Wild bees outperform managed bees in density-dependent pollination for alfalfa reproduction

Abstract

Global pollinator declines underscore the urgency of assessing bee pollination, and its impacts on crop yield. Although the independent effects of bee species and density on pollination are acknowledged, their interplay remains understudied, particularly concerning visitation quantity and quality. Using a flight cage experiment, we examined the foraging behaviors of two managed bees (honeybee [Apis mellifera], bumblebee [Bombus lucorum]), and one wild bee (megachilid [Eutricharaea manchuriana]). Additionally, we evaluated their relationships with pollen grains carried, the flower tripping rate, and seed production of alfalfa (Medicago sativa) at five bee density levels (0, 5, 10, 15, and 20 individuals per cage). Bee species significantly differed in foraging behavior and pollination efficiency, with megachilids and bumblebees outperforming honeybees with more visits, flower transitions, pollen carriage, and floret tripping. Honeybee visitation rates increased with density. Bumblebee and megachilid visits peaked at 10 and 15 individuals per cage, respectively. The amount of pollen carried and the rates of floret tripping showed similar trends to bee density. Varied effects on alfalfa seed production were observed based on bee species and density. Higher honeybee density resulted in increased pod production, but bumblebees and megachilids exhibited nonlinear effects, suggesting that there is an optimum (10–15 individuals per cage) beyond which more bees are not beneficial. While all three bee species were effective alfalfa pollinators, the megachilids were the most effective option. Determining optimal bee density can inform pollination management strategies to enhance alfalfa seed production.