Floral bagging differentially affects handling behaviors and single-visit pollen deposition by honey bees and native bees

Measurements of pollinator performance are crucial to pollination studies, enabling researchers to quantify the relative value of different pollinator species to plant reproduction. One of the most widely employed measures of pollinator performance is single-visit pollen deposition, the number of conspecific pollen grains deposited to a stigma after one pollinator visit. To ensure a pollen-free stigma, experimenters must first bag flowers before exposing them to a pollinator. Bagging flowers, however, may unintentionally manipulate floral characteristics to which pollinators respond. In this study, we quantified the effect of bagging on nectar volume in watermelon (*Citrullus lanatus*) flowers, and how this affects pollinator performance and behavior. Experimental bagging resulted in roughly 30-fold increases in nectar volume relative to unmanipulated, open-pollinated field flowers after only a few hours. Honey bees, but not native bees, consistently displayed elevated handling times and single-visit pollen deposition on unmanipulated bagged flowers relative to those from which we removed nectar to mimic volumes in open-pollinated flowers. Furthermore, we identify specific bee foraging behaviors during a floral visit that account for differences in pollen deposition, and how these differ between honey bees and native bees. Our findings suggest thatexperimental bagging of flowers, without accounting for artificially accumulated nectar, can lead to biased estimates of pollinator performance in pollinator taxa that respond strongly to nectar volume. We advise that pollination studies be attentive to nectar secretion dynamics in their focal plant species to ensure unbiased estimates of pollinator performance across multiple pollinator species.

传粉者效能的测量是传粉生物学研究的关键环节，可帮助研究者量化不同传粉者物种对植物繁殖的相对价值。当前应用最广泛的传粉者效能测量指标之一为单次访花花粉沉积量（single-visit pollen deposition），即单次传粉者访花后沉积到柱头（stigma）上的同种花粉粒数量。为确保柱头无花粉，实验人员需先对花朵进行套袋处理，再将其暴露于传粉者中。但套袋操作可能会无意中改变传粉者所能感知的花部性状。
本研究定量分析了套袋对西瓜（*Citrullus lanatus*）花花蜜体积的影响，以及该影响如何作用于传粉者效能与访花行为。仅数小时后，实验套袋处理的花朵花蜜体积相较于未受干扰的开放式传粉田间花朵，提升了约30倍。相较于我们移除花蜜以模拟开放式传粉花朵花蜜体积的套袋花，西方蜜蜂（而非本土野生蜂类）在未受干扰的套袋花上始终展现出更长的访花耗时与更高的单次访花花粉沉积量。此外，本研究明确了访花过程中导致花粉沉积量差异的特定蜂类觅食行为，并揭示了西方蜜蜂与本土野生蜂类在这类行为上的差异。
本研究结果表明，若未考虑套袋导致的花蜜人工累积效应，对花蜜体积敏感的传粉类群的传粉者效能评估可能会出现偏差。我们建议传粉研究应关注目标植物物种的花蜜分泌动态，以确保在多传粉者物种研究中获得无偏的传粉者效能评估结果。