Biological control agents can be very effective in the management of fungus gnats. There are two commercially available: Steinernema nematodes and Hypoaspis mites.
Steinernema nematodes have a simple life cycle starting with juvenile nematodes seeking a host insect. The nematode enters the fungus gnat’s body through the mouth, anus or spiracles (holes through which an insect breathes). Once in the body, symbiotic bacteria are released from the nematode, which in turn kills the host insect. The dead insect and bacteria are food for the nematode until it reaches the adult stage where it reproduces and starts the process again in the soil. Completion of the life cycle occurs in a few days. This nematode is not dangerous to humans or pets.
Hypoaspis are small (0.8mm) tan mites that feed voraciously on gnat eggs and larvae. The life cycle of the mites is 13 days from egg to death. Reproduction will occur in the potting soil, keeping them around to feed for many generations. Reapplication can be made in 2-4 weeks, if needed, and complete control can be expected in 2-4 applications. Like Steinernema these mites, dwell in the soil, and are not harmful to humans or pets.