Gnatrol Biological Larvicide. Fungus gnat larvae don’t stand a chance. Gnatrol is a highly selective, biological larvicide for use in greenhouses to control fungus gnat larvae. It may be used on a wide variety of ornamentals, as well as all bulb crops, bedding plants and vegetable sets. Once the larvae have ingested Gnatrol, they become paralyzed, stop feeding immediately and die—usually within 24 hours. Gnatrol is now available in a water dispersible granule (WDG) formulation that will not leave a residue and is easier to measure for increased application accuracy. Gnatrol WDG is listed by the Organic Materials Review Institute (OMRI) for use in organic production