Different impact of pupal predation on populations of Epirrita autumnata (Lepidoptera; Geometridae) within and outside the outbreak range

Abstract

1. We studied the effect of pupal predation on Epirrita autumnata populations within and outside the outbreak range, in northern and southern Finland, respectively. The purpose of the study was to evaluate the potential of pupal predation to contribute to the contrasting dynamics of northern and southern populations. In addition to overall pupal survival, we separately estimated the influence of small mammal and invertebrate predation. 2. Each summer for a period of 5 years, pupae inside their cocoons were buried in the ground. The pupae were re‐collected before the flight season and checked for damage. Direct and delayed density dependence of pupal survival was tested using the current and the previous year larval density indices, respectively. 3. Annual survival of pupae from both small mammal and invertebrate predation was lower in the south. Daily survival rates did not differ between the south and the north indicating that the difference in annual pupal survival is most likely due to the longer pupal period in the south. 4. In the south, pupal survival was negatively density dependent and resulted from small mammal predation. The results indicate that small mammals may prevent E. autumnata populations from reaching high densities in the south. 5. In the northern study area with a current outbreak, total pupal survival was density independent. Also small mammal predation was density independent, most likely due to predator satiation. Moreover, there was no difference in pupal survival between northern areas with and without a current outbreak. The results indicate that pupal predation was not a significant factor in termination of the outbreak, nor is it likely to contribute to the maintenance of the cyclic density fluctuations. 6. We conclude that low pupal survival and its density dependence are likely to contribute to stability of E. autumnata populations in southern Fennoscandia. On the contrary, shorter pupal period and the lack of density dependent pupal predation may make northern populations more prone to outbreaks. The potential regulating effect of pupal predation in southern populations is due to small mammals being able to respond to changes in E. autumnata densities.