Dryland organic farming increases floral resources and bee colony success in highly simplified agricultural landscapes

Abstract

Agricultural intensification has contributed to declines in wild bee populations at local and regional scales. The Northern Great Plains is a key region for small grain, forage, pulse, and oilseed production, yet the impact of available floral resources on colony success of eusocial bees is unknown. To assess the effects of farming system on bee colony success (relative growth rate and fecundity) and worker condition (lipid content and wing wear), we placed 60 mini-colonies of Bombus impatiens in six conventional and six organic spring wheat fields near Big Sandy, Montana, USA over two growing seasons. Consistent with our expectations, we found that flower density and forb richness were positively associated with colony relative growth rate and fecundity. Similarly, flower density, forb richness, and the relative growth rate and fecundity of B. impatiens colonies were greater in organic fields when compared to conventional fields. Workers from organic fields had lower mean wing wear and marginally greater average body lipid content. However, there were no differences in the number of reproductives (queens, workers, and males) between two systems. Lastly, though the overall composition of colony-collected pollen was not different across the farming systems, colony-collected pollen species richness was greater in organic fields in one of the two sampled growing seasons. We conclude that by increasing B. impatiens colony success and individual worker condition, the greater floral resources observed in organic systems can contribute to better biodiversity-based indirect ecosystem services in the highly simplified agricultural landscapes that dominates the Northern Great Plains.