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Fungus gnats

Quick facts 

  • The darkwinged fungus gnat is a common indoor fly.
  • It is a mosquito-like insect found in buildings with houseplants.
  • Fungus gnats are common on over-watered houseplants where they feed on decaying or damaged roots.
  • They do not damage houseplants and are considered to be only nuisances.
  • To control them, don't over water houseplants.

How to identify fungus gnats

Adult fungus gnats

A brownish fly-like insect with light colored wings and long legs
Adult fungus gnat
  • Small, about 1/16th inch long
  • Slender dark-colored fly with very long legs
  • Not good at flying
  • Usually found close to where larvae are developing

Larvae

  • Small, worm-like insect, no more than 1/4 inch long
  • Pale white almost translucent color
  • Black head but no legs or other appendages
  • Likes very damp conditions
  • Feeds on decaying plant material, moist organic matter and fungi

Fungus gnats cause no damage

  • Fungus gnats do not damage houseplants and are considered to be only nuisances.
  • Fungus gnats are found in houseplants that are:
    • over-watered and might have decaying roots
    • grown in potting soil high in organic matter such as peat
  • Fungus gnats rarely feed on healthy roots.
  • If you see these flies, it means that your plants might have other problems, like damping off.

Where you might find fungus gnats

Fungus gnats may be seen on wet or damp walls that can allow the growth of fungus.

They are common in new construction where green wood still has high moisture. When the wood dries, the fungus gnats go away on their own.

Sometimes, it can be difficult to tell where the flies are coming from. Gnats might be seen buzzing around in rooms and also around windows and sliding doors. In most cases houseplants are the source of these flies.

How to get rid of fungus gnats

Reduce soil moisture in houseplants

  • Allow the soil surface to dry between watering, but do not allow plants to wilt.
  • Water plants based on how heavy they feel when they're picked up.
  • Water plants when leaves look dull and limp.

Place traps to check for fungus gnats

Place sticky paper traps in the pots to tell you which plants have fungus gnats. This method might also reduce some fungus gnats but will not get rid of them completely.

Using pesticides

For larvae

The only effective product for treating fungus gnat larvae in the soil is a bacterial insecticide, Bacillus thuringiensis var. Israelensis (known as B.t. H-14).

  • This pesticide is specific to fly larvae and does not kill adult fungus gnats.
  • Look for this insecticide in garden centers.

For adult fungus gnats

Pesticides like pyrethrins are effective against adult fungus gnats. But it is a short-term solution.

If there are favorable conditions with high moisture, the fungus gnat larvae will keep transforming into adult flies.

CAUTION: Mention of a pesticide or use of a pesticide label is for educational purposes only. Always follow the pesticide label directions attached to the pesticide container you are using. Remember, the label is the law.

Layer sand on top of the soil

Placing a layer of sand on top of the soil to prevent egg laying or larvae from reaching the soil is NOT very effective.

Jeffrey Hahn, Extension entomologist

Reviewed in 2018

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