Development and Field Performance of a Sporeless Mutant of Bacillus thuringiensis Subsp. kurstaki

Commercial formulations of Bacillus thuringiensis subsp. kurstaki (Btk) consisting of insecticidal crystals and endospores are being extensively used by farmers in Karnataka for the control of insect pests on cabbage and cauliflower. The endospores of Btk persist in soil and are infectious to the mulberry silkworm, Bombyx mori. In order to lower the risk of accidental infection to silkworm we have developed sporeless mutants through chemical mutagenesis of a wild type Btk. An isolate of Btk (HR-Px91) was obtained from cadaver of the cabbage pest, Plutella xylostella. Exponential growth phase cells of Btk were exposed to the mutagen N-methyl-N'-nitro, N-nitrosoguanidine and crystalliferous but sporulation-deficient (Spo⁻ Cry⁺) mutants were isolated. Five oligosporogenous (OS) mutants were selected based on characteristics like autolysis, insecticidal crystal production and toxicity to larvae of P. xylostella, Crocidolomia binotalis and Helicoverpa armigera. Mutagenesis had not affected the quantity of toxins produced on solid medium by the five OS mutants when compared with the wildtype HR-Px91. Also there was no significant differences in the toxicity of crystals from two OS mutants and those produced by Btks of wild type and a commercial formulation ‘Bactospeine’. Field studies have unequivocally demonstrated the efficacy of OS mutant HR-Pu₇ in protecting cabbage crop against the major insect pests, P. xylostella and C. binotalis. The field efficacy was at-par with ‘Batospeine’ at an equivalent crystal-protein concentration, when evaluated based on marketable yield of cabbage obtained and pest damage. Thus, sporeless mutants of Btk could be generated by chemical mutagenesis and effectively used for plant protection in sericulture tracts with minimum risk to silkworm.