Fungus Gnats: treatment prevention and management

    Your quick guide to the treatment and management of fungus gnat infestation.












    Fungus gnats are small flies that infest soil, potting mix, other container media, and other sources of organic decomposition. Their larvae primarily feed on fungi and organic matter in soil, but also chew roots and can be a problem in greenhouses, nurseries, potted plants and interior plantscapes.

    They breed and develop in overwatered potted plants. Many people do not realize plants take up less water in the winter, continue to water regularly, thereby creating a suitable habitat in the soil for the fungus gnat to complete its life cycle.

    Fungus gnats (families Mycetophilidae and Sciaridae) are a common pest of plants grown indoors, especially where humidity and moisture are high. They’re usually first noticed when the harmless adults are seen flying around house plants or gathered at a nearby window. These non-biting adult gnats can become a flying nuisance. But it’s the larval stage, feeding in the soil, that can damage tender plant roots.


    Identification of an infestation

    Adults are delicate, grayish black, mosquito-like flies (1/8 inch long) with long legs and one pair of clear wings. They are not strong fliers and emerge from potted plants, especially when watering. Larvae or maggots (1/4 inch) have a shiny black head and an elongated, whitish to transparent body. They are most abundant in damp, rich soils and feed on root hairs, fungi and other organic materials.

    Identification of infestations will usually be obvious as the gnats are visible. You should be able to spot the gnats flying around the plant, especially when it or the soil is disturbed. You may also be able to smell a rotting or bad smell in the soil, especially if it is turned over and uncovered, which indicates that root rot is present.

    Plant symptoms that indicate fungal gnats are seen as sudden wilting, loss of vigor, poor growth, and yellowing. With severe infestations, a considerable portion of the plants may be lost. Especially prone to injury:

    • Geraniums
    • African violets
    • Carnations
    • Poinsettias


    fungus gnat sitting on green leaf

    Treatment of infestations

    Fungus gnats have a lifespan of about a week, and in that time, females can lay hundreds of eggs which can result in even more pests flying around your plants and home. Getting rid of fungus gnats once they have infested your plants isn’t difficult as they have a short lifespan and are slow flying pests. Follow these steps to effectively get rid of them.

    1 Sticky Traps

    Fungus gnats are small little flies that can be caught using sticky traps. These can be placed in areas close to your plants to catch them. Some sticky traps can be placed on a stake and placed into the soil where the fungus gnats like to lay their eggs. Use the traps placed horizontally at the soil surface to capture large numbers of egg laying adults. The gnats are attracted to yellow and are easily removed on the trap before they can lay more eggs.


    yellow paper sticky cards sitting inside a terracotta plant pot with plant inside, showing many dead small fungus gnats on the paper

    2 Drying out the Soil

    Overwatering your plants creates moist environments which the fungus gnats thrive in. They like to lay their eggs in these moist conditions which causes more fungus gnats to hatch. By following a watering routine and only watering your plants when you need to, the soil will be able to dry out enough and kill off any remaining gnats. Allow the top 2 inches to dry out completely before re-watering.

    This not only kills larvae and inhibits the development of eggs, it also makes the soil less attractive to egg-laying females. Apply a protective layer on top of the soil as a top dressing to significantly reduce pest populations.


    3 Bug Spray and Insecticides

    Using bug sprays on your plants can be another option for killing off fungus gnats. They do not usually cause harm to your plants, however they should be used with caution because they can be harmful if they are inhaled by humans.

    Azadirachtin, the key insecticidal ingredient found in neem oil, may also be used. Mix 1 Tbsp/ gallon of water and apply as a soil drench for effective control of soil-borne insect larvae. With high pest pressure make applications every 5 to 6 days. 


    Preventing further infestations

    1 Minimise Debris

    Try to clear away plant debris from the soil of the plant such as dead flowers and leaves. If these are left to decay, fungus gnats are more likely to lay their eggs in the plant debris. When potting, avoid water holding, organic material such as peat moss that may encourage egg laying.


    2 Careful Watering

    Fungus gnats love moist environments and will multiply faster in these conditions. To solve this issue, only water plants when they need watering, and this will be when the top 2 inches of soil has dried out. Watch out in the winter! Watering will be needed less frequently in the winter months as the plant will grow slower.


    3 Covering the Soil

    To prevent the fungus gnats from laying their eggs, you can add small pebbles or sand to cover the top layer of soil which acts as a protective barrier. It’s easier to prevent a pest infestation than to try and get rid of them once they have found a home in your plants.


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