Diversity of birch sawfly responses to seasonally atypical diets

Most insect herbivores are specialised on a particular plant taxon. To have a better understanding of host shift functions and consequences for insect herbivores. it is essential to gather more information on the effects of v ariation in host quality on specialists across species and environments. We examined the effects of seasonally atypical food on mortality, development al time. and final body mass of six sawfly species (Hymenoptera: Symphyta) feeding on the foliage of mountain birch (Betula pubescens ssp, czerepanovi i). whose pooled larval feeding periods form a gradient and cover the growi ng season. Insect phenology was manipulated so that the larvae of early-sea son species would feed on atypically mature leaves and mid- or late-season species would feed on atypically young leaves of their major host plant. Mo rtality increased dramatically for all species when the larval feeding sche dule was advanced or delayed. This indicates a high degree of specialisatio n not only on a particular host but also to its phenological phases. The ma in cause of mortality on novel food was a rejection of the diet by the youn g larvae and their subsequent starvation. An interesting observation was th at late-season species showed this response on nutritious young foliage. Th e effects of seasonally atypical diets on larval development and growth wer e species-specific and milder than the effects on mortality. Interestingly, for those individuals that accepted it. atypical food seemed to be most be neficial for species appearing at both ends of the seasonal gradient. which might be related to a wider exposure to variable food quality in natural c onditions compared with other species. The diversity of responses to atypic al food among closely related herbivore species with overlapping feeding pe riods on the same host plant is the most crucial finding of this study.