Look for
Small black flies around 2mm long run over the soil surface or fly slowly around houseplants, pot plants and borders. Larvae are small translucent worms, up to 1cm long. They are harder to see but can be found in the immediate area under the soil around the roots. When infestations are heavy, there may be shiny silken threads on the top of the soil.
Plants affected
Fungus gnats attack the roots of virtually all houseplants, pot and border plants including vegetables, ornamentals, fruits, fungi and even weeds.
About Black fungus gnats
- Fungus gnats are small flies around 2mm long, which are usually black in colour.
- They are extremely common pests, usually present around most house and greenhouse plants.
- There will be many overlapping generations all year round on indoor plants.
- Each female fungus gnat can lay up to two hundred microscopic eggs after mating.
- Eggs are laid into soil around the base of the plant, and hatch after five to seven days.
- The larvae are no more than 1mm long when they first emerge, but can grow to ten times that before pupating.
- The larvae are difficult to see with the naked eye due to their small size and translucent bodies.
- They will hide beneath the soil surface where it is moist, and feed mostly on dead organic matter but can damage seedlings and the base of soft cuttings.
- Larvae can also survive on patches of mould on greenhouse floors and benches.
- At room temperature, newly hatched larvae will develop into adults in 20 to 25 days.
- During hotter times of the year in greenhouses, the lifecycle can be as short as one to two weeks.
Treatment
Chemical
Products containing the following chemical ingredients are all effective on Black fungus gnats
Pyrethroids and Pyrethrin
Organic
- Place sticky traps between and around the base of plants to catch adults.
- For indoor plants, the predatory mite Hypoaspis can be applied to the soil. The mites inhabit the just area under the soil surface where fungus gnats pupate.
- Water the parasitic nematodes Steinernema feltiae into infested soil. These naturally occurring parasites will infect fungus gnat larvae with bacteria and kill them.
- Letting the soil dry out partially may help to reduce the larval population in pots.
Prevention
- Practice good plant hygiene by removing any old, dead leaves and fungal growth from the top of pots.
- Water plants only when required to prevent the build up of fungal growths.
- Cover the surface of pots with sand as a barrier against egg laying females.
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