Differences in physiological traits and resistances of Alternanthera philoxeroides after herbivory by generalists and specialists

Many researchers have surveyed damages caused by natural enemies of invasive plants in both native and introduced ranges to test the enemy release hypothesis. In this study, we report our findings on the physiological and morphological impacts of a co-evolved specialist insect (Agasicles hygrophila) and two generalist insects (Atractomorpha sinensis and Hymenia recurvalis) in introduced ranges on an invasive plant, Alternanthera philoxeroides, in both field trials and controlled environments. The resistance of A. philoxeroides against the generalists and the specialist was also studied. We obtained consistent results in both the field trials and the controlled treatments: both the generalists and the specialist decreased leaf biomass, photosynthesis, leaf nitrogen content, and total leaf non-structural carbohydrate content in A. philoxeroides. However, the specialist decreased leaf mass, photosynthesis, and leaf nitrogen content more acutely than the generalists. Moreover, A. philoxeroides increased both leaf lignin and cellulose concentrations upon the generalists’ attack but only increased cellulose concentration in response to the specialist. Our results showed that even under the same population density, the co-evolved specialists from original ranges caused more severe morphological and physiological damage to A. philoxeroides than the generalists in introduced ranges. This revealed that invasive plants released some herbivory stress before their co-evolved specialists were introduced, which may contribute to the superior performance of invasive plants in introduced regions.