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.
- 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.
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.
Pesticides for gardeners (pdf document)
Biological control suppliers (pdf document)