Fungus Gnats

Fungus gnats are small, dainty flies that commonly develop in the growing medium of houseplants.  

Order: Diptera
Family: Sciaridae

Description

  • Fungus gnats are small, mosquito-like insects often found in greenhouses, homes, and offices. 
  • Adults 3 mm long, delicate, black flies with long legs and antennae, with a distinct “Y-shaped” pattern on the forewings.  
  • The larvae are maggots and are nearly translucent, with a black head capsule and live in the soil of houseplants and plants grown in the greenhouse. 
  • They can be pests in greenhouses, but are most commonly a nuisance when infesting houseplants 

Quick Facts

  • Fungus gnats are small, delicate bodied flies that commonly develop in the growing medium of houseplants and in greenhouses.  Adult fungus gnats do not bite or feed but can be a nuisance when present in large numbers indoors.
  • Larvae of fungus gnats are generalists that can affect plants grown in the greenhouse including a wide variety of vegetables. They feed on algae, fungi, and plant roots in the growing medium, which can reduce plant health and increase susceptibility to microbial infections, especially in seedlings and younger plants. Severe injury to roots is more common on plants grown indoors. 
  • Larvae of fungus gnats feed on algae, fungi, and plant roots in growing medium. Adults do not bite or feed. 
  • Fungus gnats can be controlled by allowing the growing medium to dry between watering. 
  • Some insecticides and biological control agents can be used to control fungus gnat larvae in growing media. 
Fungus gnat larval feeding damage

Penn State Department of Plant Pathology & Environmental Microbiology Archives, Penn State University, Bugwood.org
Fungus gnat larval feeding damage on poinsettia.

Adult fungus gnat.

Whitney Cranshaw, Colorado State University, Bugwood.org
Adult fungus gnat. 

Fungus gnat larvae.

Whitney Cranshaw, Colorado State University, Bugwood.org.
Fungus gnat larvae. 

 

Life History and Habits

Fungus gnat larvae usually are in the top 5-8 cm of the growing medium, depending on moisture level. They primarily feed on fungi, algae, and decaying plant matter. However, the larvae can feed on plant roots and leaves resting on the growing medium surface. In homes, adult fungus gnats are usually seen in the vicinity of an infested houseplant. However, adult flies may disperse short distances and often accumulate around window frames. During their seven to 10-day life span, females may lay up to 200 eggs into the cracks and crevices of growing media. Moist-growing media containing high amounts of peat moss are particularly attractive to adult females.

Injury

Larvae of fungus gnats feed below the soil surface on roots. When densities of larvae are high, feeding injury can lead to stunted growth and increased susceptibility to microbial infection, especially in seedlings and younger plants.

Adult fungus gnats do not bite, but they are a nuisance indoors when their numbers are high. Adult fungus gnats tend to be most noticeable during late fall and winter. Decreased day length and cooler temperatures slow plant growth and water usage. If watering practices are not altered, particularly during fall and winter, the growing medium will remain moist, which improves conditions for fungus gnat development. Furthermore, as the growing medium ages or degrades it tends to retain more moisture, which will attract fungus gnat adults.

Monitoring

While yellow traps alone will not significantly reduce fly densities, they can be used to monitor fungus gnats when deployed in the greenhouse. Often, adults can also be observed flying or resting on surfaces.
An effective means of detecting fungus gnat larvae is to insert ¼ inch slices of skinned potato into the growing medium. Larvae will migrate to the potato and start feeding within a few days, at which point the potato can be removed and checked for larvae.

Cultural Control

  • Examining the soil of new plants can help prevent introduction of fungus gnats to clean areas.
  • The most important strategy to minimize fungus gnat problems associated with plants grown indoors is to allow the growing medium to dry between watering, especially the top 1 to 2 inches.
  • It is also recommended to re-pot plants, particularly when the growing medium is retaining too much moisture.
  • Be sure to remove any containers with an abundance of decaying plant matter such as decayed bulbs and roots, which are a food source for fungus gnat larvae.
  • In greenhouse systems, fungus gnat entry can be prevented by storing potting soil indoors or sanitizing via steam or autoclave before potting plants sensitive to fungus gnats. Populations in the greenhouse can be suppressed by reduction of breeding areas. For example, removing decaying plant matter, soil and standing water from underneath benches.

Biological Control

  • A biological control option for fungus gnat larvae is applications of insect parasitic nematodes applied as a drench to the growing medium. The nematode species Steinernema feltiae is particularly effective against fungus gnat larvae
  • Fungus gnat larvae may be killed by the microbial insecticide Bacillus thuringiensiss, Bt, when applied as a drench to the growing media. Bt is not generally available through retail outlets, but it is used in commercial greenhouses and large interior environments

Chemical Control

  • When fungus gnat populations are excessive, pesticides can be applied to the growing medium. Usually, multiple applications are required.

 

References

University of California. 2019. Pests in Gardens and Landscapes: Fungus Gnats. University of California Agriculture & Natural Resources. Available http://ipm.ucanr.edu/PMG/PESTNOTES/pn7448.html
Maccini, R. (n.d.). Fungus Gnats [Fact Sheet]. University of New Hampshire. Available https://extension.unh.edu/resource/fungus-gnats-fact-sheet

CSU Extension Fact Sheet

Download or view the CSU Extension’s PDF fact sheet for your reference.