Fungus Gnat Off is a high potency larvicide for black fly control. Fungus Gnat Off is a safe, non toxic means to elliminate fungus gnat infestations. Fungus gnats or black fly can cause damage both directly and indirectly. Direct damage results when the larvae produced nestle within the root system where they devour the roots of the plant which would prevent healthy growth. Indirect damage occurs by the larvae spreading mites, nematodes,viruses and various fungi. Young plants are particularly susceptible.