Diversity and specificity of host-natural enemy interactions in an urban-rural interface

Urbanisation and agricultural intensification cause the replacement of natural ecosystems but might also create novel habitats in urban and rural ecosystems promoting some insect communities by providing food and nesting resources. This study investigated how host-natural enemy communities change in urban and rural landscapes and their transitional zone, the urban-rural interface, by using trap nests for cavity-nesting Hymenoptera in gardens and rapeseed fields that were either isolated or paired in the urban-rural interface. Host dynamics were important for natural enemy occurrence, species richness and parasitism rates, and landscape effects were evident for natural enemy variables except for the richness of bee natural enemies. The number of parasitised brood cells was at its highest in the urban-rural interface, but the highest parasitism rates of bees were observed in isolated gardens. Parasitism rates of bees were negatively affected by host abundance, while parasitism rates of wasps were positively affected. Higher specialisation and lower connectivity of host-natural enemy interactions were found in paired habitats than in isolated habitats. This indicates that paired habitats comprise more specific natural enemies and vulnerable interactions, while isolated habitats comprise more generalist natural enemies, and thus interactions appear more stable. These results confirm that host dynamics play an essential role in the abundance and richness of natural enemies and drive parasitism. However, high habitat heterogeneity found in the urban-rural interface can also have an effect on host-natural enemy communities. This highlights that the provisioning of resources in the urban-rural interface can benefit insect communities in these areas.