Explain the Pathogens of forest insects?

Caterpillars that defoliate forest trees have main economic impacts. Many such species go through periodic outbreaks. The forces which drive the outbreaks are still hotly debated; the large spatial and temporal scales on which the outbreaks take place have severely constrained our capability to do conclusive experiments. Though, careful small-scale experimentation combined with process-based statistical models have led to recent, fast increases in our understanding of the roles of viral and fungal pathogens in controlling caterpillar populations. Understanding how variability in space, in time, and between caterpillars scales up to affect large-scale population dynamics will assist us decide when and where to target control measures like introduction of lab-raised pathogens.