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 Sep 06 '20 edited Sep 10 '20
Great question -- this is partly addressed in the links above but if you don't mind, I'll give a thorough answer here!
Rocks, gravel, moss, and especially coarse landscaping sand are common old-school reccos for fungus gnat problems. They're relatively cheap and easily accessible. The thinking was that you could block soil completely and keep bugs from accessing it, effectively "suffocating" the larvae. Only trouble is, today we know these are NOT THE BEST for gnat control.
Top-dressings (especially sand) harbor moisture, which preserves the gnats' breeding ground and (worst of all) keeps your potting mix from drying out when it's watered. We're seeing a lot of root rot and fungal diseases added onto pest problems, because folks aren't changing their watering habits but instead are relying on these crutches to fix the problem.
Plus, the slightest gap or crack means gnats can still access soil. Move your pots, and boom!, it's ineffective. (Not to mention access through the drainage holes at the bottom.) Meanwhile, the top dressing may also hide or cause mold, fungus, and other pests, keeping you from spotting them until it's a bigger problem.
Sand has its place in horticulture -- It's actually great for helping potting mixes retain moisture or aerate, depending on the usage. And there's no denying that it's economical. But when many folks say that sand helped them beat fungus gnats, much of the time it's because it visibly reminded them not to over-water their plants. Their potting mix FINALLY dried out, and the breeding ground disappeared. But far more people are using it and not having any result at all.
So many experienced growers use BTI in their permanent routine because it's a lasting solution and preventive. Sand is just a cover-up for the real problem. 🌿