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: For biological control of larva of Cnaphalocrocis medinalis and Naranga aenescens do considerable damage to crops by folding and scraping the leaf tissue of rice, a large number of Bacillus thuringiensis isolates have been obtained from soil samples in Korea and the pesticidal activity was assayed against two insect pest species described above. Among 53 Bt isolates tested in bioassay, 18 and 13 isolates showed over 90% mortality against C. medinalis and N. aenescens, respectively. Some isolates (11 isolates including CAB141) presented dual activity against C. medinalis and N. aenescens. These isolates showed over 96% control effect in pest control in laboratory against larvae of C. medinalis. Also, it was investigated that pupation, pupal length, and adult emergence of larvae exposed to Bt suspension decreased. Especially, the pupal length of C. medinalis after being fed corn seedling leaves treated Bt suspension for 10 days, were much smaller than that of control. KEY WORDS : Bacillus thuringiensis, Cnaphalocrocis medinalis, Naranga aenescens,

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