DNA metabarcoding reveals unexpected diet breadth of the specialist large-headed resin bee (Heriades truncorum) in urbanised areas across Germany

Host plants required by specialist bee species may be limited in urban areas. We investigated the effects of urbanisation on Heriades truncorum, a solitary bee species that is a specialist on the plant family Asteraceae. We examined brood cell number, host and native plant pollen proportions (via DNA metabarcoding) and the nutritional composition of pollen provisions in H. truncorum nests. Increasing urbanisation resulted in fewer brood cells and higher plant genus diversity in pollen provisions. Bees collected pollen from non-native and non-Asteraceae plant genera, including new records for H. truncorum. Increasing overall plant genus diversity in brood provisions was associated with decreased brood cell numbers. However, an increased diversity of Asteraceae plant genera was associated with increased brood cell number. The proportion of native plant genera and native plant pollen were not associated with changes in brood cell number. Pollen provision nutritional composition was not related to urbanisation or brood cell numbers. Pollen provisions averaged a P:L ratio of ~1:1, similar to Asteraceae. These results suggest that native and non-native Asteraceae are equally beneficial to H. truncorum in Germany. Moreover, fewer brood cells at more urban sites and an increased diversity of plant pollen suggests that the overall level of resources, particularly Asteraceae plants, could be limited in highly urban landscapes. While H. truncorum is not of high conservation concern, our data demonstrate the mechanisms by which a specialist bee with a more narrow or less common host plant specificity could experience nutritional stress in urban areas and provides mitigation strategies.