RELATIVE TOLERANCE OF CERTAIN POLYVOLTINE SILKWORM BREEDS OF BOMBYX MORi L. TO NUCLEAR POLYHEDROSIS VIRUS AND DEVELOPMENT OF BmNPV TOLERANT FOUNDATION CROSSES

Thirtyfour polyvoltine silkworm were screened to find out their relative susceptibility/tolerance level against BmNPV. Out of these 34 breeds, 9 breeds v.i.z, 96C, Mysore Princes, Moria, TN White, BL36, BL37, Nistari, GNP and MDR showing less than 30% mortality due to grasserie were short listed as comparatively tolerant breeds. Breeds showing more than 51% mortality due to grasserie were categorized as susceptible breeds and among them BL 45 was showing the highest mortality (98.66%). Among the short listed nine breeds, Nistari had shown highest LC 50 value of 10 8.266 and TN White the least LCso value (10 6.924 ). Based on the relative tolerance towards BmNPV and also their economic characters, four breeds v.i..z. 96 C, BL 37, GNP and MDR were used for making eight foundation crosses with six other productive polyvoltine breeds and these foundation crosses were screened against BmNPV to find out their relative tolerance. The lowest incidence of grasserie (4 %) was recorded in BL 67 x BL 37 followed by BL 65 x MDR (14%), BL 45 x BL 37 (16%) and MDR x BL 67 (17 %). The highest mortality (31.66%) was observed in 96 C x MDR followed by BL 69 x MDR (29 %) and BL 68 x GNP (27 %). Distinct superiority of these crosses over their parental breeds in terms of their relative tolerance to BmNPV was confirmed indicating genetic basis of tolerance against BmNPV in silkworm.