Growers interested in using biological control are encouraged to begin by using beneficial nematodes to manage fungus gnats. Beneficial nematodes are relatively easy to use but must be applied properly in order to be effective. What are beneficial nematodes? Nematodes are tiny (about 0.5 mm), colorless, cylindrical round worms that occur naturally in soils throughout the world. Different species work best against different target pests. Steinernema feltiae is primarily used against fungus gnat larvae and western flower thrips pupae in the soil.