The demise of enemy release associated with the recruitment of natural enemies on an invasive tree

Abstract

Abstract Salix species, commonly called Willows, are a diverse group originating from Northern Temperate regions but invasive in many parts of the world, including South Africa’s Grassland Biome. Willows impact biodiversity, disrupt ecosystems, and cause economic losses. The two most prominent invasive willows there are Salix babylonica (weeping willow) and Salix × fragilis (crack willow). Both were historically reported as the most widespread and damaging invasive tree species in the biome. We reassessed S. babylonica’s population size and demographics comparing historical aerial photographs to recent satellite images and through targeted roadside surveys. Additionally, we surveyed associated insect herbivores and plant pathogens. Results showed limited recruitment of S. babylonica , including a significant decline in adult tree numbers. Population demographics from roadside surveys suggest an aging population primarily of mature or dying trees. Surveys of associated insect herbivores and plant pathogens documented 16 insect families (including three non-native insects), a polyphagous crown gall bacterium and one rust fungus. We suggest these associated enemies contribute to the reduced vigour of S. babylonica , offering relief to landowners and conservation authorities that populations are declining. Limited resources can potentially be redirected to managing other more aggressively invading woody species in the Grassland Biome .