Population Cycles in Forest Lepidoptera Revisited

A quarter century ago, the question was posed of whether a general hypothesis could explain population cycles of forest Lepidoptera. Since then, considerable progress has been made in elucidating mechanisms associated with cyclic dynamics of forest Lepidoptera. Delayed density-related parasitism and reduced fecundity during population peaks are common influences on population dynamics, although why fecundity declines is not understood. The hypothesis that sunspots explain cycles is rejected. The influences of delayed-induced plant defenses on populations are inconsistent, but interactions between plant chemistry, pathogens, and immunity remain rich areas for future study. Population dynamics of forest Lepidoptera can be synchronous over large geographic scales, and repeatable waves of spread of outbreaks occur for some species. Climate warming could modify species distributions and population cycles, but mechanisms have not been elucidated and changes in cyclic dynamics are not generally apparent. Integration of top-down and bottom-up influences on cyclic dynamics and quantification of dispersal are necessary for progress in understanding patterns of insect outbreaks.