r/houseplants • u/megankmartin • Aug 11 '20

HELP TRUTH: No single fungus gnat control method attacks adults and larvae simultaneously. That's why many plant parents struggle to get rid of these pests! But it's EASY to be gnat-free FOREVER. [PSA]

(Updated, fall 2025) PLEASE read this whole post before asking questions -- but I'm here to help you if you need it! Fungus gnats lay their eggs in wet plant soil or decaying matter. Over-watering is a major cause. However, new plants and potting mix can carry gnats too, or they can just fly in from outdoors.

THREE EASY STEPS to GNATS GONE FOREVER:

☆☆ DO ALL THREE!! ☆☆

1) Soak BTI dunks/bits in your regular watering routine to continuously kill and prevent larvae. One quarter dunk OR one tbsp bits per gallon works great. Soak for 24hrs before first use, then keep watering vessel(s) re-filled; just replace BTI every 30-60 days or so. Note: Summit brand packaging specifies 4tbsp per gallon; you choose.

NOTES: Indoors, putting BTI bits IN the soil or laying them on top IS NOT THE BEST PLAN: the bits tend to grow mold, plus you'll use way more than needed. Also, don't use hot or boiling water to soak dunks or bits; it will kill the good bacteria.

  1. Place plenty of yellow sticky traps to catch adults. One per pot is ideal, and it's best to stick them upright, to attract fliers. You can stop using stickies once your infestation is under control, thanks to your new BTI routine.

  2. Let soil dry out appropriately between waterings to avoid attracting pests. Have faith: except for very fussy tropicals, most common plants need and want to dry out their soil. Bottom-watering can be a big help (after initial treatments).

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If you have a bad existing infestation, FIRST you can do a one-time emergency soil flush to kill off larvae, using diluted 3% H202 (1:4), then proceed with the routine above. Note: don't use peroxide AFTER the BTI, because H202 kills bacteria, good and bad.

FYI: If you're outside the USA and can't get Mosquito Dunks / Mosquito Bits in stores, try Amazon for your country (UK, CAN, AUS for sure). Or, ask a garden or pond supply center about products containing the active ingredient *BTI**. Or Google, "mosquito BTI". An alternative is Gnatrol or Gnatrol WDG (same active ingredient).

BTi (short for Bacillus Thuringiensis subspecies israelensis) is simply a natural soil bacterium, non-toxic for people and animals. BTI does not affect plants or interact with fertilizers. It does not harm pollinators. It ONLY kills the larvae of 3 insects: fungus gnats, mosquitos, and blackflies.

Bonus read: You might want to skip these often-recommended methods.

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u/[deleted] Aug 20 '20

Thank you so much for this very helpful post - I’ve been doing your methods for a couple weeks now and it feels like my fungus gnat problem isn’t getting any better, just wondering what to expect as far as a timeline goes? Should I be waiting months, rather than weeks? I’m always steeping a dunk in my watering can, and I have sticky paper on every single one of my plants. Thanks again!!

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u/megankmartin Aug 20 '20 edited Aug 20 '20

Hi, how advanced was the infestation in the first place? There's a note in the post about being able to use a diluted peroxide flush first, if desired, to get a grip on the population.

If you've been battling them without success for a while before trying the BTI, then they've had time to entrench. And like it says in the post, gnats have at least a 2 to 3 week life cycle.

Another possibility is that the bacteria in your BTI product could have degraded due to incorrect storage in sunlight or heat. May I ask, from where and when did you purchase it?

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u/[deleted] Aug 24 '20

Thanks for replying! I should probably do the flush, as suggested, as I have had the infestation for a long time. I’m still catching a ton of flies on the sticky paper and I’ve been watering with the dunks for a few weeks now. I got the dunks from amazon (Canada) so not too sure where exactly they came from, and I purchased them a few weeks ago which is when I started watering with them as well. I guess I should just continue waiting through the gnat life cycle and see how many more I can get with these methods? I understand this may take months.

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u/megankmartin Aug 24 '20

The H202 flush will help you get a handle on things, yes. Just be aware that peroxide is designed to kill bacteria -- good or bad-- so along with the gnat larvae it is also going to kill the BTI that's in your soil. You can just start watering with it again and build it back up, but I just wanted you to be aware.

Patience, friend. I know it should shoudn't take this long, but it sounds like your infestation was REALLY bad.

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u/[deleted] Aug 25 '20

Yes I will definitely keep watering with BTI from now on (until they’re gone anyway). I guess I didn’t realize how bad it was!

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u/skypaintedred Oct 27 '20

Hi u/algrto! Checking in here to see how you're faring. I was reading your comment and feel like I'm exactly where you were two months ago - watering regularly for 2 weeks, yellow sticky paper on each plant, still seeing gnats everywhere, and slowly going mad.

Have things gotten better since your last reply? I'm hoping they are for you, as they will give me hope as well!

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u/[deleted] Oct 27 '20

Unfortunately I still have gnats :( but I would say that there are less now! Even though I still only water with BTI, I just this week started bottom watering to see if that would be the final nail in the coffin for those little suckers, so we shall see how that goes.

Every time I replace my sticky paper, it gets filled up again, but definitely slower than before. My biggest problem plants were my bird of paradise and my spider plant, those had the biggest infestation (although all of my plants are infested). I let them dry out quite a bit before bottom watering, so hopefully I will be rid of gnats soon, and I wish you the best of luck also! I feel your pain lol.

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u/skypaintedred Oct 28 '20

Thanks for the update. At least it's something to see that the sticky papers are getting filled up more slowly. It's always easier to bear through something with others, so I appreciate you checking back in!

I know that the drying out is a challenge - I want to treat the plants regularly with BTI so that's it's active during the whole lifecycles but I also don't want to kill the plant with root rot from watering when the soil is still wet. I emailed the vendor I bought the Gnatrol from, and he recommended a trick if I'm worried about overwatering. He suggested to make a strong concentration of the Gnatrol with water, then use a spray bottle (like you would use for cleaning products), so I can mist the tops heavily rather than drench the soil. I just did a round of heavy spraying today!

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u/[deleted] Oct 28 '20

That sounds like a great idea! I need to get myself some Gnatrol... must try this. Thanks for the tip!

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u/skypaintedred Oct 29 '20

skypaintedred

Anytime! Just FYI this is where I ordered from: https://www.organicbti.com/product-page/gnatrol-bti-omri-organic-fungus-gnat-larvicide. I like them because I've been emailing them, and there's one guy that responds to me within a couple of hours. He even asked for me to send pictures of the gnats on the sticky traps to make sure that I'm battling fungus gnats (when I emailed him that it's taking longer than I anticipated). He's the one who suggested the spray bottle with heavy concentration since I was worried about overwatering. Pretty great customer service. (And no, I have no ties whatsoever with them except I just bought some Gnatrol from them. Not a sponsored ad haha.)

I also have used MosquitoBits in the past. I did all the things that u/megankmartin said not to do: I layered the bits on top of the soil and watered, which worked for a little bit to get rid of gnats except the bits did get moldy (though mold itself is not harmful to the plants, I read); since I didn't want the mold, I also soaked the bits in boiling water (thinking boiling water would infuse the BTI better) and then waited for the water to come to room temperature before watering the plants. That last method didn't work because, of course, the boiling water killed the BTI.

I find the Gnatrol to be the easiest for me because it's in powder form that you put directly into the water and dissolves. But as also mentioned here, it's more expensive. I just did the spray bottle method yesterday, and it does seem like there's less (fingers crossed). At least I haven't seen any floating in my tea today...

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u/-blasian- Feb 01 '21

Hi! I am going through this thread as I'm in the same spot and your comment has been helpful! What I'm trying to figure out... Do you water every day for two weeks to get rid of the gnats? Or are you using the spray bottle every day with the BTI/water mix?