Q&A: Controlling Fungus Gnats

Can you tell me how to get rid of fungus gnats in my houseplants and seedlings?
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Question: Can you tell me how to get rid of fungus gnats in my houseplants and seedlings?

Answer: Fungus gnats are tiny black flying insects. They are weak fliers and tend to stay close to potted plants rather than flying around the house. They can be seen sitting on or hopping across leaves, debris, the soil surface, windowsills, walls, etc. They will fly up when the plant is disturbed or watered.

Fungus gnats cause the most damage as larvae. They hatch under the soil surface and feed on fungi that grows in potting mix that is wet, warm and decaying quickly. The larvae can also feed on plant roots, which stunts the plant's growth. Adult flies do little damage—aside from creating more larvae. Even considering root damage by larvae, fungus gnats are largely just a nuisance.

To control fungus gnats:

  • Allow the surface of the growing mix to dry between waterings, and provide good drainage. Fungus gnats thrive in moist conditions.
  • Empty suacers of water and clean humidity trays regularly, as they may like these conditions too.
  • Use sticky traps. (See article on controlling whitefly.)
  • Spread a quarter to a half inch of horticultural sand on top of the soil. When you water, the sand will drain and dry quickly. The adult gnats will not want to lay their eggs in a dry environment, and it likely won't occur to them to burrow deeper to the moist soil.
  • Replace the top inch of potting soil with fresh soil.You may also need to repot the plant entirely, because the gnats can live in the medium surrounding the holes in the side/bottom of a pot. Note: Fungus gnat larvae are often already present in storebought potting mixes. Letting the mix dry out after you open the bag, before you use it, might help.

    If you have a lot of plants you suspect have fungus gnats, use a beneficial nematode found in the products Scanmask or Beneficial Sf Nematodes (a Gardens Alive product).

Reader tip: Steve Aegerter, of Denver, Col., reports having good success against fungus gnats with Bayer Advance Triple Action Ant Killer. He spreads it on top of the soil then works it in.