Climate change is causing European temperate climates to shift toward mediterranean conditions, a phenomenon called ‘mediterranization’. Mediterranization is altering biological communities, facilitating expansion of species pre-adapted to mediterranean habitats. Colonization by novel species can drastically affect local biodiversity, particularly in freshwater ecosystems. Lethocerus patruelis is a large predatory aquatic insect whose distribution spans from the Balkans to south-eastern Asia. In the last decades, L. patruelis sightings outside its known range have increased, particularly in Italy. The aim of this study was to assess the drivers of this expansion and the potential for future spread.

We collected records of L. patruelis using published literature, citizen-science platforms, and social media. These data were used to test for directional expansion and to compare native and novel niches. Secondly, we used native observations to create a suitability model using MaxEnt, testing it using observations from Italy. Lastly, we projected the model under three future climatic scenarios to assess the potential for future expansions.

We found a significant westward and southward expansion of L. patruelis in Italy. Niche conservatism between native and novel ranges was observed. Nonetheless, we found limited overlap and high level of niche unfilling, suggesting an ongoing colonization process. The suitability model showed good predictive performances, indicating a preference toward mediterranean climates and a selection against agricultural areas. Suitable areas were predicted to increase under all three future climatic scenarios.

This study suggests an ongoing spread of L. patruelis, and a strong expansion potential in Europe due to mediterranization. Future studies are needed to assess the means of this expansion and its consequences on local biodiversity.