Scale-dependent effects of honeybee abundance on Lima bean pollination and yield

Abstract

High pollinator abundance can hinder crop productivity due to accumulated reproductive costs associated withrepeated visits. We combined observational and experimental approaches across flower, plant, and field scales toevaluate how honeybee (Apis mellifera) abundance affects the pollination service, net reproductive benefit, andcrop yield in Lima bean (Phaseolus lunatus). Natural pollination resulted in lower fruit and seed set than optimalhand cross-pollination, indicating a limitation in the pollination service at the plant level. At the flower scale, netreproductive benefit increased asymptotically with increasing visits, driven primarily by increased fruit set, withno detectable change in seed set. In contrast, crop yield at the field scale declined linearly with increasinghoneybee visitation, with each additional 1.2 increase in visits per flower per hour reducing seed production by21 %. Overall, our results indicate that high honeybee abundance can constrain Lima bean reproduction andyield. Furthermore, these findings underscore the need for pollination studies to use standardized, multi-scaleapproaches to accurately assess and manage the impact of this superabundant pollinator in agroecosystems.