Classification of Bacillus sphaericus Strains and Comparative Toxicity to Mosquito Larvae

Unlike Bacillus thuringiensis, Bacillus sphaericus was first recognized as a saprophytic microorganism, the spores of which currently contaminate soil and aquatic environments. Since the first reports of strains moderately toxic to mosquito larvae (strain K of Kellen and Meyers 1964; strain SSII-1 [= 1321] of Singer 1973), researchers have isolated a series of very potent B. sphaericus strains, beginning in 1977 with strain 1593 (Singer 1977). Strain 1593 was followed by strain 2297 ( = MR-4) (Wickremesinghe and Mendis 1980), strain 2362 (= 290-8) (Weiser 1984), and strain Iab 59 (de Barjac et al. 1988). In addition to the numerous atoxic strains, in 1989 more than 150 mosquito pathogenic isolates with various toxicity levels have become available. This has created problems in their identification and differentiation—first from the atoxic B. sphaericus, and second from each other within the toxic group.