Those tiny flies hovering around your houseplants aren’t fruit flies—they’re fungus gnats, and they’re breeding in your potting soil. While the adults are merely annoying (they don’t bite or spread disease to humans), their larvae feed on organic matter in soil and can damage plant roots, particularly in seedlings and young plants. An unchecked fungus gnat population means hundreds of larvae per pot, steadily undermining root health beneath soil that looks perfectly fine from above.
The good news: fungus gnats are entirely controllable and preventable with the right approach. The key insight most guides miss is that you must attack both adults and larvae simultaneously—targeting only one life stage while ignoring the other guarantees the population bounces back within a week.
Key Takeaways
- Fungus gnats breed in consistently moist potting soil—allowing soil to dry between waterings is the most effective preventive measure
- Yellow sticky traps catch flying adults but don’t address larvae in soil—you need both surface and soil treatments
- BTI (Bacillus thuringiensis israelensis) in Mosquito Bits/Dunks is the most effective biological larvicide for soil
- Hydrogen peroxide soil drenches kill larvae on contact without harming plants
- Complete elimination takes 3-4 weeks (one full lifecycle) of consistent treatment
Understanding the Enemy
Lifecycle (Why They Keep Coming Back)
A single adult female lays up to 200 eggs in moist soil. Eggs hatch in 3-6 days into translucent larvae that live in the top 2 inches of soil, feeding on fungus, algae, and organic matter for about 2 weeks. Larvae pupate in the soil for 3-5 days, then emerge as flying adults that live 7-10 days—long enough to mate and lay hundreds more eggs. The complete lifecycle takes about 3-4 weeks.
This lifecycle explains why killing adult flies alone doesn’t work—hundreds of larvae and eggs in the soil produce replacement adults within days. And why soil treatment alone isn’t enough—flying adults can lay fresh eggs in treated soil. You must attack both stages continuously for one full lifecycle (3-4 weeks) to break the cycle.
Treatment Plan: The Two-Pronged Attack
Prong 1: Kill Flying Adults
Yellow sticky traps are the simplest and most effective adult control. Fungus gnats are strongly attracted to the color yellow and stick irreversibly to the adhesive surface. Place traps horizontally at soil level (not hanging high above plants) for maximum effectiveness—adults fly close to the soil surface where they lay eggs.
Apple cider vinegar traps provide a supplementary catch method. Fill a small container with apple cider vinegar, add a drop of dish soap (breaks surface tension), and place near affected plants.
Prong 2: Kill Larvae in Soil
Method A — BTI treatment (most effective):
BTI (Bacillus thuringiensis israelensis) is a naturally occurring soil bacterium toxic to fungus gnat larvae but completely harmless to plants, pets, and humans. Available as Mosquito Bits (granules) or Mosquito Dunks (rings). To use: soak one tablespoon of bits in one gallon of water for 30 minutes, then use the strained water for regular watering. Repeat with every watering for 3-4 weeks.
Method B — Hydrogen peroxide drench:
Mix one part 3% hydrogen peroxide with four parts water. Water the affected plant thoroughly with this solution. The hydrogen peroxide kills larvae on contact, then breaks down into water and oxygen—completely harmless to plants and actually beneficial for root health by increasing soil oxygen. Repeat every watering for 3-4 weeks.
Method C — Neem oil soil drench:
Mix neem oil solution (1 tablespoon neem oil + 1 teaspoon liquid castile soap per gallon of water) as a soil drench. Neem disrupts insect development and is toxic to gnat larvae. Best used as a supplementary treatment.
Prevention: Stop Them Before They Start
Let Soil Dry Between Waterings
This is the single most effective preventive measure. Fungus gnats cannot breed in dry soil—eggs desiccate and larvae die without consistent moisture. Follow our watering guide for plant-specific drying intervals.
Top-Dress Soil With Sand or Gravel
A half-inch layer of coarse sand, decorative gravel, or perlite on the soil surface prevents adults from reaching the soil to lay eggs. Simple, attractive, and indefinitely effective.
Sterilize New Potting Soil
New potting soil from bags frequently contains fungus gnat eggs. Sterilize by microwaving moist soil in a microwave-safe container (3-5 minutes until steaming) or baking at 180°F for 30 minutes.
Inspect New Plants
Quarantine new plants for 2 weeks before placing them near your existing collection. Preventively treat new plants with a BTI drench before introducing them to your plant family.
Treatment Timeline
| Week | Actions | Expected Results |
|---|---|---|
| Week 1 | Place sticky traps + first BTI/H₂O₂ drench + let soil dry | Traps catch many adults; larvae start dying |
| Week 2 | Repeat soil treatment + maintain traps | Noticeably fewer flies; trap catches declining |
| Week 3 | Repeat soil treatment + maintain traps | Very few adults; remaining are late-emerging pupae |
| Week 4 | Final soil treatment + maintain traps | Zero or near-zero adults; cycle broken |
| Ongoing | Proper watering + monthly preventive BTI | No recurrence |
Frequently Asked Questions
Are fungus gnats harmful to plants?
Adults are harmless—just annoying. Larvae primarily eat decomposing organic matter and fungus in soil, but large populations can damage fine roots. Established plants tolerate moderate larval populations, but heavy infestations slow growth and can cause yellowing leaves from root damage.
Why do fungus gnats keep coming back after treatment?
Inconsistent treatment (missing a watering cycle lets surviving larvae repopulate) or reintroduction from contaminated soil or new plants. Treat consistently for a full 4-week cycle and maintain preventive practices.
Can I use cinnamon to control fungus gnats?
Cinnamon has mild antifungal properties that may slightly reduce the fungus that larvae feed on, but it doesn’t kill larvae or eggs. Not an effective standalone treatment. Stick with BTI or hydrogen peroxide.
Will repotting in fresh soil eliminate fungus gnats?
Only if you remove all old soil from roots, use sterilized new soil, and sterilize or replace the pot. If any contaminated soil remains, the problem transfers to the new pot.
Are fungus gnats the same as fruit flies?
No. Fruit flies are stockier with red eyes and hover around ripe fruit. Fungus gnats are smaller, darker, and hover around plant soil. Different insects requiring different control strategies.
Is hydrogen peroxide safe for all plants?
At the recommended dilution (1:4 with water using 3% H₂O₂), yes. This concentration kills larvae without harming roots or beneficial soil microbes. It actually adds oxygen to waterlogged soil.
