Rediscovery of <i>Stomaphis radicicola</i> Hille Ris Lambers, 1947 (Hemiptera, Aphididae, Lachninae) in Europe ‐ potential past and future distribution of a large but cryptic, arbicolous aphid species

Aphids of the genus Stomaphis are among the largest aphids, living on tree trunks in the Palaearctic, always with an obligate mutualistic relationship with ants. Some species are very common and abundant, but a few of them are very rare and known only from single sites. Stomaphis radicicola was described in 1947 and since its description, it has been found only three times prior to 2024, when an abundant population was discovered in Germany, close to the UNESCO biosphere reserve Upper Lusatia Heath and Pond Landscape. Detailed studies on the species’ biology and modelling of its potential past, current and future distribution indicate its relatively narrow habitat specialisation towards riparian forests, with Alnus glutinosa as a predominant host plant. With such a level of specialisation, its distribution during the Last Glacial Maximum was very limited, and the current rarity across Europe may result from a bottleneck effect. The species’ future range may shift eastwards under climate change, but it will depend on finding riparian habitats and host ant species.