Fungus gnats are mainly attached to decomposing plant tissues of Haworthias and organic components in the growing medium. Adult fungus gnats don’t harm plants, but larvae can damage roots and stunt plant growth, particularly for seedlings.
For a heavily infested plant, clean as much of the growing medium out of the roots and completely discard it. Some roots may be torn off but the plant will be fine. Remove any rotten plant tissues, and soak the plant in 1% hydrogen peroxide solution for 15 minutes and rinse in water. This kills any fungus gnat larvae, eggs and fungus on which their larvae primarily feed on. If a major wound is left on the plant, treat it accordingly. Otherwise, let the plant dry out in a shaded and cool place before planting it in a fresh growing medium.
For a less infested plant, add 1% hydrogen peroxide to the solution in the next several waterings. For remaining adult fungus gnats, place yellow sticky traps around the plants to capture them.
Using an inert growing medium which is resistant to decomposition is an excellent preventative measure.