Seasonality of bumblebee spillover between strawberry crops and adjacent pinewoods

Abstract

In agricultural landscapes, differences in floral resources provided by crops compared with adjacent habitats promote the spillover of pollinators seeking to fulfil their feeding needs. These foraging patterns play an important role in both crop production and wild plant fitness. However, in classical observational studies, pollinator spillover patterns may be confounded by differences in pollinator phenologies and population sizes. To avoid these confounding effects, we quantified the combined effect of relative floral availability and season on pollinator spillover between strawberry crops and adjacent pinewoods by using commercial bumblebee colonies. We used 60 colonies that were placed in 6 open polytunnel strawberry crops and in 8 pinewoods adjacent to crops in landscapes with either low or high berry (mostly strawberry) crop cover. We repeated the experiment in winter and in spring to account for differences in flower resource availability. While strawberries were in bloom during the entire study period, wild flowering plants were scarce and abundant in winter and in spring, respectively. Spillover in crops was quantified as the percentage of bumblebee individuals that carried pollen from non-berry flowers, while spillover in pinewoods was quantified as the percentage of individuals that carried pollen from berry flowers. Overall, 526 bumblebees were collected. We found them carrying the pollen of 15 plant taxa in winter and 39 in spring, in accordance with seasonal floral availability. In crop colonies, around 30% of bumblebees spilled over into non-berry habitats in winter and 78% in spring, regardless of crop cover in the surrounding landscape. However, in pinewood colonies, we found an interaction effect between season and crop cover: even though on average 34% of bumblebees spilled over into berry crops, in winter it was almost twice in landscapes with high crop cover. Spillover patterns mainly mirrored seasonal changes in floral availability between habitats.