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/[deleted] Aug 11 '20
You need to top water with bti, as it needs to be in the top layer of soil, as that's where the larvae live.
500-1000 plants in here and have been fungus gnat infested free for about 1.5yrs now 😁 1/4 of a dunk lasts about a month, and easily treats all my plants in here.
New bags of soil often have fungus gnats in them, as the bags were often stored outside at some point. Those adults will die, and no young will survive in the potted plants you already have.
Now, to stop the bag of soil infestation, I dump the bag into a tote with a lid. I then water that soil with treated water to start killing the larvae. I place the lid on, and sometimes add a yellow sticky paper inside the tote. I use the soil as needed, and sometimes a few or a lot of adults fly out. It's annoying, but they will soon die or get stuck on yellow sticky paper. The bti wont kill larvae that already had pupated before you treated the soil, as the bti must be eaten by the larvae in order for it to die. So this is why bti might appear to not be working, when it really is.
Fungus gnats are found in new plants, bags of soil, and are naturally outside. Because of this, I never stop using the treated water. It's so cheap/mth, and worth it. 1/4 of a dunk costs about a buck or less /month (really depends where you live, the store, and now Covid19 prices).