Comparison of invertebrate herbivores on native and non-native Senecio species: Implications for the enemy release hypothesis

The enemy release hypothesis posits that non-native plant species may gain a competitive advantage over their native counterparts because they are liberated from co-evolved natural enemies from their native area. The phylogenetic relationship between a non-native plant and the native community may be important for understanding the success of some non-native plants, because host switching by insect herbivores is more likely to occur between closely related species. We tested the enemy release hypothesis by comparing leaf damage and herbivorous insect assemblages on the invasive species Senecio madagascariensisPoir. to that on nine congeneric species, of which five are native to the study area, and four are non-native but considered non-invasive. Non-native species had less leaf damage than natives overall, but we found no significant differences in the abundance, richness and Shannon diversity of herbivores between native and non-native SenecioL. species. The herbivore assemblage and percentage abundance of herbivore guilds differed among all Senecio species, but patterns were not related to whether the species was native or not. Species-level differences indicate that S.madagascariensis may have a greater proportion of generalist insect damage (represented by phytophagous leaf chewers) than the other Senecio species. Within a plant genus, escape from natural enemies may not be a sufficient explanation for why some non-native species become more invasive than others.