Management of nucleopolyhedrovirus (NPV) in silkworm

The conventional NPV management strategy of chemical disinfection has limited success. Persistent NPV outbreaks continue to cause heavy cocoon losses which brought the need to pursue other potential options for virus control. NPV persistence was investigated by assessing the impact of sublethal infection of NPV in 5th instar larvae on yield performance of both the inoculated larvae and their progeny. Analysis of variance and separation of means by DMRT revealed that low levels of infection did not affect directly the inoculated individuals. Instead, it caused a dose-correlated mortality and lower yield in their offspring. This result implies a carry over effect or vertical transmission of the virus. The effectiveness of indirect ELISA in detecting NPV was verified when the assay was utilized in virus indexing of silkworm eggs. It also confirmed the occurrence of latent infection and indicated transovarian transmission of NPV. Resistance to NPV of seven silkworm inbred lines and ten hybrids was assessed in a virus challenge test by oral inoculation with 1x10 to the 6th PIBs/ml from November, 2002 to December, 2003. Results revealed that resistance was significantly affected by silkworm variety and rearing season. Hence, the best performing hybrids with resistance in specific rearing season may be recommended and utilization of hybrids in seasons when they are most susceptible can be avoided. The careful selection of individual silkworm immune to disease after artificial induction of infection in bivoltine purelines resulted to a remarkable reduction of mortality in the second generation indicating a development of resistance in the population. An integrated approach of early disease detection, inoculum control by routine disinfection techniques and destruction of infected insects, the use of resistant breeds including practices to enhance silkworm health, may prove effective in the control and prevention of NPV.