Fungus gnats in terrariums: yes it happens, here's how to handle it

Fungus gnats are tiny black flies (1-3mm) that hover around moist soil. They’re annoying but rarely harmful to terrarium plants. Here’s how to deal with them.

How they get in

• Through open windows
• On new plants you bring in
• In contaminated substrate
• On your hands if you’ve been handling other plants

Once they’re in a moist terrarium, they’ll multiply fast.

Why they’re not usually a problem

Fungus gnat larvae live in the top layer of substrate and eat:

• Fungus (yes, the same fungus that terrariums grow)
• Decaying organic matter
• Algae

They don’t typically eat living plant roots (unless the population is huge).

When they ARE a problem

• Massive populations (hundreds visible)
• They’re damaging seedling roots
• They’re flying into your face constantly

In a typical terrarium, fungus gnats are an annoyance, not a crisis.

The fixes

Step 1: Let it dry out

Fungus gnat larvae need moisture. If your terrarium is overwatered, drying it out will kill larvae.

• Open the lid for a few days
• Stop misting
• Let the top inch of substrate dry

Step 2: Sticky traps

Yellow sticky traps placed in or near the terrarium catch adults. Break the breeding cycle.

Step 3: Beneficial nematodes

Steinernema feltiae nematodes eat fungus gnat larvae. Available online, terrarium- safe. Mix with water and apply to substrate.

Step 4: Bottom watering only

If you have an open terrarium, water from the bottom (in drainage layer) instead of top. The top stays dry = no gnat breeding.

Step 5: Hydrogen peroxide mix (emergency)

For severe infestations:

• Mix 1 part 3% hydrogen peroxide to 4 parts water
• Apply to substrate surface
• Kills larvae on contact
• Repeat weekly until cleared

Use sparingly — can harm beneficial microbes.

Prevention

For new builds:

• Use fresh, sterile substrate (don’t reuse old)
• Quarantine new plants for 1-2 weeks before adding
• Don’t overwater
• Add springtails (they compete with gnat larvae)

The bottom line

A few fungus gnats = annoying, not dangerous. Hundreds of fungus gnats = act, follow the steps above.

In a bioactive terrarium with healthy springtails, fungus gnats rarely become a problem — the springtails outcompete the larvae.

When to start over

Fungus gnats alone rarely warrant a rebuild. Only if combined with:

• Major plant decline
• Smell
• Mold

Then see the diagnosis flowchart.

Ask in the Discord if your infestation feels overwhelming.