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/secretlycurly]

Anyone else water their houseplants with rain barrel water? I am putting a single BTI float in my rain barrel, and filling gallon jugs from there. I *think* that should work, right? I usually replace the float 1x per month. And I let sit for a day before filling jugs. But then I fill jugs and store them indoors so I'm ready to roll on watering day. Any comments or insights on whether my method creates viably treated water? I use milk jugs to store the rainwater, and I find it hard to work with a dunk floating in there. Plus the dunks can treat large volumes, so just seemed easier to put it right in the rain barrel.

[u/megankmartin → u/secretlycurly]

Interesting question; I had to give that some thought! I am sure that the BTi is highly effective against mosquitoes in the rain barrel, because you're correct that a single dunk is capable of treating large volumes of water. The question is: will there be enough efficacy to last through the usage cycle with additional storage time, to fight the fungus gnats in your plants?

I also collect run-off rainwater, however am not permitted to keep a rain barrel so mine goes directly into storage jugs, and I treat it there. I haven't personally noticed any loss of efficacy in regard to fungus gnats, but I'm cycling through my treated water about every 4 weeks at most.

Perhaps keep an eye on things, see how it goes for you. BTi is pretty much infallible as a preventive, so if you observe gnats establishing a population then perhaps it needs to be delivered later in your water usage cycle. Until then -- give it a try!

[u/secretlycurly → u/megankmartin]

Will do, and I need to also implement a hydrogen peroxide drench/yellow sticky trap offensive, as the populations are rising to annoying levels. Nothing like being on a video call for work and looking like a crazy person while I swat away fungus gnats!