How to Kill Fungus Gnats

A gnat on a white background.
Fungus gnats wreak havoc by feeding on fungi, decaying plant matter, plant roots, and leaves. Dorling Kindersley: Debbie Maize / Getty Images/Dorling Kindersley

Fungus gnats are about one-eighth of an inch long and are dark grey or black with thin wings, long legs and long antennae. Although they have a very short life span, they wreak havoc by feeding on algae, fungi and decaying plant matter, as well as plant roots and leaves. They don't fly very far and tend to cluster around the plant that they are infesting. Fungus gnats are usually most noticeable in the late fall and winter, hovering around indoor plants [source: Cransaw, Cloyd]. Read here for some ideas of how to get rid of this nuisance.

  • Make sure the flies you see are really fungus gnats by trying the raw potato test. Place a piece of raw potato on the soil near an infested a plant. If these are fungus gnats, you will see them migrating to the potato within a few hours. If you lift the piece of potato, you should see the larvae.
  • Drench the soil with microbial insecticide, such as bacillus thuringiensis, more commonly called Bt. This is the most effective way to kill any fungus gnat larvae.
  • Allow the soil to dry out for a few days, so that the top 2 inches (5 centimeters) are really dry. The larvae of fungus gnats cannot survive in dry soil. However, they will remain dormant and begin their development once the soil is moist again. Water your plant with a mixture of one part hydrogen peroxide and four parts water. The solution will kill the larvae, but is harmless to your plant. Reapply the hydrogen peroxide solution once a week until you see that the fungus gnats are gone.
  • Place yellow sticky traps near the infested plant. These 3-by-5-inch (7.6-by-12.7-centimeter) adhesive-based traps attract and kill many, though not all. fungus gnats [source: UCDavis].