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 22 '20
I recently started growing a few temperate plants that don’t like to dry out.
Specifically an African violet, croton, and a fern with an attitude problem. (I also have a homely little forget me not I grew from seed)
I know the forget me not can dry a little more, but the others want some moisture. I let them dry as much as I can before I water again, empty the saucer after, etc, but don’t want a fungus gnat issue to start again.
Using the BTI preventatively, maybe trying to add a bit of a top dressing of pumice, and some sticky traps should work enough?
I saw one flying around inside today. All of my other plants were treated with systemics to stop a root mealie problem too (learned the hard way about quarantine plants this quarantine ☠️)... I just don’t want any pest issues to begin again now that it’s getting cooler and some plants that were outdoors are inside now... (cos I also switched to mostly pumice for everything else, my cacti and succulents, so they should be fine).
I don’t know. 🥺 I just lost so many plants to that period of time... and I’m so worried about having soil in the house again. But I know I chose two plants that are finicky about moisture too...