Temperate plants show a rapid seasonal turnover in various leaf traits and defenses. Such trends in plant defenses can potentially drive seasonal shifts in the specialization of insect herbivores. We quantified how non-volatile leaf metabolites, inducible volatile organic compounds (VOCs), C:N ratio and leaf toughness changed between the early, middle, and late seasons in five Salicaceae species and one Salix hybrid. We also explored seasonal trends in overall trait variation among the studied plants. We tested whether seasonal changes in dietary specialization of leaf-chewing larvae and adult beetles related to changes in the studied host-plant traits. Trait turnover occurred mainly through changes in VOCs and seasonal increase in traits that directly lower herbivore feeding efficiency. The overall variation in leaf traits was highest in the early season, with seasonal intraspecific variation being 33% smaller than the variation among species sampled at one time point. Although less frequently than we expected, the two groups of insect herbivores showed seasonal changes in specialization. The significant trends in herbivore specialization included peaks in the middle season for larval specialization based on VOCs and host phylogenetic relatedness and for adult beetle specialization based on C:N ratio plus leaf toughness. The detected species-specific trends in host-plant traits, their intraspecific variability, and differential trends among insect herbivores highlight the importance of considering seasonal variation when predicting trends in plant-herbivore interactions.

Bioanalytical Laboratory Institute of Biomedicine University of Turku 20500 Turku Finland

Biology Centre of the Czech Academy of Sciences Branisovska 31 37005 Ceske Budejovice Czech Republic

Department of Forest Nature Conservation Faculty of Forest Sciences and Forest Ecology Georg August University of Göttingen Göttingen Germany

Faculty of Science Department of Zoology University of South Bohemia Ceske Budejovice Czech Republic

Natural Chemistry Research Group Department of Chemistry University of Turku 20500 Turku Finland

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