How to Kill Gnat Larvae in House Plants

Fungus gnats are tiny flying insects that often swarm around houseplants and sunny windows. Although they are annoying house guests, fungus gnats in the adult stage are harmless to people and plants. They lay their eggs in moist potting soil, and the larvae that hatch a few days later feed on houseplant roots. Symptoms of a houseplant affected by fungus gnat larvae include pale color, sluggish growth and dropping leaves. The gnat larvae thrive in moist soil with plenty of decaying organic matter, and the easiest way to kill them is to make their environment less hospitable.

Things You'll Need

• Sterile potting soil or soil-less potting medium
• Houseplant soil treatment containing Bacillus thuringiensis (Bt)

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Instructions

1. • 1

     Allow the top 1 or 2 inches of soil to dry before watering houseplants. Dry potting soil is unattractive to egg-laying female fungus gnats, and their larvae can’t survive without moisture.

   • 2

     Re-pot plants that have been growing in the same soil for 1 or more years. Plant them in sterile potting soil or a soil-less potting medium. Over time, organic potting medium breaks down, providing food for fungus gnat larvae.

   • 3

     Apply a houseplant soil treatment containing Bacillus thuringiensis (Bt). Verify that the product is labeled for use against fungus gnats. Bt is a bacterium that kills fungus gnat larvae but is harmless to humans and other animals.