Differences in interactions of aboveground and belowground herbivores on the invasive plant Alternanthera philoxeroides and native host A-sessilis

Plant invasions may result in novel plant-herbivore interactions. However, we know little about whether and how invasive plants can mediate native above- and belowground herbivore interactions. In this study, we conducted greenhouse experiments to examine the interaction between a native defoliating beetle, Cassida piperata, and a native root-knot nematode, Meloidogyne incognita, on the invasive alligator weed, Alternanthera philoxeroides. We also included their native host A. sessilis in the experiments to examine whether the patterns of above- and belowground herbivore interaction vary with host plants (invasive vs. native). We analyzed total carbon and nitrogen in leaves and roots attacked by M. incognita and C. piperata. M. incognita slightly negatively affected feeding by C. piperata on A. philoxeroides, and the leaf area damaged decreased as the number of M. incognita increased. M. incognita had a negative impact on total leaf nitrogen, but had no impact on total leaf carbon. M. incognita egg production on A. philoxeroides roots decreased as the amount of damage caused by C. piperata increased. Herbivory by C. piperata did not affect total root carbon or nitrogen. M. incognita and C. piperata did not affect each other on the native plant A. sessilis. These results suggest that invasive plants can mediate native above- and belowground herbivore interactions. The knowledge of how invasive plants affect those interactions is crucial for better understanding the impacts of biological invasions on native above- and belowground organisms.