When a cloud of tiny, black flies rises from the pot of a houseplant, it's a clear sign of trouble--and it's downright annoying. That cloud may consist of hundreds of adult fungus gnats, and while individually they are barely noticeable, in large numbers they are hard to miss. Fungus gnats become problematic when their populations reach conspicuous levels. Their gregarious nature makes them a classic example of a nuisance pest.
Mature gnats measure a mere 1/8 inch and live for about a week. During this time, females lay eggs in soil fissures. The resulting translucent, minute larvae feed largely on the assorted fungi growing in the potting soil, though they can also feed on fine roots and plant debris. In a few weeks, they pupate into adults within the soil and the cycle continues, with several generations occurring together at any given time.
Overwatering often leads to a problem with fungus gnats. Constantly damp soils promote fungal growth, which serves as an excellent food source for the larvae. Simply cutting down on watering has solved many a fungus-gnat issue. Water infested houseplants deeply, but not frequently, and only when the soil is dry. Be sure the pot itself has good drainage and the saucer underneath doesn't house standing water.
If cutting back on water doesn't clear up the problem, repot the plant with new, sterile potting soil, gently removing as much of the old soil as possible without decimating the roots. Adult fungus gnats can also be trapped on yellow or blue sticky cards placed an inch or two above the soil surface.