Raw data and code for: Effects of flower patch additions and urbanisation on cavity-nesting bees and wasps.

Abstract 1. Cavity-nesting bees and wasps provide essential ecosystem services such as pollination and pest control but can be negatively affected by urbanisation. Within urban environments, cavity-nesting hymenopterans use greenspaces to forage for food and nesting resources, and thus may benefit from urban greenspace management interventions, such as the addition of flower patches and "bee hotels" (artificial nests). 2. While additions of floral resources and artificial nests are common interventions to support urban pollinators, empirical validation of these tools in heterogeneous urban landscapes is still limited. Consequently, we lack empirical evidence which would justify their widespread use, and which could help optimise conservation interventions for these taxa. 3. To understand how the addition of floral resources influences cavity-nesting hymenopteran communities, we conducted a 2-year, city-wide study in urban greenspaces used for horticulture (allotments) in the U.K which can be variable in their management. We manipulated floral and nesting resources by adding nectar-rich flower patches and artificial nests and compared nest colonisation by cavity-nesting bees and wasps in sites with and without floral additions. Our sites were configured along an urbanisation gradient, allowing us to test whether interventions mitigate the complex effects of surrounding urbanisation. 4. We found no significant difference in artificial nest uptake in sites where floral resources were added, compared to control sites. Cavity-nesting hymenopteran abundance showed a 6-fold decrease over our urbanisation gradient, but the negative effect of urbanisation was not mitigated by experimental addition of floral resources or by existing variation in floral resources at an allotment site-level. While the benefits of floral enrichment could likely be enhanced through an improved understanding of floral nutrition, our results suggest that, as commonly deployed, these interventions may carry fewer benefits for cavity-nesting solitary bee and wasp communities than currently assumed. 5. Although enhancing floral resources in urban areas is a widely used strategy to support pollinators, particularly bees, our results show that the benefits of such interventions are complicated and hard to detect. We demonstrate that the effectiveness of floral supplementation depends on the spatial scale of the intervention, the diversity of resource requirements across insect groups, and the surrounding urban landscape. Our findings highlight the need for conservation actions that move beyond single-taxon approaches and consider both local resource provision and landscape context to support diverse insect communities in cities.