Multi-generational effects of simulated herbivory and habitat types on the invasive weed Alternanthera philoxeroides: implications for biological control

Long-term pre-release evaluations of how invasive plants respond to herbivory in introduced ranges can help identify the most effective biological control agents. However, most evaluations have been conducted within only one generation of introduced invasive species. This study tested effects across seven generations of simulated herbivory (i.e., defoliation) and habitat types on the invasive weed Alternanthera philoxeroides. We found total biomass of A. philoxeroides was decreased by defoliation during the first three generations when grown in a simulated aquatic habitat, but was decreased by defoliation only in the first generation when grown in a simulated terrestrial habitat. Defoliation significantly decreased stem diameter and collenchyma thickness and increased cortex thickness and total phenol production in A. philoxeroides grown in a simulated terrestrial or aquatic habitat during the first generation, but showed little effect during the following six successive generations. The associations between stem anatomical structural parameters and biomass significantly differed between non-defoliation and defoliation treatments in the simulated aquatic habitat, but not in the simulated terrestrial habitat. Our results suggest simulated herbivory exerted successful biological control on A. philoxeroides during the first generation in a simulated terrestrial habitat and the first three generations in a simulated aquatic habitat, but failed to restrain the vegetative offspring of A. philoxeroides. This failure of long-term biological control on A. philoxeroides might be caused by changes in the stem anatomical structure and compensatory growth. Our study highlights the need for long-term pre-release evaluation when testing the efficiency of biological control agents.