Insect of the Month - Fungus Gnats and Shore Flies

  Hort Snacks - February 2017
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 Fungus Gnats | Shore Flies

Fungus Gnats

Orfelia spp. and Bradysia spp.

Crops Affected: Range of plants may be affected

Life Cycle:

  • Small nuisance flies that infest soil, growing medium, as well as other sources of decomposing organic matter
    • Don’t typically damage plants unless present in very high numbers
  • Adults are dark, delicate flies that resemble mosquitos, somewhat
    • May be attracted to light but often stay near plants or sources of organic matter
    • Weak fliers
    • Typically 1/16 to 1/8 inch long
    • Slender legs
    • Light grey to clear wings
      • Bradysia spp. have a Y-shaped wing vein
    • Segmented antennae which are longer than their head; Long antennae distinguish them from shore flies
  • Adults lay eggs in moist organic matter, such as potting soil, compost, etc., hatching within about 3 days
  • Larvae
    • Shiny black head with long, white to clear coloured, legless body
    • Feed on mulch, leaf mould, grass clippings, compost, root hairs and soil borne fungi
    • Develops for about 10 days before pupating
  • Multiple generations might be expected in a season in a greenhouse
Fungus gnat adult (Bradysia spp.)
Photo by David Cappaert,

  • Adults may be observed flying near windows or running across organic matter
  • In moist or humid conditions, with high populations, larval slime trails will be visible on the surface of organic matter
  • In some situations, plant wilting or a decline in health may be due to larval root feeding, alone or in combination with other factors
  • Watch for the presence of adults either at rest or flying
  • Monitor areas of the greenhouse where moist conditions and high organic matter may occur, specifically watching for evidence of larval activity
  • Yellow sticky traps may be used for adults or other attractants for larvae (e.g. potato slices)
  • Control immature larval stages rather than adults
  • Focus on reducing excess moisture and organic debris
    • Avoid overwatering, ensure good drainage in all areas of the greenhouse, fix leaks and puddles of water
    • Clean up debris in the greenhouse
  • Registered insecticide or biological control agents may be available for use, as required
Shore Flies

Scatella stagnalis

Crops Affected: Greenhouses, but not the plants within them

Life Cycle:
  • Feed on algae and are found near places where algae occurs
  • Nuisance pests, however they may serve as vectors for various root disease pathogens (e.g. Pythium)
  • Adults are small, dark grey flies, approximately 1/8 inch long, with a sturdy body
    • Resemble a fruit fly, somewhat
    • Short bristle-like antennae (distinguish them from fungus gnats)
    • Wings have 5 distinctive white spots
  • Larvae are small, whitish maggots which feed on algae (not plants)
  • Adults lay up to 300 eggs on moist soil, with life cycle completing in less than 2 weeks
Shore Fly adult
Photo by David Cappaert,

  • May cause “fly-specking” on foliage in high population situations
  • The presence of pupae or other life stages may be observed in association with seedling diseases, although they may not be connected
  • Yellow sticky traps can help capture adults
    • Traps should be oriented horizontally at the soil surface or vertically just above the plant canopy
  • Close examination of algae-fied areas can reveal larvae and pupae
  • Manage algal growth
    • Reduce over-fertilization and over-watering
    • Deal with areas where water accumulates
    • Clean up areas and equipment where algae grow, including walls, benches, floors, etc.
  • Insecticides may be used to control populations, but most can be controlled culturally or by using biological control agents
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For more information about the content of this document, contact Robert Spencer.
This information published to the web on January 31, 2017.