When generalists become picky: Regional and seasonal specialization in a pimpline Darwin wasp (Hymenoptera, Ichneumonidae) with a supposedly broad host range

The parasitoid Darwin wasp Acrodactyla degener (Haliday) (Ichneumonidae, Pimplinae, Ephialtini) is a koinobiont ectoparasitoid of sheet-web-building spiders, for which knowledge of its ecology and distribution remains limited. We compiled our data from 2017 to 2023 across 209 sampling sites in Central and Southern Europe to assess elevational patterns, host specificity, seasonality, and phylogenetic context. Parasitoidism by A. degener did not follow a continuous elevational gradient but was predominant in montane habitats and shaped by orogeographically discrete host assemblages of linyphiid spiders (Linyphiidae). The species relied primarily on Mughiphantes mughi , a montane spider with abundance peaks in autumn and spring. In contrast, Entelecara and Neriene served as secondary or facultative hosts, with substantial regional variation. Seasonal patterns indicate that the highest levels of parasitoidism occurred in autumn and persisted through overwintering hosts, resulting in spring emergence, consistent with a univoltine cycle in montane regions. Bivoltinism may occur at lower elevations and in warmer habitats. Phylogenetic analysis confirmed that A. degener is distinct from its cryptic congener Acrodactyla similis , which tends to parasitize lowland hosts. There was no evidence of cryptic host-specialized lineages within A. degener . These findings indicate that A. degener is a flexible but montane-biased parasitoid whose life cycle and host associations are structured by elevation, orography, and seasonal host turnover. This species provides a valuable model for investigating ecological adaptations, host shifts, and the evolution of host use in polysphinctine wasps.