Evaluation of six multivoltine silkworm breeds of Bombyx mori (Lepidoptera: Bombycidae) for their resistance to Bombyx mori nucleopolyhedrovirus (BmNPV)

The present study investigates the variation in disease susceptibility in six multivoltine silkworm breeds of Bombyx mori—4 Nistari strains and 2 developed breeds. The effect of BmNPV and disease resistance was observed both in infected and in stages without BmNPV load (control). The BmNPV polyhedra were isolated from the hemolymph of the grasserie-infected larvae. The eggs were incubated at room temperature and underwent block boxing for uniform hatching. Pathogen suspensions were transferred to the treated worms with a pipette (1×106 OBs/ml) onto the surface of the leaves and gently smeared and allowed to air dry. Maximum mortality (66%) due to grasserie, flacherie, and other diseases was observed in the breed Nistari–Plain. The BmNPV-infected breeds showed higher mortality (49%) and less pupation (98.07%) than the breeds of control (mortality 7% and pupation 99.4%). Results of ranking analysis based on Mano’s Evaluation Index (EI) indicate the highest (51%) degree of tolerance (DT) in the Nistari–Marked breed and the lowest (28%) in the Nistari–Plain breed. As per the multiple trait evaluation index (EI) analysis, Nistari–Debra is placed at 1st rank (Cumulative EI - 59.95) and Nistari–Plain at last rank (Cumulative EI - 43.13). The present study can help to develop BmNPV-resistant varieties for commercial use in the future.