Nucleopolyhedroviruses of forest and western tent caterpillars: cross-infectivity and evidence for activation of latent virus in high-density field populations

1. Cyclic population dynamics of forest caterpillars are often associated with epizootics of nucleopolyhedrovirus, but it is not known how these viruses persist between generations or through the fluctuations in host population density. 2. To explore the question of virus persistence at different phases of the population cycle, the nucleopolyhedroviruses of two species of tent caterpillar that co-occur in British Columbia, Canada, Malacosoma californicum pluviale (western tent caterpillar) and Malacosoma disstria (forest tent caterpillar), were characterised. The cross-infectivity of the viruses in these two host species was investigated to determine whether there might be a route for virus persistence via the alternative host species. Any virus produced in the cross-infections was characterised to confirm true cross-infection or to ascertain whether cross-inoculation triggered latent virus persisting within the population. 3. The virus associated with forest tent caterpillars (MadiNPV) did not infect western tent caterpillars from low-density populations, nor did it trigger a latent virus infection; however, inoculation of forest tent caterpillars from high-density populations with virus from western tent caterpillars (McplNPV) resulted in viral infection, but without a dose-response relationship. 4. Analysis of DNA profiles of virus resulting from cross-infection of the forest tent caterpillar with McplNPV, revealed that 88% of these infections were caused by MadiNPV rather than McplNPV; however the virus from all 44 infected individuals was identical and differed in DNA profile from the stock MadiNPV used for cross-infection. This suggests strongly that forest tent caterpillars from high-density field populations harbour a latent, persistent, or sublethal form of MadiNPV that was triggered by exposure to nucleopolyhedrovirus from the western tent caterpillar. 5. Virus was not activated in western tent caterpillars collected over 2 years of late population decline and the first year of population increase.