Novel pollen analogue technique shows bumblebees display low floral constancy and prefer sites with high floral diversity

Abstract

Context: There have been dramatic global declines in pollinating insects. A common land management intervention to support wild pollinators is to plant non-crop flowering plants (‘pollinator planting’). However, there are limited data on which species or spatial arrangement of planting provide maximum benefit to wild pollinators. Objectives: Here we investigate which flowering species and locations are visited by free-foraging Bombus terrestris (buff-tailed bumblebees) in species-rich semi-natural grassland and woodland. Methods: Two study nests of buff-tailed bumblebees were established in Wytham Woods, UK. Pollen analogue pigments were sprayed on open flowers in the study area over a period of two months, with unique colours used to identify separate sections of the study area. Pollen load analysis was used to identify forage species and foraging locations. Results: Bumblebees showed low flower constancy, visiting five flower species per trip on average, and as a group the sampled bumblebees visited 36 of the 49 plant species identified in study area surveys. Many individuals foraged in multiple, spatially-discrete locations during single trips. Conclusions: The positive relationship between floral diversity and pollen load species diversity, and the positive relationship between site floral diversity and frequency of visitation, suggest behavioural strategies that maximize the diversity of flower species visited, in line with the energetic costs and benefits hypothesis. This supports recommendations for pollinator plantings with high species diversity, potentially spread across many small forage areas across the landscape.