Pollination services to squash insufficient despite abundant specialist bees

Abstract

Animal-mediated pollination is a critical ecosystem service required by 75 % of crop species for optimal yield. Yet landscape-level pollination assessments for key crops remain limited as many studies focus only on pollinator visitation events, producing an incomplete understanding of the ecological factors that may be driving pollen deposition. In this study, we assess how pollinator visitation as well as local and landscape-level land management contribute to pollen deposition rates for squash (Cucurbita pepo) in one of the largest, yet most understudied squash production regions in the United States. We documented contrasting visitation patterns for generalist and specialist bees, where generalist honey bees exhibited a landscape dilution effect in response to natural habitat cover and specialist squash bees exhibited a resource concentration effect in response to male squash flower abundance. Despite relatively high crop visitation rates overall, we found that pollen deposition was insufficient for optimal crop yield across the study region. Further, our single-visit experiments revealed that most pollinator visits did not result in pollen deposition, suggesting that visitation rates alone may overestimate pollination. Specifically, we identified that specialist squash bee, Xenoglossa strenua, contributed the most to pollen deposition as they were the most frequent visitors, were more likely to deposit pollen on a visit, and deposited more pollen grains per visit than other bee species. Our results highlight the importance of quantifying pollination metrics across ecologically distinct taxa and multiple points in the ecosystem service cascade to more accurately characterize service provision.