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If you're seeing tiny black flies hovering around your plant soil, you have fungus gnats. Their larvae live in the top 2 to 3 inches of moist potting mix, and a single female can lay up to 200 eggs in a seven to ten-day lifespan per Colorado State University Extension. The full egg-to-adult cycle is three to four weeks at room temperature per Colorado State, which is why by the time you notice the adults you already have overlapping generations developing in the soil. Most online advice sells you on solutions that don't address the larvae, which is where the damage actually happens.
Fungus gnats are tiny dark flies, about 1/8 inch long per Colorado State University Extension, that breed in moist potting soil and whose larvae feed on plant roots. The adults you see flying are annoying but harmless; the larvae cause real damage to seedlings and young plants. Treatment requires letting the top 1 to 2 inches of soil dry between waterings (Colorado State's advice) and applying Bacillus thuringiensis israelensis (BTi, sold as Mosquito Bits) to the soil surface to kill larvae.
The treatment ladder
Start at step 1. If you don't see results, move to the next step. Step 5 is for repeat offenders.
- 01Free
Let the top 1–2" dry between waterings.
Fungus gnat larvae need moist soil. Colorado State University Extension calls letting the growing medium dry between waterings, especially the top 1 to 2 inches, the most important strategy for houseplants, and MBG advises letting the soil dry as much as possible, without damaging the plant, to kill larvae. This alone won't end a bad infestation — there are still adults laying eggs — but the other steps build on it.Ongoing · then assess - 02Inexpensive
Mosquito Bits — the actual fix.
Active ingredient: Bacillus thuringiensis israelensis (BTi). Per the EPA, BTi produces toxins that specifically affect the larvae of mosquitoes, black flies, and fungus gnats — and has no toxicity to people. Sprinkle a thin layer on top of your soil and water through it, or water with a soak made from the granules. The bacteria release into the wet soil, where the larvae feed. Summit's directions are to repeat weekly for three weeks, which spans the egg-to-adult cycle that Colorado State puts at 3–4 weeks.
One BTi product is Summit Mosquito Bits (30oz).
Apply every 7 days · 3 weeks total - 03Inexpensive
Yellow sticky traps — for monitoring, not killing.
Stick them in the soil near your plants. They catch adult gnats and let you watch the population drop over the 3-week BTi treatment. Don't use these instead of Mosquito Bits — yellow sticky traps catch adults only per UC IPM, and a single missed female can lay up to 200 more eggs, per Colorado State. Use them with the BTi treatment so you can see your progress.
One option is Garsum yellow sticky traps (42-pack). Replace when half-covered.
Replace when half-covered - 04Inexpensive
Sand or grit top-dressing as prevention.
After the infestation clears, top-dress your plants with coarse sand or fine gravel. UW Extension suggests a ½ to 1 inch layer to keep the surface drier and less attractive for egg-laying, and UMN Extension says a layer at least half an inch thick is best.Permanent · refresh as needed - 05Inexpensive
Hydrogen peroxide soil drench (only if BTi failed).
Cornell Cooperative Extension mixes 1 part 3% hydrogen peroxide with 4 parts water and pours it over the soil; the mixture fizzes as it attacks the larvae. The research behind this is thin: UF/IFAS Extension found only one published study on hydrogen peroxide in soil and says its unintended effects are unknown. Use it once, not weekly.
If you suspect root rot has started, peroxide is unlikely to fix it. See the root rot guide for the full unpot-and-repot protocol.
One drench · then return to BTi maintenance
What doesn't work
These common remedies don't end an infestation on their own:
| Method | What it does | Why it doesn't end an infestation |
|---|---|---|
| Yellow sticky traps alone | Catches adult gnats | Doesn't touch the larvae. The eggs already laid will hatch and continue the cycle. |
| Apple cider vinegar traps | Drowns some adult gnats | Same problem as sticky traps. Catches a fraction of the adult population. Larvae untouched. |
| Cinnamon sprinkled on soil | Mild fungicide | Folk remedy. Doesn't kill gnat larvae. May reduce fungal food source slightly. |
| Dish soap drench | Suffocates some larvae | Inconsistent results. Can damage roots at high concentrations. BTi is safer and more effective. |
| Repotting in fresh soil | Removes current larvae | Adults can lay eggs in the new soil. Doesn't break the cycle without BTi or top-dressing. |
| Letting soil dry out completely | Kills larvae in the dry layer | Reduces population but stresses the plant. Works best as part of treatment, not alone. |
Prevention checklist (after you've cleared an infestation)
- Bottom-water your plants when possible. Sit the pot in water and let the soil wick it up. Top of the soil stays dry — gnats have nowhere to lay eggs.
- Top-dress with coarse sand or fine gravel on every pot — a ½ to 1 inch layer, per UW Extension.
- Quarantine new plants. A new plant can bring gnats in with its potting mix. Keep it apart with a yellow sticky trap nearby and check it before integrating.
- Don't reuse old potting mix. If you repot, throw the old soil — even if it looks fine, it can carry eggs.
- Address overwatering at the source. Chronic moist soil is the root condition.
- Keep BTi on hand. Sprinkle preventively after a heavy water if you've had gnats before.