Abstract
A total of 189 isolates of Bacillus thuringiensis producing parasporal inclusions (PIs), obtained from soils of Japan, were examined for their oral toxicity against larvae of the silkworm, Bombyx mori, and the mosquito, Aedes aegypti, and adults of a chrysomelid coleopteran, Pyrrhalta tibialis. Of these isolates, 68 (36.0%) were toxic to insects tested: 48 were Lepidoptera-toxic and 20 were mosquito-toxic. The Lepidoptera-specific isolates were referable to B. thuringiensis serotypes 3a (2 isolates), 5a:5c (1 solate), and 10 (44 isolates), while one isolate was untypable by the known B. thuringiensis H antisera. Unlike PIs of the known Lepidoptera-toxic B. thuringiensis strains, PIs produced by these untypable and serotype 10 isolates were spherical in shape. Of 20 mosquitotoxic isolates, 7 were referable to B. thuringiensis serotype 11a:11c, and 13 were untypable. Although 121 isolates (64%) produced PIs with various morphologies including typically bipyramidal ones, they did not demonstrate any toxicity against insects of three orders, suggesting that nontoxic PI-forming bacteria predominate in natural environments rather than toxic ones.
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