There is a naturally occurring soil bacterium called Bacillus thuringiensis subspecies Israelensis (BTI) that kills the larvae of fungus gnats, mosquitoes and black flies . When BTI is eaten by these larvae toxins are released into their gut which causes the larvae to stop eating. Without food the larvae die. This interrupts the life cycle of these pests and the population then dies off.
BTI is not harmful to humans, birds, fish or mammals. Deemed Fit For Organic production by the USEPA in accordance with the USDA's National Organic Program.
Floating a dunk in your watering container will condition the water with Bacillus thuringiensis subspecies Israelensis (BTI) that can be used as a soil drench to get rid of fungus gnat larvae. Each dunk will condition water for several refills.