Fungus gnats (sciarid flies)

Fungus gnats, also known as sciarid flies, are associated with damp composts especially in 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 and can be a nuisance, the compost-dwelling larvae can sometimes damage seedlings and cuttings. These insects also occur out of doors where they cause no damage. Cultivated and wild mushrooms can also be affected. There are many species of fungus gnats, or sciarid flies, most of which are entirely harmless. There are also many (perhaps 1000's of ) species of small fly that occur in Britain most of which are part of natural ecosystems and do not damage garden plants, or feed on blood. 

 

Symptoms

Adult fungus gnats are greyish brown flies that are mostly 3-4mm long. They can often be seen running over the surface of seed trays and pots, or flying slowly around plants. The larvae are slender whitish 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 soil or potting compost. This insect thrives in damp composts containing high levels of organic matter. 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 indoors. 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 seedling roots and the base of soft cuttings. Established plants are unlikely to be damaged by fungus gnat larvae.

Control

Fungus gnats are really only of concern if they are causing damage to seedlings or cuttings; established plants are not harmed.
Check indoor plants frequently, ensuring they are not over watered enabling fungus gnats to breed. Regular inspection will mean that action can be taken before a damaging infestation has developed. When choosing control options you can minimise harm to non-target animals by starting with the methods in the non-pesticide control section. If this is not sufficient to reduce the damage to acceptable levels then you may choose to use pesticides. Within this group the shorter persistence pesticides (that are usually certified for organic growing) are likely to be less damaging to non-target wildlife than those with longer persistence and/or systemic action.

Non-pesticide control

  • In the open garden fungus gnats are not a problem and part of a well balanced garden
  • 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 can be used. Pathogenic nematodes (Steinernema feltiae), predatory mites (Hypoaspis miles, Stratiolaelaps scimitus and 'Mighty Mite', Macrocheles robustus) and a predatory rove beetle (Atheta coriaria) are sometimes available by mail order from various biocontrol supply companies. Formulations of nematodes may also be available in some plant centres. These biocontrols are added to the potting compost where they will help control the eggs, larvae and pupal stages of the flies.

Pesticide control

Control of fungus gnats should be aimed at reducing larval numbers by non-pesticide methods. Contact insecticides may control the adult flies but this may give only temporary respite if more adults emerge from pupae in the compost
  • Organic sprays, such as natural pyrethrum (e.g. Bug Clear Gun for Fruit & Veg, Neudorff Bug Free Bug and Larvae Killer), fatty acids (e.g. Solabiol Bug Free, Doff Greenfly & Blackfly Killer) or plant oils (e.g. Vitax Plant Guard Pest & Disease Control, Bug Clear for Fruit and Veg) can give good control of earwigs. These pesticides have a very short persistence and so may require reapplication to keep earwig numbers in check. Plant oil and fatty acid products are less likely to affect larger insects such as ladybird adults. 
  • More persistent contact-action insecticides include the synthetic pyrethroids lambda-cyhalothrin (e.g. Westland Resolva Bug Killer), deltamethrin (e.g. Provanto Ultimate Fruit & Vegetable Bug Killer, Provanto Sprayday Greenfly Killer) and cypermethrin (e.g. Py Bug Killer)
  • The systemic neonicotinoid insecticide acetamiprid (e.g. Bug Clear Ultra) is also available
Follow label instructions when using pesticides. On edible plants make sure the food plant is listed on the label and follow instructions on maximum number of applications, spray interval and harvest interval. There are no synthetic pesticides available to the home gardener that are approved for use on cultivated mushrooms
Plants in flower should not be sprayed due to the danger to bees and other pollinating insects.
Inclusion of a pesticide product does not indicate a recommendation or endorsement by RHS Gardening Advice. It is a list of products currently available to the home gardener.

Downloads

Pesticides for gardeners (pdf document)
Biological control suppliers (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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