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Fungus gnats

Fungus gnats, or sciarid flies, are a nuisance more than a threat on established plants, but their larvae can damage the roots of seedlings and cuttings. They are a reliable sign the compost is kept too wet.

Pests
Small dark fungus gnats on houseplant compost
Small dark fungus gnats on houseplant compost

What it looks like

You see the flies; the larvae do the damage.

  • Weak-flying dark flies about 3 mm long around the compost.
    Flies over pots Weak-flying dark flies about 3 mm long around the compost.
  • Flies run over the compost surface.
    Running adults Flies run over the compost surface.
  • White maggots with black heads in the top few centimetres.
    Translucent larvae White maggots with black heads in the top few centimetres.
  • Seedlings wilt with chewed roots.
    Failing seedlings Seedlings wilt with chewed roots.

How to confirm it

Check the compost.

  1. Look at the surface

    Flies rise when you water.

  2. Potato slice test

    Lay a slice of raw potato on the compost; larvae gather underneath in a day or two.

  3. Wet compost

    The compost is consistently damp.

What to do

Dry them out.

  1. Let compost dry

    Allow the top few centimetres to dry between waterings.

  2. Yellow sticky traps

    Catch adults and break the cycle.

  3. Top-dress with grit

    A layer of grit or sand discourages egg-laying.

  4. Biological control

    Steinernema nematodes or Hypoaspis mites in the compost control larvae.

Prevent it next time

  • Water from below and less often.
  • Use fresh, sterile compost for seedlings.
  • Do not leave pots standing in water.

What not to do

Do not keep watering to 'drown' them; the larvae thrive in wet compost.

In the growing guides

Where the guides mention it, with plant-specific advice.

Sources

  1. Fungus gnats, Royal Horticultural Society
  2. Fungus gnats, University of Maryland Extension