There are several small flies, or gnats, that can be found inside the home. The ones associated with houseplants are called fungus gnats. These small, non-biting flies are completely harmless, except for the annoyance of their presence. Fungus gnats look dark gray in color. The wings also appear smoky-gray. The body is long and slender, unlike a fruit fly that has a short, robust body shape. At a little more than 1/16th inch in length, fungus gnats usually go unnoticed outdoors where they can be quite plentiful in fungi, damp soil and decayed vegetable matter. Though fungus gnats occasionally wander in from outdoors, a persistent problem 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 often occurs in the potting soil of houseplants.
Household insecticide sprays or fogs for the adults are of limited, short-term benefit. Spraying would have to be repeated quite frequently to catch all of the gnats as they emerge. Controlling the maggots by locating and eliminating the breeding site gives the best results.
When houseplants are infested it may mean they are overwatered. Fungus gnats do not survive if the soil becomes dry so one management option is to permit the soil to dry out to the extent tolerable between waterings. Otherwise, houseplant insecticide spray can be applied to the surface of the soil and around the edges of the pot. As a last resort, a solution of malathion insecticide can be applied to the soil surface as a soil-drench treatment in place of a regular watering. This must be done after taking the plant outdoors. Additional pesticides are available to commercial growers. Some, such as Bt israelensis products (Gnatrol, for example) may be available to home gardeners through specialty catalogs and organic gardening supply sources.
This article originally appeared in the April 12, 1996 issue, p. 51.