Effects of flower patch additions and urbanisation on cavity‐nesting bees and wasps

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 may benefit from the addition of flower patches and ‘bee hotels’ (artificial nests).

2. While the addition of floral resources and artificial nests are common interventions to support urban pollinators, empirical validation of the tools in heterogeneous urban landscapes is still limited. Consequently, we lack empirical evidence to justify their widespread use, 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 United Kingdom. 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 and their associated parasitoid communities 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 six-fold decrease along 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. 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 bee and wasp communities than currently assumed.

5. Practical implication: Enhancing floral resources in urban areas is a widely used strategy to support pollinators, particularly bees. However, our results suggest that the benefits of such interventions depend on environmental variation and resource requirements among insect groups. Our findings highlight the need for conservation actions that move beyond single-taxon approaches and consider both local resource provision and landscape context.