While adults don’t cause any lasting damage, the fungus gnat larvae of that family will move on to plant roots once their preferred foodstuff is gone. The larvae will chew holes in the roots and can cause yellowing, wilting, and even plant death in large numbers. Not only do they damage roots, but the Sciaridae can spread disease.

Fungus gnats are also capable of spreading the plant pathogen that causes damping off and the eventual death of seedlings. Fungus gnat damage will appear similar to that of any other root-related issue, such as root rot. Lower leaves may turn yellow and drop, and the plant’s growth may slow down or stop completely.