Stand age-structure influence in a low population peak of Epirrita autumnata in a mountain birch forest

Population densities of Epirrita autumnata caterpillars were recorded from 1984 to 1990 in four mountain birch stands of two age categories, young and old. Caterpillar populations peaked in 1986-87, and peak densities were higher in the old than in the young stands. Recorded peak densities were low compared with synchronous populations with outbreak densities in neighbouring areas. Differences in caterpillar densities between young and old stands could not be explained by differences in survival of early instar caterpillars or in parasitoid-caused larval mortality. Instead, tree-age related differences in amounts of suitable oviposition sites and possible changes in food quality due to ageing and lower vigour of mature trees are suggested to explain why E. autumnata populations tend to be higher in old stands.