Fungus gnats are very common, small, non-biting flies that normally go unnoticed. They are completely harmless, except they are an annoyance by their presence. Fungus gnats are frequently quite plentiful outdoors in fungi, damp soil and decayed vegetable matter. Though fungus gnats occasionally wander in from outdoors, a persistent problem with this nuisance in the house indicates an indoor breeding site. The immature stage of the fungus gnat is a small white maggot that lives in very moist areas high in decaying organic matter. This habitat may occur indoors with houseplants or in slow-running drains, moisture-accumulating cracks and crevices, refrigerator drain pans, and other places where fungi and slime accumulate.
Indoor household insecticide aerosols can be used to control the adult gnats. However, this treatment has to be repeated to catch all of the gnats as they emerge and does not usually result in complete control. Controlling the maggots by locating and eliminating the breeding site gives the best results.
When houseplants are infested, it is often because they are overwatered. Fungus gnats cannot survive in houseplants if the soil is permitted to dry out almost to the leaf-wilting point between waterings. Otherwise, houseplant insecticide spray can be applied to the surface of the soil and around the edges of the pot.
Sources of maggots other than houseplants (drains, water-soaked areas, etc.) should be cleaned or dried. In the case of slow-moving drains, physically cleaning the drains with a stiff brush will be of more benefit than chemical drainer openers.