Effects of increased content of leaf surface flavonoids on the performance of mountain birch feeding sawflies vary for early and late season species

Summary. The effects of artificially added flavonoid aglycones to birch leaf surfaces on the larval performance of six species of leaf-chewing sawflies were investigated. Significantly negative effects of increased contents of both total flavonoid and individually fed flavonoid compounds were found for the larval performance of certain mid to late and late, but not early season, sawfly species. Species-specific variations in the quantity of faecal flavonoid glycosides, which were examined to investigate whether effective glycosylation of foliar flavonoid aglycones in larvae correlated with varying tolerance to these compounds, also yielded significant species-specific differences between early and late season species. The results suggest seasonal adaptations in host plant use by sawflies feeding on mountain birch, such that phenologically earlier species are better adapted to coping with leaf surface flavonoid aglycones, which occur in the highest concentrations in young leaves.