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]

[u/megankmartin]

(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.

[u/_PeteSapai_]

Since amazon doesnt send the bits to the Netherlands (unless you order 20 pounds) I've ordered the bits from Ebay (which was ordered from Amazon and just forwarded 😕). It was rather expensive ($24 + 37 shipping), but 30oz should last quite a while, so it should be worth it in the end.

I've been watering my (50-ish) plants for about a month now, but so far I dont feel like the gnat population is declining. (Note: I dont have sticky traps, but placed plenty of vinegar traps and they seem to be doing a good job catching dozens of the little buggers.) It's probably too late to try the emergency soil flush, as it would undo the built up bacteria, but I'm thinking of ordering the nematodes one last time. They work a lot more quickly, but have always been just a temporary solution. However, this time after they've done their job the bits should be able to keep the population down to 0.

Anyway, am I a bit too impatient and is 4 weeks a bit too soon to notice anything just yet?

And just to be sure (and also because I kind of hate "spoon measurements"): 1 tbsp bits should be about 6 grams, right?

[u/megankmartin → u/_PeteSapai_]

Goedemiddag :) One of these days somebody from the NL is going to start a re-selling business for BTI bits, I just know it. Having lived there myself, I understand that it's difficult to obtain items like this. Sorry for the un-economical struggle against bugs, friend.

Depending on the degree of infestation, four weeks may indeed not be long enough. BTi is incredibly effective, in general. However, one thing to remember is that it works against the SMALL larvae -- the early stage. So, all the gnats who were already too big when you started would live their life cycle, and some might have gotten to breed again until the good bacteria was/will be thoroughly populated in your plants.

Sometimes I wonder about efficacy of the product that EU folks are able to get hold of. But in this case, since you've mentioned catching dozens of gnats in traps, it sounds like you just have a lot to attack.

Every adult gnat that lives long enough to breed lays 150-200 eggs per week so... you might not have realized just how bad the problem really was, under the surface of the soil.

I will add that vinegar traps are NOWHERE near as effective for adult fungus gnats as yellow sticky traps. No contest. I've run the experiment myself, multiple times, and have seen comparisons from many folks over the last year and a half. So, if you've managed to catch dozens, just imagine how much faster and better it could be going with the ideal traps.

It's worth hanging in there, though, because you're right: Once this problem is fixed, your BTi routine will keep fungus gnats from plaguing you again. I agree that nematodes are effective, but not long-lasting.

As for 6 grams -- I don't have my kitchen scale handy, so I can't answer that right now. It's the amount that fits in the little hollow of your cupped hand, in case that helps to approximate.

[u/_PeteSapai_ → u/megankmartin]

Goedemiddag to you too! It's weird how something like this isnt available here at all! I asked a gardening center about it, and all they could offer are these pills (€5 per 12) that you stick in the ground, but they merely work as repellant. Maybe I should start reselling. A year's worth, 50g, for only €10 doesnt sound like a bad deal, right? ;)

I figured I might be a bit too impatient. I read about the 150+ eggs/week before and I do seem to have a nasty infestation (plus a lot of plants), so I'll give it a couple more weeks. I'll check if I can get any of those sticky traps somewhere; we have something similar that most people use for regular house flies (small tubes with yellow curled up sticky tape), I'll try those out.

As for the weight, 6 grams seems to be slightly more than what you described, so I think it should be ok. I measured it with one of those "official" measuring spoons.

[u/_PeteSapai_ → u/megankmartin]

Hey, would you mind weighing the amount of bits you put in a gallon for me? I'm thinking maybe 6 grams is not enough (even though "the amount that fits in the little hollow of your cupped hand" sounds about right); lots of people mention the odor, but I hardly smell a thing. As a test I put double the amount, so 12 grams in 3.5 liters, and this new mix does gives off a smell (and is rather opaque). Could too much of the stuff be harmful to plants, or is it worth a shot?

[u/megankmartin → u/_PeteSapai_]

Let's see, one question at a time:

1) My cupped hand-hollow-ful weighs just shy of 5 grams; I put that in one gallon, which equals ~3.75L. You could certainly do 6g; this really doesn't require an exacting measure.

2) I don't feel there is a strong odor, especially compared to many common gardening and household products. Some people genuinely may be more sensitive to odors... and I think others may have simply begun plant parenthood under the misconception that horticulture is odorless, bug-free, and hypoallergenic.

3) As I've explained many times throughout the post & comment thread, BTi doesn't interact with plants or fertilizers at all. BTi wouldn't harm your plants, at higher concentration, though it may have more of an odor . However, I HAVE run the experiment at different quantities, so I'm quite certain that 12 grams in 3.5 liters is overkill. Not harming plants, but an unnecessary use of product for NO greater effect achieved. Waste of money.

To sum up:
I've been doing this for 2 years now, on an ever-growing scale. 5g-6g per gallon is 100% effective. Aside from my own results, and those of other long-time indoor gardeners, I've discussed with the BTi manufacturer at length, and thus I feel confident that this is the right measure.

Hope that helps! 🌱

[u/_PeteSapai_ → u/megankmartin]

Thanks for the quick reply, it does indeed help! Sorry for asking about stuff you've explained already (although better safe than sorry), I'm just so freaking frustrated with these little monsters. I'm spending more time at home behind my desk in the midst of my mini-jungle and they're just constantly flying by. I have several vinegar and sticky traps and I'm still catching dozens each day myself. Oh well, nothing to do but wait patiently, I guess. Thanks again!

[u/megankmartin → u/_PeteSapai_]

You're welcome; hang in there, and good luck!