Fungus gnats, also known as sciarid flies, are often associated with house plants and seed trays.
What are fungus gnats?
The adults are small flies but it is the compost-dwelling larvae that sometimes damage seedlings and cuttings. There are many species of fungus gnats, or sciarid flies, most of which are entirely harmless.
Adult fungus gnats are greyish brown flies that are mostly 3-4mm long. They can be seen running over the surface of seed trays and pots, or they fly slowly around plants. The larvae are slender white maggots, up to 6mm long, with black heads. Their bodies are semi-transparent and it is often possible to see the dark coloured gut contents. They live in the soil or potting compost. Unfortunately this insect thrives particularly well in certain peat-free media that contains composted green waste. Potting media formulated for houseplants is the best choice for indoor plants.
Adult fungus gnats do not damage plants but they can cause annoyance when they are flying around house plants. The larvae feed mainly on dead roots and other decaying plant material and associated fungal growth. Some species of fungus gnats may also feed on soft plant growth, such as seedlings and the base of soft cuttings. Established plants are unlikely to be damaged by fungus gnat larvae.
Fungus gnats are really only of concern if they are causing damage to seedlings or cuttings; established plants are not harmed. If the adult flies are a nuisance, their numbers can be reduced by placing some yellow sticky traps near the plants. These are available from garden centres as Agralan Yellow Sticky Traps, Bayer Greenhouse Fly Catcher, Growing Success Greenhouse Whitefly Traps and Vitax Whitefly Traps.
Biological control is also an option. There are pathogenic nematodes (Steinernema feltiae), predatory mites (Hypoaspis miles) and a predatory rove beetle (Atheta coriaria) available by mail order from various biocontrol supply companies. These biocontrols are added to the potting compost where they will help control the eggs, larvae and pupal stages in the fly's life cycle.
Contact insecticides such as pyrethrum (Py Spray Garden Insect Killer Spray, Doff All in One Bug Spray or Scotts Bug Clear Gun for Fruit & Veg) or deltamethrin (Bayer Sprayday Greenfly Killer) will control the adult flies but this may give only temporary respite if more adults emerge from pupae in the compost.
Pesticides for gardeners (Adobe Acrobat pdf document outlining pesticides available to gardeners)
Fungus gnats can breed all year round in greenhouses and houses. The females deposit eggs in the surface layer of the potting compost and these hatch within a few days under warm conditions. The larvae feed on fungal growth and decaying plant material but some species can also damage the roots of seedlings or tunnel into the base of soft cuttings. When fully fed, the larvae pupate in the soil. During the summer the life cycle can be completed in about a month.