Insect invasions track a tree invasion: Global distribution of black locust herbivores

Aim: Many invasive plant species benefit from enemy release resulting from the absence of insect herbivores in their invaded range. However, over time, specialized herbivores may `catch up' with such invasive plants. Black locust is a tree species with a relatively limited native range in North America but has invaded large areas in virtually every temperate continent including North America. We hypothesize that both intra - and intercontinental spread of black locust leads to a parallel, though delayed pattern of intra - and intercontinental spread of insect herbivores. Location: Global. Taxon: Black locust, Robinia pseudoacacia, and its insect herbivores. Methods: We compiled historical records of the occurrence of insect herbivore species associated with R. pseudoacacia from all world regions. Based on this list, we describe taxonomic patterns and investigate associations between environmental features and numbers of non-native specialist herbivores in the portion of North America invaded by R. pseudoacacia. Results: A total of 454 herbivorous species are recorded feeding on R. pseudoacacia across the world, with 23 of these being specialized on Robinia. From this group, seven species have successfully expanded their range beyond North America. Within North America, the richness of specialists is explained by a combination of road density, R. pseudoacacia density, distance from the R. pseudoacacia native range, and climate. Main Conclusion: Non--native herbivore species have accumulated on invasive R. pseudoacacia in both North America and in other continents. The steady build-up of invasions likely has diminished the enemy release that this invasive tree species has benefited from - a trend that will likely continue in the future. These findings support the hypothesis that invasive plants promote parallel though delayed invasions of specialist insect herbivores.