Fungus gnats (Bradysia sp.) are small insects that resemble fruit flies and are attracted to moist soil. Although adult fungus gnats won't harm your houseplants, the larvae feed on root hairs and can stunt plant growth. The insects develop in potting soil, according to entomologists with Colorado State University, and any plant can become a host.
Limit soil moisture by allowing the surface of the soil to dry out prior to watering.
Avoid fertilizing with manure, bloodmeal or other organic fertilizers, as these serve to attract fungus gnats.
Control the fungus gnat larvae by drenching the soil with the biological insecticide Bacillus thuringiensis, the H-14 strain. Specialists at Colorado State University mention that commercially available pesticides, such as Gnatrol and Knock-Out Gnats, contain this substance.