Green patches among a grey patchwork: the importance of preserving natural habitats to harbour cavity-nesting bees and wasps (Hymenoptera) and their natural enemies in urban areas

Urbanisation is fast-growing worldwide characterised by the conversion of natural vegetation ecosystems into densely paved areas with a high concentration of human constructions and few remnants of natural habitats. This phenomenon may threaten wildlife, especially high trophic level organisms such as predators and parasitoids, which are known to be more sensitive to habitat fragmentation. We investigated the influence of urbanisation on the community of trap-nesting bees and wasps as well as on their parasites in an urban area. Trap-nests were installed in 11 areas within the perimeter of the city of Ribeirao Preto, Brazil. Fourteen land cover categories were distinguished and their percentages calculated for each area from satellite images. The community sampled consisted of 20 wasp and 12 bee species, as well as 25 natural enemy species that attacked 9.75% of the nests. The highest diversity of bees and wasps was observed in areas with higher percentages of natural vegetation, i.e., forests, wastelands, and neighbourhoods with extensive green areas. Rates of parasitism, measured by the abundance and richness of parasites, was positively correlated with the proportion of green areas in the landscape. Even though predatory wasps constitute a higher trophic level than bees they were not more negatively impacted by urbanisation. Our results demonstrate that natural habitats and extensive green areas can host diverse communities of cavity-nesting bees, wasps, and their parasites within a city. The conservation of green areas in urbanised landscapes should be considered as essential to maintain the populations of these important insects.