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Fungus gnats

Lord of the Flies!

Fungus gnats
Yellow sticky trap in a holder

A year-round annoyance for me in the conservatory is a host of tiny black flies on and above the soil surface of tomato grow bags.

They don’t seem to do much damage, but they are irritating.

I was under the impression they were thrips, but further investigation has led me to believe they’re sciarid flies, also known as fungus gnats (Bradysia paupera).

The flies are about 2mm long and run over the soil surface or fly slowly around pots. The larvae are the bad guys, small translucent worms, up to 1cm long and very hard to see, living just under the surface. When infestations are heavy, there are shiny silken threads on the top of the soil.

Fungus gnats can breed all year under glass and in house plants. Larvae feed on fungal growth and decaying plant material but some damage the roots of seedlings or soft cuttings.

Adults don’t cause damage. The life cycle can be completed in a month, speeding up to one-two weeks in hot weather.

Larvae attack the roots of virtually all houseplants, pot and border plants including vegetables, ornamentals, fruits and fungi.

Organic treatment

  • Place yellow sticky traps between and around the base of plants to catch adults.
  • Biological control: pathogenic nematodes (Steinernema feltiae), predatory mites (Hypoaspis miles) and a predatory rove beetle (Atheta coriaria) are available. These biocontrols are added to the potting compost where they will help control the eggs, larvae and pupal stages in the flies’ life cycle.
  • Letting the soil dry out partially may help to reduce the larval population in pots.


Fungus gnats
The conservatory in August – biggest problem time for fungus gnats
  • Good hygiene: remove any dead leaves and fungal growth from the top of pots or benches – larvae can survive in patches of mould.
  • Water plants only when required to prevent the build-up of fungal growth.
  • Cover the surface of pots with sand as a barrier against egg-laying females.