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Fungus gnat

Fungus gnats are small, dark flies whose larvae commonly develop in moist potting media, soil, and other decaying organic material.

By Ethan Anderson| Reviewed by Olivia Bennett, Standards & Fact-Checking Lead| Updated July 27, 2026|2 min read| Fact-checked

Fungus gnats are small, dark flies whose larvae commonly develop in moist potting media, soil, and other decaying organic material. In home and greenhouse settings they are usually a nuisance and minor pest, especially around houseplants, where overwatering often promotes infestations.

Taxonomy and description

Fungus gnat is a common name used for several groups of small flies, particularly within Sciaridae and related families. They are dark, delicate, mosquito-like insects usually found near damp growing media.

Life cycle and habitat

Adults lay eggs in moist soil or potting mix, and the larvae develop in wet, organic-rich media. The insects are associated with decaying vegetation, fungi, algae, and greenhouse or houseplant settings.

Damage and significance

Larvae may feed on roots and tender plant tissue, which can lead to poor growth, yellowing, and stunting. Adults do not bite people, and in most home settings they are mainly considered a nuisance.

Management and prevention

The main control methods are moisture management, sterile or well-drained potting media, sanitation, and allowing the soil surface to dry between waterings. Sticky traps and biological controls are also used to reduce adult and larval populations.

Key facts

  • The term fungus gnat covers several small fly groups rather than a single species.
  • Larvae feed on fungi, decaying organic matter, and sometimes plant roots.
  • Infestations are strongly associated with moist, poorly drained soil or potting mix.
  • Control and prevention focus on drying the growing medium, improving drainage, sanitation, and trapping adults.
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Canadian context

Fungus gnats are common indoor and greenhouse pests in Canada, especially around houseplants and commercial growing operations. Canadian extension services recommend moisture management, sanitation, sterile potting media, and sticky traps as practical control measures.

Frequently asked questions

What are fungus gnats?
Fungus gnats are small flies, usually associated with moist soil, potting mix, and decaying organic matter. Their larvae are the stage most closely linked to plant damage.
How to get rid of fungus gnats?
Reduce soil moisture, let the top layer dry between waterings, improve drainage, remove decaying plant debris, and use yellow sticky traps. Severe infestations may also be treated with biological controls or targeted insecticides.
What causes fungus gnats?
They are usually caused by persistently moist growing media, poor drainage, overwatering, and organic-rich potting mix or decaying plant material that provides breeding sites.
Are fungus gnats harmful?
Adults are generally harmless to people and do not bite. Larvae can damage plants by feeding on roots or tender tissue, and infestations may weaken seedlings and houseplants.
Fungus gnats vs fruit flies
Fungus gnats are usually found around moist soil and houseplants, while fruit flies are attracted to ripening or fermenting fruit and kitchen waste. Fungus gnats have longer legs and a more mosquito-like appearance.
How to prevent fungus gnats?
Use sterile, well-draining potting soil, avoid overwatering, let soil surfaces dry between waterings, remove plant debris, and prevent standing water. Good sanitation and drainage are the most effective preventive measures.

References

  1. UNH Extension — https://extension.unh.edu/resource/fungus-gnats-fact-sheet
    Supports: Fungus gnats comprise several fly families; larvae feed on roots and stems; overwatering and wet areas promote breeding; adults are a nuisance.
  2. Ohio State University Extension — https://ohioline.osu.edu/factsheet/ENT-59
    Supports: Overwatering, water leaks, and poor drainage encourage infestations; sterile potting soil and moisture management are primary control approaches.
  3. Missouri Botanical Garden — https://www.missouribotanicalgarden.org/gardens-gardening/your-garden/help-for-the-home-gardener/advice-tips-resources/insects-pests-and-problems/insects/flies/fungus-gnats
    Supports: Fungus gnats occur around damp, decaying vegetation, algae, and fungi; they do not bite people or animals; control includes reducing moisture and sanitation.
  4. Royal Horticultural Society — https://www.rhs.org.uk/biodiversity/fungus-gnats
    Supports: Fungus gnats are small flies in the family Sciaridae; many species feed on rotting organic matter; wet compost encourages breeding; sticky traps can reduce adult numbers.
  5. Colorado State University Extension — https://extension.colostate.edu/resource/fungus-gnats-as-houseplant-and-indoor-pests/
    Supports: Fungus gnats develop in houseplant growing media; larvae are minor pests; adults are harmless and do not bite.
  6. North Carolina State Extension — https://beaufort.ces.ncsu.edu/news/fungus-gnats-tiny-pests-in-houseplants-and-how-to-control-them/
    Supports: They are often mistaken for fruit flies; they are attracted to moist soil; top-drying soil, sticky traps, and repotting with fresh soil help control them.