Assessing invasion risk of two North American Saperda species (Coleoptera, Cerambycidae) in Europe using joint bioclimatic-habitat species distribution models and maritime-inland connectivity

The introduction and establishment of non-native wood-boring insects pose a significant threat to forest ecosystems, urban trees and agricultural systems worldwide. In this study, we assessed the invasion risk of two North American longhorn beetles, Saperda candida and Saperda tridentata, to Europe by integrating joint bioclimatic–habitat species distribution models with maritime connectivity data. Both species can infest live trees; S. candida is primarily associated with hosts in the Rosaceae, whereas S. tridentata mainly attacks Ulmus species, making both species of primary phytosanitary concern to Europe. We developed ensemble models that separately quantified the effects of broad-scale climatic variables and fine-scale habitat factors, including land cover and host plant availability and then combined them into overall suitability maps. These were further integrated with a proxy of propagule pressure, based on international shipping connectivity to generate spatially explicit invasion risk maps. Both species showed strong model performance and high transferability. Results indicate that large areas of Europe, particularly central and western regions, offer suitable environmental conditions for establishment, although climatic constraints limit suitability in southern and northern extremes. Host plant availability emerged as a key determinant of habitat suitability, while climate acted as the primary limiting factor in Europe. When combined with maritime connectivity, invasion risk was highest in central Europe, especially in regions with dense trade networks and high environmental suitability. Our findings highlight that invasion risk is driven by the interaction between environmental suitability and human-mediated pathways, emphasising the importance of integrating ecological modelling with trade data. This approach provides a robust framework for improving surveillance, prioritising high-risk areas and supporting preventative phytosanitary strategies against invasive wood-boring insects.