Phenology and abundance in relation to climatic variation in a sub-arctic insect herbivore-mountain birch system.

The two forest-defoliating geometrid moth species Operophtera brumata and Epirrita autumnata are known to exhibit different altitudinal distribution patterns in northern birch forests. One possible explanation for this is that altitudinal climatic variation differentially affects the performance of two species through mismatching larval and host plant phenology. We explored this hypothesis by investigating the relationship between larval phenology and leaf phenology of Betula pubescens, which is the main host plant of both moth species, along ten replicate altitudinal transects during two springs with contrasting climate in northern Norway. There was a distinct monotonous cline in host plant phenology with increasing altitude in both years of the study, but the development of the leaves were generally 14 days later in the first of the 2 years due to cold spring weather. We found that larval development of both species closely tracked host plant leaf phenology independent of altitude and year. However, at the time of sampling, E. autumnata was approximately one instar ahead of O. brumata at all altitudes, probably reflecting that E. autumnata has faster early instar growth than O. brumata. The abundance of O. brumata was lowest at the altitudinal forest-line, while E. autumnata was lowest near sea level. Our results do not indicate that the altitudinal distribution patterns of the two moth species is due to any phenological mismatch between larval and host plant phenology. We suggest rather that natural enemies at low altitudes limit larval survival and thus abundance of E. autumnata, while an early onset of winter at the forest limit reduces survival of late eclosing adults of O. brumata.