Fungus gnats or Sciarids are small grey-black flies that run over the soil surface of pot plants or fly slowly around them. The adults do not feed on plants. Their larvae are thin white maggots up to 1/2in long with black heads. They live in the soil and feed mainly on rotting plant material but sometimes damage the roots of seedlings and plants that are in poor health, allowing other pathogens access into the plant. They may also tunnel into the base of soft cuttings and cause them to rot, or feed on algae on rockwool blocks.
Control by not over watering, covering soil surface with a layer of sterile medium. Biocontrol with Hyopaspis predatory mite or Steinernema feltiae nematode.