Abstract: 【Aim】 Bombyx mori nucleopoyhedrovirus (BmNPV) causes nuclear polyhedrosis which is one of the most important prevalent diseases and results in significant economic losses in sericultural areas in Yunnan, southwestern China. This study aims to determinate the virulence of BmNPV isolates from Yunnan and to investigate the epidemic trend of BmNPV diseases in Yunnan, so as to provide a basis for surveillance and control of the B. mori nuclear polyhedrosis. 【Methods】 The infectivity of 19 BmNPV isolates from Yunnan was estimated by the mortality of the 5th instar larvae of B. mori and the virulence to BmN cells caused by BmNPV. The bro-d genes of 19 BmNPV isolates were cloned, sequenced and subjected to phylogenetics analysis. 【Results】 The per os infectivity varied in different BmNPV isolates from Yunnan by comparing the mortality of the 5th instar larvae of B. mori caused by BmNPV isolates. The virulence test results showed that the titers of budded virus (BV) of the 19 BmNPV isolates were significantly different. Phylogenetic analysis showed that the 19 BmNPV isolates from Yunnan were separated into two main clades, i.e., clade I and II. Clade I isolates were distributed in eastern, central and western parts of Yunnan province, which are sub-tropical regions with high temperature, while clade II isolates were mainly distributed in northern part of Yunnan province, which has plateau moosoon climate with relatively low temperature. 【Conclusion】 There are abundant BmNPV strains in sericultural areas of Yunnan. The virulence of BmNPV isolates from Yunnan to B. mori varies widely, and the evolutionary relationship of these isolates is related with geography and climate in a certain degree.

Key words: Bombyx mori, sericultural area, BmNPV, bioassay, bro-d; Yunnan