Utah Plant Pest Diagnostic Lab

Gnat Infestation


I have gnats flying around my house which seem to be coming from my potted plants. What can I do to control these insects?


Fungus gnats are a common nuisance pest associated with a cool, damp environment. They are usually brought into your home via a new potted plant or seasonal plant (poinsettias). The small adults emerge from the potting soil and fly over to other indoor plants, laying eggs on the potting media surface. Eggs hatch and larvae feed on fungi and plant roots for 2 to 3 weeks

Recommendation: Always quarantine new indoor plants in a separate room until you feel sure they harbor no pests. Prevent fungal growth by letting potting media dry slightly between waterings. Never allow water to stand in pot saucers. Move the potted plant to a warmer room. Switch to an un-glazed clay pot thatdries out faster. Repot with a sterile mix. Treat with B.t.i. (Bacillus thuriengiensis var. israelensis) to control larvae.

Posted on 13 Jun 2006
Karl Hauptfleisch
Salt Lake Master Gardener


If you have tiny, flying pests that have made your plants their home, you are not alone. These pests, called fungus gnats, are a common indoor plant problem. The gnat larvae live in the potting soil and feed on the roots of the plants. This can cause poor plant health as well as root rot. The adult gnats are a dark black. After they breed, they fly around the plants and land on the soil to lay their eggs. Consider these tips for control.

  • The gnat larvae will not survive in soil that dries thoroughly between waterings. The first and most important step is to watch your watering pattern. To avoid water stressing your plants, water thoroughly each time but only after the soil has completely dried. You may need to water different plants on different schedules depending on pot size and how quickly the soil dries between waterings.
  • If you have too much organic matter in your potting mix, it can hold the water too long. You may need to repot your plants and use a soil mixture with more sand or soil and less organic matter.
  • The best chemical control for the pests is targeted at the larvae. Products that work well for larvae control can be applied directly to the soil. Some of theme include: Gnatrol, a natural bacteria that attacks insects; beneficial nematodes, parasites that feel in insects; or Diazinon. Chemicals that target the adult gnats will quickly reduce their numbers, but usually work best when applied in combination with chemical control for the larvae. Some of them are Diazinon, insecticidal soap, Permethrin, or Pyrethrin. These can be applied directly to the leaves of the plants.

Posted on 14 Sep 2006
Diane Alston
Hort-Entomologist Specialist