Fungus Gnats – a Common Problem of Over-Watered Houseplants

— Written By and last updated by Nancie Mandeville
en Español

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black fungus gnat

Adult fungus gnats are delicate, gray, dark-gray, or black fly-like insects about 1/8” long that resemble fruit flies. They are often seen as they swarm up out of a plant when you water the plant. Adult fungus gnats do not damage plants but are objectionable and a nuisance to the homeowner. In heavy infestations, they can be very annoying by flying around your home or office and into your face. I have worked in an office where one of my co-workers was overwatering his plants in the office and everyone in the office had these little boogers flying in their face! The immature fungus gnat lives in the soil and is a white, translucent larva with a shiny blackhead. The larvae feed on any organic matter and can attain a length of about ¼”.

The fungus gnat larvae cause damage to the root systems of the infested plants by burrowing in the soil and feeding on the roots and sometimes the crowns of plants. Seedlings, rooted cuttings, and young plants can be severely damaged or killed by fungus gnat larvae feeding on root hairs or roots.

Avoid bringing these pests home in new plants by inspecting any plants carefully before you purchase. Once fungus gnat adults and larvae are discovered on the plant, control and prevention come in several forms. For infested plants, allow the soil to thoroughly dry between watering. This will kill the larvae through desiccation as well as help prevent future problems. If the plant is of a type that cannot be allowed to dry out, immerse the pot in water and allow it to stand for an hour or so. This should drown the larvae. If this doesn’t work, you can drench the soil with a pesticide labeled for use on houseplants and for control of fungus gnats. Always read and follow label directions when using pesticides.

The best way to prevent new or future infestations of fungus gnats is to follow proper watering practices. Houseplants in the winter normally do not require as much water as at other times of the year. Fungus gnats are more of a problem in the winter and are usually a result of overzealous watering. Whenever possible, allow soil surfaces to dry completely between watering.