Landscape and local factors influence solitary bee nesting, but reported effects show little consistency across studies

Abstract Solitary bees constitute 75% of bee species but are understudied compared with social bees. Because of global pollinator declines, it is critical to develop effective conservation measures for solitary bees. Consequently, a thorough understanding of the local and landscape factors that influence the nesting of above‐ground nesting (AGN) and below‐ground nesting (BGN) species is required. We performed a systematic review of the scientific literature with respect to solitary bee nesting behaviour. A screening of ~15,000 articles resulted in 91 articles providing meaningful data for AGN bees and 55 articles for BGN bees. For AGN bees, we extracted information on factors, such as nest orientation, nest height, cavity size and nest materials. For BGN bees, we extracted information on factors, such as ground aspect and slope, bare soil and vegetation height. The effects of floral resources, prevalence of semi‐natural habitat (SNH) and wider habitat type were recorded for both AGN and BGN bees. Considering the factors examined in the existing literature, both AGN and BGN bees displayed higher nest occupancy in sites associated with SNH and/or high floral abundance. For AGN bees, trap nest occupancy was associated with nest‐site characteristics, such as the construction material and cavity diameter, while for BGN bees, nesting was associated with the prevalence of bare ground. Notably, however, the reported effects of most explanatory factors on solitary bee nesting behaviour were highly inconsistent across studies. Synthesis and applications. Collectively, these findings indicate that agri‐environment schemes aiming to support solitary bees should integrate the provision of nesting resources with proximity to floral resources. The heterogeneity among studies highlights the need for solitary bee conservation measures to be tailored to local conditions and to the local fauna. Where possible, nest‐site interventions, such as the placement of trap nests and substrate properties, should be tested experimentally across biogeographic zones to clarify their effects and assess the consistency of responses under different environmental conditions.