Without a doubt, fungus gnats are very destructive greenhouse pests. I have seen the carnage myself across countless species. Much time and effort is spent on their control. Not all Bt is the same. Bt products like Gnatrol that specifically target fungas gnats and other closely related insect species have been available since at least the 1980's. I believe that other strains of Bt have since been isolated that target other insect groups. Also, "both larvae and adults can spread plant pathogens and may promote disease in commercial crops." From: http://ipm.ucdavis.edu/PMG/PESTNOTES/… Nathan