Fungus gnats

Also known as sciarid flies, this common greenhouse pest damages small roots of cuttings, seedlings, young plants and pot plants worldwide.

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  • Eggs (0.25mm) are deposited on growing media in humid areas with organic matter
  • Larvae can be found in growing media near plant roots
  • Fully grown larvae (5mm) are transparent with distinctive black heads and pupate in the soil
  • Pupae (2-5mm) are yellow to brown in colour
  • Adult flies (3-5mm) are dark with long, slender antennae and long legs – wings have Y-shaped venation
  • Adults typically spotted in warm, humid areas of greenhouses near plants

Life cycle

  • Fertilized females lay 50 to 200 eggs which hatch in 2 to 3 days
  • Fungus gnats have 7 development stages: egg, 4 larval stages, pupa and adult
  • Egg to adult takes 3 to 4 weeks at temperatures above 24 °C/75 °F. Development stops below 10 °C/50 °F or above 35 °C/95 °C
  • Adults live up to 1 week

Damage caused

  • Active year round – larvae feed on rotting organic material and living plant material, such as root and stalk tissue
  • Larvae pierce roots and stalks of cuttings, seedlings, young plants and pot plants
  • Secondary plant diseases, such as Pythium, can then gain entry and spread - leading to wilting, slow growth and even plant death
  • Adult flies can create large swarms under optimal conditions


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