Fungus gnats are tiny, annoying pests that thrive in damp soil—exactly the kind of environment you want to avoid when growing succulents. These small, dark flies might seem harmless, but their larvae feed on organic matter and, worse, can damage succulent roots. If you’ve noticed tiny black flies buzzing around your plants or white worm-like larvae in the soil, you’re dealing with fungus gnats.
The good news? They’re preventable and treatable. Let’s get rid of these pests for good.
Dig in!
What Are Fungus Gnats?
Fungus gnats (Sciaridae family) are small, mosquito-like flies that lay eggs in moist soil. The larvae feed on fungi, decaying plant matter, and—in severe cases—succulent roots.
Why Do They Love Succulent Soil?
- Overwatering creates the damp conditions they thrive in.
- Organic-rich soil (like peat-based mixes) provides food for larvae.
- Poor drainage keeps the soil moist longer, encouraging infestations.
Signs of a Fungus Gnat Infestation
Before treating, confirm you have fungus gnats and not another pest (like fruit flies or aphids). Look for:
✅ Tiny black flies hovering around plants or soil.
✅ Larvae in the soil (tiny white worms with black heads).
✅ Yellowing or wilting leaves (if larvae are damaging roots).
✅ Sudden plant decline despite proper care.
Quick Test: Place a slice of raw potato on the soil. If larvae are present, they’ll crawl to it within hours.
How to Prevent Fungus Gnats
The best defense is making your succulent soil inhospitable to gnats. Here’s how:
1. Water Properly
- Let soil dry completely between waterings (gnats need moisture to breed).
- Bottom-watering reduces surface dampness where gnats lay eggs.
2. Use a Gritty, Well-Draining Soil Mix
- Avoid moisture-retentive mixes (e.g., peat-heavy soils).
- Ideal mix: 50% cactus/succulent soil + 50% perlite/pumice.
3. Add a Top Dressing
- A ½-inch layer of sand, gravel, or diatomaceous earth blocks gnats from laying eggs.
4. Quarantine New Plants
- Isolate new succulents for 2 weeks to prevent introducing gnats.
5. Improve Air Circulation
- Use a small fan to keep soil surface dry.
How to Eliminate Fungus Gnats
If gnats are already present, use a multi-step approach to break their life cycle.
Step 1: Kill Adult Gnats
- Yellow sticky traps catch flying adults (place near soil).
- Apple cider vinegar trap (fill a jar with ACV + drop of dish soap; cover with plastic wrap poked with holes).
Step 2: Destroy Larvae in Soil
- Let soil dry out completely (most effective for succulents).
- Drench soil with hydrogen peroxide mix (1 part 3% H₂O₂ + 4 parts water) to kill larvae.
- Apply Bacillus thuringiensis (Bt) (a natural bacteria that targets larvae).
- Use neem oil drench (mix 1 tsp neem oil + 1 quart water; apply to soil).
Step 3: Repot if Necessary
- If infestation persists, replace contaminated soil and rinse roots before repotting.
Natural vs. Chemical Treatments
| Method | Pros | Cons |
|---|---|---|
| Sticky traps | Non-toxic, easy to use | Only catches adults, not larvae |
| Hydrogen peroxide | Kills larvae quickly | Can harm roots if overused |
| Neem oil | Organic, deters future gnats | Smell can be unpleasant |
| Bt (Mosquito Dunks) | Targets larvae, safe for plants | Takes a few days to work |
| Insecticidal soap | Fast-acting | May need repeated applications |
| Chemical pesticides | Strongest option | Harmful to pets/beneficial bugs |
For severe infestations, a combination of sticky traps + Bt or neem oil works best.
Common Mistakes That Make Infestations Worse
❌ Overwatering (keeps soil damp, encouraging gnats).
❌ Using organic-heavy soils (provides food for larvae).
❌ Ignoring early signs (small infestations grow fast).
❌ Not treating all plants (gnats spread easily).
Final Thoughts
Fungus gnats are a nuisance, but with the right approach, you can eliminate them and prevent future outbreaks. Dry soil is your best defense—adjust watering habits, use a gritty mix, and deploy traps or natural treatments at the first sign of gnats.