Photosynthetic performance of water hyacinth as affected by Neochetina spp. and Megamelus scutellaris herbivory

Water hyacinth, Pontederia crassipes Mart. Solms, is invasive throughout tropical and subtropical freshwater bodies. Multiple tools are utilised to control water hyacinth including classical biocontrol with the agents, Neochetina eichhorniae, N. bruchi and Megamelus scutellaris. These have been released in several countries including the United States, where they are widely established. Post-release evaluations of impacts understandably focused on plant metrics indicative of impact including biomass, reproduction, and coverage. These metrics are coarse and may not fully capture the plant and leaf-level impacts of herbivores on their host. This study focused on leaf-level physiological parameters to evaluate the impact of M. scutellaris and its combined interaction with Neochetina spp. Leaf-level gas exchange and chlorophyll fluorescence were measured after two and four weeks of exposure to herbivory using a portable photosynthetic gas exchange system (LiCoR LI 6800). Treatment (herbivore type × infestation level) and exposure time have a significant reduction in transpiration rate and leaf conductance. However, there was no significant reduction in net photosynthetic rate or intercellular CO2 concentration. Similarly, no significant interaction was observed between treatment and exposure time across all parameters. For fluorescence, exposure time significantly reduced maximal fluorescence, maximum quantum efficiency of PSII photochemistry, and performance index, but not minimal fluorescence. However, treatment and its interactions with exposure time were not significant across all fluorescence parameters. The findings of this study indicate that gas exchange and chlorophyll fluorescence parameters are effective metrics for understanding the impact of biological control on invasive species.