The impact of large-scale nitrogen (N) fertilization on alder species for herbivorous insects has never been studied. Alder species have N₂-fixing traits and a high potential to accumulate N in plant tissue. To evaluate the relationship between alder canopy trees traits (plant growth, leaf nutrients (N and C/N ratio) and plant defenses (condensed tannin and LMA)) and their herbivore responses (chewer, galler, and miner) to N addition, we conducted the large-scale N fertilization experiment (ca. 9 ha, 100 kg N ha^-1 year^-1) for 3 years including prior to the N fertilization in a cool-temperate forest in northern Japan. The N fertilization decreased LMA, although the others (N and condensed tannin) remained unchanged, and increased leaf damage by chewer of herbivorous insect with feeding guilds. As a result, the chewer was significantly increased with decreasing LMA. Our results suggest that large-scale N fertilization has positive indirect (plant-mediated) effect on chewer through the decrease in LMA of alder species. Thus, it implies that N₂-fixing plants in the same with other plants (e.g., non-fixing plants such as oak species) under elevated N deposition, although the mechanisms are considerably different, will lead to greater feeding activities of herbivorous insects.