Range Expansion of the Giant Water Bug <i>Lethocerus patruelis</i> (Stål, 1854) in Europe

Climate change is altering freshwater ecosystems, causing extinctions, range expansions, and facilitating biological invasions. Colonization by novel species can drastically affect local biodiversity, particularly in aquatic habitats. <i>Lethocerus patruelis</i> (Stål, 1854) is a large predatory aquatic insect whose distribution spans from the Balkans to south-eastern Asia. In the last decades, <i>L. patruelis</i> 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 <i>L. patruelis</i> using published literature, citizen-science platforms, and social media. These data were used to test for directional expansion and to compare historical and novel niches. Second, we used observations from the historical range of the species to create a suitability model using MaxEnt, testing it using observations from Italy. Finally, we projected the model under three future climatic scenarios to assess the potential for future expansions. We detected a significant westward and southward expansion of <i>L. patruelis</i> in Italy. Niche conservatism between historical and novel ranges was observed. Nonetheless, we found limited overlap and a high level of niche unfilling, suggesting an ongoing colonization process. The suitability model showed good predictive performance, 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 <i>L. patruelis</i> and a strong expansion potential in Europe facilitated by climate change.