Fungus gnats: how to spot and manage them

Fungus gnats are tiny dark flies that hover around seedlings and pot plants. The adults are mostly just annoying — it's the larvae in the soil that nibble roots and can set young seedlings back.

How to spot them

1. Small dark flies running across the soil surface and drifting around seedlings.
2. Tiny clear larvae with black heads in the top of damp mix.
3. Sluggish, yellowing seedlings that aren't thriving.

Plants they target

Growing any of these? See our guides to lettuce and basil.

Seedlings and anything grown in constantly damp mix, especially indoors and in punnets.

How to manage them, safely

1. Let the soil surface dry out between waterings — this is the single best fix, as the larvae need constant moisture.
2. Set yellow sticky traps to catch the adults.
3. A Bti-based product (a different Bt strain sold for gnats and mosquitoes) treats the larvae, and a dry layer of sand or grit on top puts the adults off laying.

This one is mostly a watering-habit fix. For the bigger picture, see our guide to managing pests and problems safely.

Plan your patch: our free planting calendar shows what to plant now where you live. Ready to grow? Browse our raised garden beds or build your own with the garden bed builder.

Image: Katja Schulz, CC BY 2.0, via Wikimedia Commons.