Effects of simulated herbivorous insect feeding on the competitiveness of three Xanthium species

Competitiveness is the key factor to determine whether foreign plants can successfully invade new habitats. In invasion ecology, the evolutionary hypothesis of improving competitiveness believes that the pressure of growth and reproduction of alien plants in the place of introduction is reduced because they escape from restrictive factors such as parasites, pathogens, and animals in the country of origin, thus making the resource allocation of invasive plants transition from defense to growth and promoting the invasion of alien plants. In this study, Xanthium spinosum, X. italicum, and X. sibiricum were used as the research objects for comparing the effects of nonporous, light, moderate, and heavy herbivorous insects on the relative growth rate and biomass of the three species of X. sibiricum. The results showed that the relative growth rate and biomass of X. spinosum and X. sibiricum increased with the light feeding of phytophagous insects, thus increasing their competitiveness; however, the relative growth rate and biomass of X. italicum significantly decreased (P < 0.05). The feeding of phytophagous insects increased the root-to-shoot ratio of X. sibiricum, changed its growth－defense strategy, and reduced the input of nutrient growth to enhance its defense ability. The effects of herbivorous insects on the three species of X. sibiricum were Italian X. italicum, X. sibiricum and X. spinosum. Since phytophagous insects feed on three kinds of X. sibiricum and other plants, further experiments are required to demonstrate whether they can be used to control X. spinosum and X. italicum.