Supporting the implementation of a conservation action plan for three sympatric hoverflies

Hoverflies (Diptera: Syrphidae) are important pollinators, benefiting up to 30% of crops produced globally. However, the populations of specialist hoverfly species are particularly vulnerable. For example, some species in the genus Neoascia likely depend on Butterbur (Petasites hybridus) plants for reproduction, making them vulnerable to habitat loss. We aim to: (1) study the distribution of the three Butterbur-dependent fen flies, Neoascia annexa (Müller 1776), Neoascia obliqua (Coe 1940); and the globally endangered Neoascia unifasciata (Strobl 1898) in Luxembourg, and (2) identify variables that affect their encounter rates (individuals caught/hour). From April to June 2025, we sampled 54 Butterbur stands in Luxembourg and the Greater Region (cross-border regions of Belgium, France, and Germany). We collected data on five local and six landscape variables at six spatial scales, analyzing them with generalized linear mixed-effects models. We found that all three species are more widely distributed than previously thought, and that Butterbur stand area was the strongest positive predictor of combined fen fly encounter rates. For N. unifasciata , encounter rate was also positively associated with that of the other two species (but not the other way around) and canopy openness. Protecting and restoring Butterbur stands, while ensuring a sufficient light regime in alluvial forests, will likely bring conservation benefits for these specialized species. Implication for insect conservation Our data will inform the implementation of the conservation action plan of N. unifasciata , bridging the gap between conservation knowledge and action while benefiting other species relying on Butterbur, such as N. annexa and N. obliqua .