Effects of clonal integration on the invasive weed Alternanthera philoxeroides and its native congener varied with plant–herbivore coevolutionary history: implications for management

Clonal invaders can cope with herbivory via clonal integration, but how the impacts of clonal integration are affected by plant-herbivore coevolution history is still poorly understood. Alternanthera philoxeroides (Caryophyllales: Amaranthaceae) is an amphibious clonal invader. The beetle Agasicles hygrophila (Coleoptera: Chrysomelidae) was introduced to control A. philoxeroides, and also feeds on the native Alternanthera sessilis (Caryophyllales: Amaranthaceae) in China. We grew the apical and basal parts of A. philoxeroides and A. sessilis, connected or disconnected with the stolon, and exposed apical parts to A. hygrophila or the native beetle Cassida piperata (Coleoptera: Cassididae) that also feeds on both plants. Both beetles significantly reduced the stem length, belowground biomass and photosynthesis of both plants without clonal integration, and flea beetles had a stronger negative impact on A. philoxeroides than did tortoise beetles. The stem length, stem diameter, belowground and/or aboveground biomass and photosynthetic parameters of apical parts of plants under attack by their corresponding coevolved beetle were 27% ~ 391% higher with clonal integration compared to without. In contrast, when the two plants were attacked by the corresponding beetle without coevolution, the same parameters in either apical or basal parts were unchanged or significantly lower under clonal integration than no integration. These results indicate that artificial disrupting of clonal integration combined with biological control may suppress clonal plants like A. philoxeroides more effectively. Compared with coevolved monophagous herbivores, some oligophagous herbivores in invaded ranges might have a stronger potential to influence the clonal integration capacity of invasive aliens.