what is a Fungus Gnat

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what is a Fungus Gnat

What is a Fungus Gnat?


Gnats (Bradysia sp. Sciaridae pron. Nat) are small black flies about 5 mm in length with long legs and antennae. When the gnat is viewed through a microscope a Y shape pattern can be seen on the end of their wings. Their larvae are a transparent to white worm about 5-8 mm long and have a small black head.

The larvae can often be found near the surface of pots as they feed on seedlings, roots, soil, fungi and algae.

Fungus gnats worsen in cool wet weather usually in autumn and prefer compost mixes and sawdust that is high in organic matter.

Gnats are a common garden pest, but it is the larvae that cause more of an issue for gardeners  rather than the adult flies. The larvae chew the plant roots preventing nutrient and water uptake and suck the sap from the roots infecting the roots with fungal disease.

Life Cycle

The life cycle of a Fungus gnat ranges from 18 to 27 days.

Control Measures

  • Identification
  • Inspection
  • Sanitation
  • Mechanical control/exclusion
  • Insecticide application

The key to controlling gnats is finding the location and eliminating of the breeding source. All possible sources need to be inspected.

Inspection Areas

  • Moist damp areas
  • Soil
  • Potted plants
  • Atriums
  • Compost heaps
  • Plants
  • Garden beds
  • Note: adults are attracted to lights

If there is an issue inside offices or health care facilities, it is recommended to replace live plants with artificial plants

Infestations have been known to develop in wall voids where damp building materials have trapped moisture allowing fungal growth.

Leaking roofs which has caused moisture inside the roof cavity could also create a breeding ground.

Once the source has been located it needs to be removed or dried out, to prevent fungal growth. Steps that can be taken included

  • Turning over the top 5-8 cm of soil
  • Using fans will speed up the drying process
  • Insect light traps can help reduce the adult population
  • If all else fails, chemical control

Chemical Measures

As gnats are a garden pest, many avid gardeners may not like the use of chemicals, especially in their veggie patches. If you do decide you need a chemical treatment, a mild pesticide and fungicide is the way to go.

  • Apply pesticide application ( check good nurseries for best pesticide options)
  • Drench the entire root zone with a fungicide solution 2 days after pesticide application

Stick with safe, effective products that will allow your plants to recover quickly.