Sciarid flies are also known as Fungus Gnats or Mushroom Flies. They are small, dark coloured flies (3 to 5mm long) that are usually seen erratically flying around the surface of compost, particularly in the warmer humid environment of a greenhouse. There are around 600 different species of Sciarid Flies within Europe although the species that cause most problems for gardeners and growers are Bradsysia paupera (the Black fungus gnat) and Lycoriella auripila (the Mushroom sciarid). Although the adult flies are the most obvious stage of the Sciarid fly’s life cycle, they do not damage plants and can only ingest liquid. It is the Sciarid fly larvae that cause the problems when they feed on roots and other under-soil parts of plants.
After mating, the adult males die whilst the females lay between 50-200 minute transparent eggs into moist compost where plants are growing. Pots, troughs and tubs as well as commercial mushroom houses provide an ideal environment for eggs to be laid. After two or three days the eggs hatch into small translucent white larvae with shiny black heads, that immediately start to feed. Over the following two to three weeks, these larvae rapidly grow to a length of about 5mm and can sometimes be seen on the surface of the compost where they will eat leaves that touch the surface. They then pupate within the compost to emerge as adult flies a few days later.
Plants that are attacked by Sciarid fly larvae are likely to grow slower than plants that are not infested. They may also wilt and collapse, especially if they are young, tender plants or seedlings. Older, more robust plants can often cope with Sciarid fly, but will provide a constant source of these pests to other plants if they are not controlled. However, the damage caused by their feeding can become infected with fungal diseases which may then kill the larger plants.
Cyclamen, Begonias, Ferns, Poinsettia, Azaleas, Impatiens and Primulas are particularly prone to Sciarid fly attack, as are mushrooms, where the larvae will burrow into the stalks and up into the cap.
Chemical control of sciarid flies is no longer an option, but a number of biological control agents are now available for indoor use. Initially, sticky traps should be used to confirm the presence of sciarid flies as well as reduce the numbers of egg laying females. The predatory mite Hypoaspis miles can then be released on to the surface of pots where they burrow down and feed on the sciarid larvae. The predatory beetle Atheta coriaria can also effectively be used in this manner. Another option is to water the affected compost with Steinernema feltiae, an entomopathogenic nematode that burrows into Sciarid fly larvae and kills them.
A layer of perlite, vermiculite or fine grit on the compost around the stems of susceptible plants will also help to reduce sciarid problems. It provides a dry barrier that prevents Sciarid flies from laying eggs into moist compost which is required for them to hatch.