1. In two subsequent years and under standardized experimental conditions, we bioassayed the leaf quality of the same 20 subarctic mountain birches Betula pubescens ssp. tortuosa with 14 sawfly and one lepidopteran species to study the effects of annual and seasonal leaf quality variation on herbivore growth, and potential species interactions. The larval periods of the 15 species covered the whole growth season of birch.
2. Insect growth rates were higher in 1987 than in 1988, except for two late-season species. In spite of the large annual variation in foliage quality, the relative ranking of individual trees for growth of the herbivore species remained constant between the 2 years, Differences in insect growth between the years seemed to be explained by plant vigour.
3. Birch leaf quality decreases rapidly within a season. The early species had two to three times higher relative growth rates (RGR) than species feeding in mid- and late season. Growth rates of the early season species also varied more than those of late season, both among insect species and among individual trees.
4. We compared tree-specific herbivore growth between pairs of species to investigate similarity of the trees for herbivores. All significant correlations within a year between the RGR of the herbivore species were positive, but significant correlations were found between early and mid-season species only, i.e. at the time when foliage quality was temporally best and most variable among trees. When split into years, these positive correlations were more common (44% of cases) in 1988, when foliage quality was poor, than in 1987 (1%).
5. Growth of several insect species differed significantly among the 20 trees in 1987 and in the early season of 1988. This happened when larvae were consuming high-quality foliage, and indicated that the herbivore species were able to adjust their growth performance to take advantage of high-quality diets. But they did this in species-specific ways, as indicated by the fewer significant correlations in growth between herbivore species on high-quality than on low-quality diets. Accordingly, growth of different herbivore species converged under conditions of poor leaf quality, and herbivore species showed some specialization when leaf quality was high.