Pollen is often consumed by the very pollinators required for its movement. In particular, bees rely on pollen as their primary source of protein and lipids. Thus, plants that attract bees as floral visitors are often subject to pollen foraging. Pollen lost to consumption translates to lost plant reproductive opportunities. However, roughly 20,000 angiosperms provide pollen as their sole floral reward. Whether or how pollinator behavior differs on pollen-only plants relative to plants that offer other rewards, such as nectar, holds implications for patterns of plant reproduction. To characterize how the pollen-only character state affects forager behavior and plant reproductive success, we performed a nectar addition experiment in a population of Lupinus argenteus, a pollen-only species. Daily during the 2016 flowering season, we deposited 2 µl of artificial nectar (40% v/v sucrose) into all open flowers on 49 individuals (treatment), and probed all open flowers of 49 additional individuals with a dry micro-pipette (control). Subsequently, we tracked bee foraging behavior, recording time spent per-flower, flowers visited per plant, and total approaches received by each plant. At the end of the flowering season, we collected fruits from all experimental plants and assessed seed set, seed mass, and number of aborted seeds.
Results/Conclusions
Over 35 days of nectar additions (14 observation periods), we observed 169 foraging bouts. All foraging observations were of bees from 9 species (Bombus: 6, Andrena: 1, Megachile: 1, Osmia: 1). Foragers spent >27% longer per-flower on nectar-addition relative to control plants (T40.040 = 4.064, P < 0.001). Bees collected both pollen and nectar from the nectar-addition plants. Despite this additional reward, there was no significant difference between nectar-addition and control plants in total number of flowers visited per plant or total number of approaches observed. Longer time spent per-flower in nectar-addition plants did not translate into differences in female components of plant reproduction, with no significant difference in seed set, seed mass, or number of aborted seeds between nectar-addition and control plants. These results suggest that offering multiple versus single rewards may have implications for pollinator foraging behavior, but that this does not necessarily translate into differing effects on female components of plant reproductive success.