Gnats are commonly seen on indoor plants, according to Colorado State University. There's almost nothing more irritating to a home gardener than being welcomed by a rising mass of these tiny insect pests every time you go to water your plant. These small black bugs are most likely fungus gnats (Bradysia species), which thrive in the damp soil of overwatered indoor plants.
Adults gnats are small, flying insects about 1/8 of an inch long. They have long legs and look somewhat like a mosquito, according to Colorado State University. The larvae, which hatch and live in the soil, are shaped like tiny worms, with opaque bodies and black heads.
Adult gnats live for only about 10 days and rarely stray from the plant. During that time, the females lay up to 200 tiny eggs in the growing medium. The eggs hatch a week later. In two or three weeks, they pupate. A week after that, they are adults. In this way, the cycle of gnats perpetuates itself unceasingly if the home gardener does nothing to stop it.
Overwatering is the primary cause of a fungal gnat problem. The adult flies are attracted to the wet planting medium. The larvae feed on the organic content found in the rich, moist soils. Such overly wet conditions encourage the growth of soil fungi, which the gnats feed on as well. In many cases, the indoor plants are left outdoors during the summer, where gnats discover them. Then, when the plants are brought indoors, the gnats come with them.
These insects are considered minor indoor plant pests. They even have some beneficial value; they help decompose decaying plant materials in the soil, according to Oregon State University. Still, they can damage the roots of many of the less hardy indoor plants such as African violets. Affected plants may begin to wilt, and the root damage can lead to other, more serious problems, such as root rot.
The best way to prevent gnats is to make sure you don't overwater your indoor plant. Let the top 2 inches of the soil in your plant dry out before watering it again. This is especially important in the fall and winter, when most plants go dormant and need less water anyway. It is during these periods that most home gardeners overwater their plants, according to Colorado State University. Adults can be trapped with yellow sticky paper, or sprayed with insecticide.