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 extensi vely used by farmers in Karnataka for the control of insect pests on c abbage and cauliflower. The endospores of Btk persist in soil and are infectious to the mulberry silkworm, Bombyx mori. In order to lower th e risk of accidental infection to silkworm we have developed sporeless mutants through chemical mutagenesis of a wild type Btk. An isolate o f Btk (HR-Px91) was obtained from cadaver of the cabbage pest, Plutell a xylostella. Exponential growth phase cells of Btk were exposed to th e mutagen N-methyl-N'-nitro, N-nitrosoguanidine and crystalliferous bu t sporulation-deficient (Spo(-) Cry(+)) mutants were isolated. Five ol igosporogenous (OS) mutants were selected based on characteristics lik e autolysis, insecticidal crystal production and toxicity to larvae of P. xylostella, Crocidolomia binotalis and Helicoverpa armigera. Mutag enesis had not affected the quantity of toxins produced on solid mediu m 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 commerci al formulation 'Bactospeine'. Field studies have unequivocally demonst rated the efficacy of OS mutant HR-Pu7 in protecting cabbage crop agai nst 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 ob tained and pest damage. Thus, sporeless mutants of Btk could be genera ted by chemical mutagenesis and effectively used for plant protection in sericulture tracts with minimum risk to silkworm.