Herbivore accumulation on invasive alien plants increases the distribution range of generalist herbivorous insects and supports proliferation of non-native insect pests

Invasive alien plant species have become dominant components of many landscapes, where they are indicators of ecological disequilibria. They coexist and compete with native plants, disrupting a wide range of trophic interactions, and must cope with a new array of herbivores. We examined factors influencing the occurrence of invertebrate herbivores on two invasive alien plant species, Acacia dealbata and Carpobrotus edulis, and surrounding native vegetation in the northwestern Iberian Peninsula. The aims were to assess the accumulation of herbivorous insects, including the distribution and frequency of herbivorous insects at different invasion levels (low, medium and high), and to evaluate whether introduced plants favour native or exotic, and generalist or specialist herbivorous insects in newly evolving plant–herbivore networks. To achieve these objectives, we surveyed A. dealbata along transects in five mixed pine forests and four shrublands, and C. edulis in nine coastal areas using quadrats. We identified nine herbivore species feeding on one or both of these species and determined that herbivore species composition in forest and coastal areas is directly tied to host preference, as well as related to invasion levels of A. dealbata in forest areas. On average, the introduced species in both the forest and coastal areas had approximately threefold more herbivore species (~ 50% of them exotics) than native plants. The introduced plants in forest areas had approximately eightfold more species interaction strength between herbivores and plants (a measure of the importance of each plant species to each herbivore). Overall, generalist herbivores were favoured by the invasion of the introduced plants. The results also demonstrate that exotic insects are supported by introduced plants, increasing the local occurrence and range of insect pests. Considering our observations, management strategies should be implemented to favour the restoration and protection of native areas for conserving biodiversity.