Herbivore pressure on goldenrod (Solidago altissima L., Asteraceae): its effects on herbivore resistance and vegetative reproduction

Abstract

Summary1. Ecological and physiological costs of resistance to herbivory are core concepts in the study of plant–insect interactions, but identifying them remains challenging. These costs are most obvious when trade‐offs in resource allocation occur between different growth and resistance traits.2. We used plant genotypes collected from long‐term herbivore exclusion plots and from plots with natural herbivory. We evaluated putative trade‐offs between resistance to two different herbivore species (the larvae of the beetle Trirhabda virgata and the moth Spodoptera exigua), and between resistance and growth as a potential outcome of differential natural selection.3. We hypothesized that long‐term exclusion of herbivores would result in differential natural selection on plant resistance traits that are expressed by marked differences in mean resistance between plants from populations with and without herbivores. The results supported this hypothesis. Genotypes from herbivore exclusion plots were less resistant to the more common herbivore in the system, T. virgata, than genotypes from control plots. In contrast, the genotypes from the two herbivory regimes were equally resistant to the rarer S. exigua caterpillars. As a consequence, resistance to one herbivore species did not correlate with resistance to another, suggesting independent evolution of resistance to multiple herbivores.4. Plant growth correlated positively with resistance to Trirhabda, but not to Spodoptera, and only in plants from herbivore exclusion plots, leading us to reject the hypothesis of a growth resistance trade‐off.5. Synthesis. Our results suggest that correlations between growth and resistance are context dependent and may only be apparent in populations relieved from certain natural pressures, such as in natural populations relieved from natural selection by herbivores.