Population cycles: generalities, exceptions and remaining mysteries

Population cycles are one of nature's great mysteries. For almost a hundred years, innumerable studies have probed the causes of cyclic dynamics in snowshoe hares, voles and lemmings, forest Lepidoptera and grouse. Even though cyclic species have very different life histories, similarities in mechanisms related to their dynamics are apparent. In addition to high reproductive rates and density-related mortality from predators, pathogens or parasitoids, other characteristics include transgenerational reduced reproduction and dispersal with increasing-peak densities, and genetic similarity among populations. Experiments to stop cyclic dynamics and comparisons of cyclic and noncyclic populations provide some understanding but both reproduction and mortality must be considered. What determines variation in amplitude and periodicity of population outbreaks remains a mystery.

Interactions between pupae of the pine processionary moth (Thaumetopoea pityocampa) and parasitoids in a Pinus forest

Parasitoids are significant enemies of many economically important insects and there is some evidence to suggest that their actions have a role in terminating the outbreaks of forest Lepidoptera populations. In this study, we examined the impact of parasitoids on the pupae of the pine processionary moth, and highlighted the presence of several parasitoid species for this developmental stage. A higher rate of parasitism was found when the pupal density in the soil was reduced, but the rate of parasitism was not influenced by pupal morphological traits or by the presence or absence of a cocoon around a pupa. Of the external factors examined, a delay in the time of descent of larvae from the trees had a positive effect on the level of parasitism. Observational data indicated that dipteran and hymenopteran were the most abundant parasitoids to emerge from moth pupae. Our study highlights the complexity of the parasitoid–host dynamics, and stresses the importance of carefully determining environmental effects on host–parasitoid relations.