Bacillus sphaericus is a commonly occurring spore-forming bacterium found worldwide in soil and aquatic environments. Certain strains of this bacterium have been found effective against some species of mosquitoes. Like the larvicide Bti (Bacillus thuringiensis var. israelensis serotype H-14) this bacterium forms protein crystals that can destroy the mosquito larva’s gut lining causing death. Unlike Bti, Bacillus sphaericus only affects larval mosquitoes and is virtually non-toxic against black flies. Most Culex spp. and Anopheles spp. mosquitoes may be effectively controlled while many species of Aedes, especially Salt Marsh mosquitoes, are not susceptible.

Bacillus sphaericus is also different from Bti in that it can control mosquito larvae in highly organic aquatic environments, especially wastewater ponds, animal waste ponds and septic ditches. Also in contrast to Bti, Bacillus sphaericus has shown environmental persistence for two to four weeks, with the ability to grow and reproduce, though persistence is low in saline or highly organic environments.