Fungus gnats (sciarid flies)

Fungus gnats, also known as sciarid flies, are often associated with house plants and seed trays.

Fungus gnat adults

Quick facts

Common name Fungus gnats or sciarid flies
Scientific names Bradysia and other species
Plants affected Seedlings, soft cuttings  in greenhouses and pot plants
Main cause Maggots feeding on decaying organic matter; adult flies can be a nuisance in houses
Timing All year round on house plants and in greenhouses

What are fungus gnats?

The adults are small flies but it is the compost-dwelling larvae that sometimes damage seedlings and cuttings. They also occur out of doors where they cause no damage. Cultivated and wild mushrooms can also be attacked. There are many species of fungus gnats, or sciarid flies, most of which are entirely harmless.

Symptoms

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. This insect thrives in composts containing high levels of organic matter. 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.

Control

Non-chemical control

Fungus gnats are really only of concern if they are causing damage to seedlings or cuttings; established plants are not harmed. Fungus gnats are often more numerous in composts that are constantly wet, allowing compost to dry can reduce infestations, provided this does not affect the health of plants. Potting media formulated for houseplants is the best choice for indoor plants. If the adult flies are a nuisance, their numbers can be reduced by placing yellow sticky traps near the plants. These are widely available from garden centres. 

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.

Chemical control

Contact insecticides such as pyrethrum (e.g. Py Spray Garden Insect  Killer Spray, Vitax House Plant Pest Killer or Bug Clear Gun for Fruit & Veg),deltamethrin (e.g. Bayer Sprayday Greenfly Killer) or lambda-cyhalothrin (e.g. Westland Resolva Bug Killer) will control the adult flies but this may give only temporary respite if more adults emerge from pupae in the compost.  There are no synthetic pesticides available to the amateur that are approved for use on cultivated mushrooms.

Downloads

Pesticides for gardeners (Adobe Acrobat pdf document outlining pesticides available to gardeners)

Biological control suppliers (Adobe Acrobat pdf document)

Biology

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.

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