Moving your houseplants outdoors during the summer gives them a welcome boost of sunshine and washes the winter dust from their leaves. By early fall, they are refreshed and ready to come inside. They may, however, bring a souvenir of their time outdoors---fungus gnats. These small, non-biting insects lay eggs in the top 2 or 3 inches of your plant's potting soil. The larvae mainly feed on fungi and other organisms in the growing medium but they sometimes feed on plant roots. Fortunately, fungus gnats are easily controlled.
Confirm the diagnosis. To be sure you're facing a fungus gnat outbreak, insert a ¼-inch slice of raw potato into the growing medium. Fungus gnat larvae will begin to feed on this tempting delicacy within 2 or 3 days. Remove the potato and look for small, translucent larvae happily munching on the slice.
Allow the top 1 to 2 inches of soil to dry out between waterings. This will slow the growth of soil fungi and kill some existing larvae. Additionally, adult fungus gnats look for damp, dark spots to lay their eggs and a drier environment discourages this.
Place sticky traps to capture adult gnats. They are attracted to the yellow color of these traps where they become stuck in the non-toxic adhesive and die.
Treat severe fungus gnat outbreaks with a pyrethroid-based insecticide. Sprays will kill adult fungus gnats but will not have much effect on larvae. Retreatment every 3 to 4 days may be needed to newly hatched adults.
Apply Bacillus thuringiensis (Bt), a microbial insecticide, as a soil drench to kill larvae. These organisms are harmless to people, pets and plants but lethal to larvae.
Consider introducing Steinernema feltiae, a parasitic nematode. Nearly microscopic, these larvae killing roundworms are through many organic garden product suppliers. They are usually mixed with water and applied as a soil drench.