Hybrid vigour of six bivoltine silkworm breeds was assessed based on eight quantitative traits. The heterosis was estimated using multiple evaluation indices (EI), mid parent heterosis (MPH), better parent heterosis (BPH), and sub-ordinate function indices (SF). The results showed highly significant (p<0.01) difference in rearing performance of inbred and hybrid breeds. Hybrid JL04xCL04 showed superior performance in the fecundity (596.50±4.03 eggs per female), larval body weight (3.55±0.07 g), pupation rate (94.80±1.23%), cocoon yield (14.85±0.22 kg per 10,000 larvae) and cocoon weight (1.42 ±0.03 g), while hybrid C02x05M was superior in the shell weight (0.37±0.01 g) and shell ratio (26.47±0.04%). The EI values ranged between 40.96 and 61.92. All hybrids except 05POx06M resulted in greater than 50 EI values. Four hybrids showed positive MPH for the cocoon yield, cocoon weight and shell ratio, while three hybrids showed positive BPH for the cocoon yield, cocoon weight and shell ratio. The cumulative SF values ranged between 2.91 and 5.55. Based on mean EI and cumulative SF values, hybrids JL04xCL04 and 05POx06M were ranked first and sixth, respectively. The study identified hybrids JL04xCL04 and C02x05M as potential breeds for commercial rearing.

Key words: Mulberry silkworm, hybrid vigour, heterosis, multiple evaluation index, sub-ordinate function