Old Mountain Birches at High Altitudes Are Prone to Outbreaks of Epirrita autumnata (Lepidoptera: Geometridae)

Abstract

We studied which site characteristics were associated with susceptibility to outbreaks of Epirrita autumnata Borkhausen in a mixed coniferous forest area in Finnish Lapland during a high density phase of the population cycle from 1992 to 1995. This information is important in understanding population dynamics of the species and as a background in studies of biotic interactions between E. autumnata and its host plants and natural enemies. We determined relative densities of larvae living on birch, Betula pubescens Ehrhart, at 45 sites covering an area of ≈3,900 km2. Association of larval density with 5 site characteristics was evaluated: altitude-related winter minimum temperature, birch age, birch density, overall tree density, and forest type. Of these variables, local winter minimum temperature and birch age together accounted for nearly half of the variance in larval density; the rest of the variables were not important. There is a clear mechanism for the effect of winter minimum temperature: E. autumnata eggs are killed at temperatures below -36°C. Because of inversion, cold air accumulates in topographically low sites, killing eggs there. We have, however, no simple explanation for why sites with old birches had high larval densities.