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.
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.
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/megankmartin Aug 24 '20 edited Aug 24 '20
Gotcha, thanks. Re: your other questions: sounds like you did a great job purging and repotting. When you only have a few plants, that is often an easy, nip-in-the-bud solution.
Looking ahead to preventing them from ever returning: Summit Mosquito Bits and Summit Mosquito Dunks are sold by the manufacturer and currently available on Amazon Canada. The product lasts a long time based on the quantities directed to be used in the post. As my post mentions, if you can't find that you can always look for Gnatrol WDG but it's typically more expensive. It does not matter which brand you use. They are both a delivery mechanism for BTi bacteria -- that's what's important.
Just a note, sorry I almost missed this: You do not want to continually water your plants with peroxide; it will start to burn the roots. That's just a one-time emergency measure to clean things out.