Transmission Dynamics of Bombyx mori Nuclear Polyhedrosis Virus and Its Impact on the Economic Traits of Silkworm, Bombyx mori Linn

Disease-free layings of Bombyx mori L. silkworm breed DUN22 were reared up to 3rd instar using standard methods. Following the 3rd moult an experiment was conducted with four treatments (T1–T4) and five replications of 100 larvae per treatment. In T1, 1ml of 1x106 POB/ml of Bombyx mori nuclear polyhedrosis virus (BmNPV) was inoculated through mulberry leaves immediately after the 3rd moult whereas in T2, the same inoculum was sprayed on 100 larvae and rearing seat paper; in T3 BmNPV infected silkworms (carriers) were introduced into a healthy population of silkworms at the ratio of 8:92, and T4 served as the untreated control. The data regarding various economic traits viz., larval weight, 5th instar larval duration, total larval duration, cocoon yield by number, cocoon yield by weight, single shell weight, single cocoon weight, shell ratio and pupation, raw silk percentage, silk filament length, renditta, filament size, and disease incidence were recorded and analyzed statistically. Results revealed that significant differences were observed in larval duration, larval weight, cocoon yield, pupation, disease incidence, and shell ratio across treatments. However, silk filament length, renditta, and filament size showed no significant differences but were marginally lower in treated groups compared to the control. Results indicated that BmNPV spreads through ingestion (T1), surface contamination (T2), and contact with infected larvae (T3), with varying degrees of infection. Infection rates were highest in T1, followed by T3 and T2, likely due to the ingestion of higher doses of polyhedral bodies, leading to rapid dissolution and tissue invasion. The study concludes that all three modes effectively transmit BmNPV, with ingestion being the most severe.