Eradicates Fungus Gnat (black fly) High Potency Larvicide for Blackfly control. Fungus Gnats or Black Fly lay their larvae in the root zone of a plant, these larvae then burrow into the root zone to feed carrying with them infection. Fungus Gnat Off works by killing the Larvae in the root zone thus eradicating the problem. 100% Natural organic concentrate, fully compliant to agricultural legislation.