Most species of insect herbivores are restricted to low densities, but some display large-scale density fluctuations, including periodic outbreaks (Faeth 1987, Mason 1987, Hanski 1990, Hunter 1995). The tendency to reach high densities has been related to certain life history traits (Hunter 1991, 1995, Tammaru and Haukioja 1996). However, all populations of a given outbreaking species do not necessarily display high densities. In those cases, outbreaks are frequently more pronounced in populations in physically severe and marginal habitats (Wallner 1987, Myers and Rothman 1995). The autumnal moth, Epirrita autumnata (Borkhausen) (Lepidoptera: Geometridae) is an example of a species with both outbreaking and nonoutbreaking populations. In mountain birch [Betula pubescens ssp. czerepanovii (Orlova) Hämet-Ahti] forests of northern and mountainous Fennoscandia (hereafter northern populations), E. autumnata displays fluctuations with a statistically significant periodicity of 9-10 years (Tenow 1972, Haukioja et al. 1988, Bylund 1995). During outbreaks, forests may be totally defoliated and trees may even die over large areas (Tenow 1972, Lehtonen and Heikkinen 1995). In more southern parts of the species' Holarctic distribution (hereafter southern populations), outbreaks are absent and populations remain at low densities. Cycles of northern E. autumnata populations vary in their amplitude (Tenow 1972). Outbreak densities that produce conspicuous defoliation are typically reached in only some areas, and often in different areas during successive peaks (Tenow and Bylund 1989). Empirical data indicate a fairly regular pattern of fluctuations, that is synchronous on a regional scale, also in populations with moderate or low peak densities (Bylund 1997). Thus, there are two main questions regarding population regulation of northern and mountainous E. autumnata—what causes the cycles, and what causes spatial variations in outbreak severity? In southern populations, the main question is what prevents outbreaks? Larvae of E. autumnata hatch early in spring at the time of birch bud break. Birches (Betula spp.) are the main host plants, although larvae are able to feed on many deciduous trees and shrubs (Seppänen 1970).
Consequences of defoliation on phenological interaction between
Epirrita autumnata
and its host plant, Mountain Birch
