Effects of Insect Herbivores With Different Feeding Modes on the Exotic Plant Solidago altissima: Changes in Induced Responses and Resource Allocation

Herbivory, particularly by insects, significantly affects plant growth, reproduction, and survival. To cope with insect herbivory, many plants exhibit induced responses, including changes in elemental composition and the production of secondary metabolites. These responses can also occur in exotic plants that have established new interactions in the introduced regions and may vary depending on the herbivore's feeding mode. This study examines how the exotic plant Solidago altissima in Japan responds to herbivory by sap-feeding aphids (Uroleucon nigrotuberculatum) and leaf-chewing lace bugs (Corythucha marmorata). Using greenhouse experiments, we analyzed plant growth (dry mass) and quality (water, nitrogen, carbon, and phenolics) under controlled conditions. Results showed that lace bug herbivory reduced leaf dry mass by 22% and stem water content by 13%. Additionally, nitrogen content increased by 26%–81% in non-leaf organs, particularly in stems and roots. In contrast, aphid herbivory had no significant effects on plant growth or chemical composition. Regarding resource allocation, lace bug herbivory reduced nitrogen allocation to leaves by 6% while increasing allocation to stems and roots. To explain these differences, we considered three factors: (1) insect feeding modes, (2) intensity and timing of feeding damage, and (3) co-occurrence history between plants and insects. Lace bugs, which consume leaf mesophyll, likely trigger stronger plant defense responses than aphids, which feed on phloem sap and cause relatively mild stress. These findings provide insights into how exotic plants exhibit different induced and resource-allocation responses to different herbivores, particularly highlighting the strong responses of S. altissima to leaf-chewing insects like lace bugs.