Different effects of local and landscape context on pollen foraging decisions by two managed orchard pollinators, Osmia cornuta and Bombus terrestris

Abstract

Populations of managed pollinators are often introduced in agricultural fields to increase pollination levels. Ideally, managed pollinators should show a strong affinity for the target crop, which depends on innate preferences, but also on the availability of alternative co-blooming flower resources. In this study we compared pollen collection in Osmia cornuta and Bombus terrestris populations introduced in orchards, and we analysed how it is affected by local and landscape floral context. Based on pollen preferences and foraging rates of the two species, we expected O. cornuta to collect more fruit tree pollen than B. terrestris and to be less influenced by landscape factors. We sampled 16 orchards of various fruit tree species (almonds, apricots, cherries, apples). In each orchard, we characterized local flower availability and landscape composition, and identified pollen samples from 20 to 30 individuals of each pollinator species. O. cornuta collected a greater proportion of fruit tree pollen than B. terrestris (92% vs 81%), irrespective of the availability of alternative flower resources at the local and landscape levels. The diversity of pollen collected by B. terrestris increased with the availability of alternative flower resources, both at the local and landscape levels, suggesting that some of the foraging by this species took place away from the target orchard. Our results demonstrate that wild flowers do not compete with fruit tree flowers for O. cornuta visitation. Therefore, the use of managed O. cornuta populations to supplement pollination services in orchards is compatible with the implementation of agri-environmental measures (flower strips, ecological focus areas) to promote wild pollinator communities.