used the ones from Amazon and they worked amazingly.
Good to know, thank you! I’ll keep it in mind if it happens again.
How often do you treat your plants with the Mosquito dunks/bits? Thanks!
Hey Kimberly, I water infrequently with now set schedule and let the plants dry out pretty well between watering. When I had the gnats, I watered using the bits probably four times over a two-month period.
So my plants are infested. I repotted in my plants in new soil and then that soil started growing mushrooms and then got this white layer on top and now I have the gnats. Do I need to get new soil and start over and do this process as well just in case? Or should I just do this process?
Hi Crystal, I would just do this process now and be careful not to water your plants too often. I let mine dry out pretty well between watering. Fungus gnats LOVE consistently wet soil. It may take several weeks to get rid of them but stay the course.
I had the same experience, trying a variety of solutions, of which none worked until I discovered Mosquito Bits. I think it may have taken 4 weeks to get no more bugs on the sticky yellow traps. I keep traps around to alert me to any new infestations. Beware of new potting soil because it can have fungus gnat eggs in it. I am in the process of researching easiest way to treat new soil to rid of eggs.
Hi Alison! Yes, it takes some time to get rid of all of them. The gnats either came in with some new potting soil or in some poinsettia plants sent to me. I think using mosquito bit water the very first time when you’re repotting a plant should help with any eggs in the soil. The larva won’t have a chance. Thanks for stopping by the blog!
I got the bugs in the new soil as well, now they have spread to all of my plants, 40 of them! I used the soil to separate my two peace lilies into 10 pots, this is how it all started 🙁 About two months later I used the same leftover soil to pot some cuttings and placed those two pots in the upstairs room that I don’t use. Would you believe it that those got the gnats too! It seems like the eggs can survive for months in dry soil and come to life when watered. Hard to believe, but maybe it’s possible. I thought about repotting all of my plants, but that will be last resort.
I have used the mosquito dunks in water, but haven’t had any success so far, maybe I’m using too much water and not enough dunks. I also have multiple sticky traps in every pot and fly traps all around. These bugs just seem to keep getting worse and worse. My plants are so sad because I stopped watering them frequently, but that hasn’t slowed down the bugs any! The moment I water, it’s like a bug party up in here!!! And all up in my face! I wish they would go party somewhere else 🙂
Anyway, I thought that maybe putting like a 2 inch layer of sand on top of the soil could help with the issue, has anyone tried this? Can adult gnats go through sand to lay eggs? Can hatched ones come up through the sand?
Thanks! Happy bug elimination all!!!
Oh no! Sounds like quite a buggy situation! I’m not sure about the sand method but you have to use a multiply pronged approach and it may take a few months to get things under control. The mosquito bits work in tandem with the sticky traps. The bits kill the larvae, the traps kill the adults. With your popularity 😂 it may take several waterings with the mosquito bit water over a period of a few months to get it under control. Do let your plants dry out between watering. Good luck!
Hi there!
Landscape designer in Los Angeles, CA working on an indoor landscape project, where we have a fungus gnat issue. I found this blog post and I just wanted to thank you for sharing, and the maintenance crew will try your methods!!
All my best,
Melissa
Hi Melissa! It may take a few applications of the BTI water but doing it in combination with the sticky traps and allowing soil to dry out between watering should do the trick. Would love to know how it goes!
Omg, you have NO idea how happy I am to have found this article. You can’t even imagine what we’re all dealing with here. So, this isn’t our first rodeo with these gnats! We have about 30 to 35 plants in our home, and my husbands prize possesion, a 9ft money tree that he has been growing for 12 years. We were scared to put anything in the soil in fear that we would kill the money tree. These gnats have taken over our home!!! We had a TERRIBLE infestation 6 months ago and our only option was we had to re soil EVERY plant in the house. Our yellow sticky traps were so covered that it looked like it was a black piece of paper taped to it! After re potting, I thought we were done with this mess….. but NOPE! They’re back with a vengeance. We have an infestation again! I’m hoping afar you suggested will do the trick. How many bags would you suggest buying? I’ll need to do all the plants because I don’t know where they’re laying eggs. Some plants I see thousands of them moving around in the soil, and some plants I see a few. So they’re making homes everywhere and living rent free in my house. As I’m writing this, 4 flew by me already. 😠
Does this mix go on top of the soil, or does it get mixed with water? HELP!!!!
Oh my, sounds like you’ve been invaded! You should let the granules steep in the water for at least 30 minutes before applying to the soil. You may have to do it a few times and it works in tandem with the stocky traps. The traps are for the adults, the Bti is for the larval stage in the soil. Depending on how many plants you have, you may need more than one bag. I like to keep one on hand just in case. But remember, the best line of defense is to not overwater your plants. The gnats love moisture so it’s best to let your plants dry out a good bit before reaching for the watering can. Good luck and do keep me posted! Heather
This was the best article I have ever seen for those knats.I have been trying to kill for a couple years now. No one ever said it correctly so you understood you needed all the different things
GJHare
Thank you so much! They are such a nuisance and I hope it helps you rid yourself of them for good!
I have been propagating a fig tree. I stuck the cuttings in soil and covered the top with a plastic bag to keep the soil moist and it was going great until I noticed the gnats. What is your suggestion to deal with this when you are trying to keep the soils moist?
I would use the same technique I use in this blog post and apply the Bti as a soil drench. That will take care of the larva. You’ll need some sticky traps for the adults too.
LOL thanks! I babysat my nephew’s plants for a season at my office which has great indirect light, good climate control, no animals. When he asked originally, I asked they don’t have gnats do you? Before long, every staff meeting involved persistent swatting and clapping of hands. Then they moved into the next office. When my nephew returned, he denied his plants being “the source” of our bug problem. Then he woke to one buzzing around his nose as he was trying to sleep. Mosquito bits solved his problem. And on my end, the office became a soil free zone. The only plants allowed to stay now are hydroponic
Hilarious! And the hydroponic method is brilliant!
I already have mosquito dunks. Would the procedure be the same as using the mosquito bits – steep them in water first? Thanks!
Yes, it would be the same. As long as it contains Bti (Bacillus thuringiensis israelensis) it will work.
“I will not accept blame for this as they showed up only after I had repotted a string of pearls that had outgrown it’s container.”
Its – possessive of it (as in a string of pearls)
It’s = it is, every time!
Also thanks for the tips!
You’re welcome! And thank you for the grammar catch.
Heather, I left the bits in the water overnight because I forgot. Will the water still be ok to water my houseplants?
Thank you!
Absolutely! It won’t hurt anything.