Pest

Fungus gnats

Small dark flies that breed in damp potting mix; mostly a nuisance.

What you'll see

• Small dark flies running over the potting mix or fluttering near pots and windows.
• See-through larvae with shiny black heads in the top layer of damp mix.
• In heavy cases, seedlings or cuttings wilt because larvae nibble tender roots.

Why it happens

Fungus gnats breed in moist, rich potting mix. Their larvae eat fungi and rotting matter, and sometimes tender roots.

How to fix it

1. Let the top of the mix dry out between waterings, and empty any water left in saucers.
2. Catch adults with yellow sticky cards placed at pot level.
3. Water Bti, a soil bacterium that kills the larvae, into the pot as the label says, keeping kids and pets away while it soaks in.
4. Repot badly infested plants into fresh, sterile mix and throw out the old mix.

Keep it from coming back

• Water only when the plant needs it, in pots with drainage holes.
• Use fresh potting mix, and avoid unfinished compost in pots.
• Remove dead leaves from the soil surface.

Read more

• Overwatering vs underwatering
• Watering houseplants in winter
• Potting mix basics
• A 5-minute pest check
• How to pick a healthy plant

Sources

• UC IPM: Fungus gnats
• RHS: Fungus gnats
• UC IPM: Houseplant problems

General guidance, not a diagnosis. For valuable, edible or outdoor crops, check with a local extension service or plant expert. Use any product exactly as its label says, and keep children and pets away while it dries.