Does bees’ transport of pollen differ from their scopal pollen collection?

Background/Question/Methods

When bees incidentally transfer pollen to stigmas as they forage, they facilitate gamete dispersal and plant reproduction. However, when bees collect and feed pollen to their larvae, they destroy male plant gametes and exact fitness costs on their host plants. The trade-off between bees’ transport and consumption of pollen appears to have been a major force in bee-plant coevolution, and has been well-studied at the scale of interactions between single plant species and their flower visitors. This process is poorly understood in speciose natural communities, though, because logistical constraints typically prevent measurement of fitness outcomes for bees’ interactions with more than one host plant. Here, we approximate those fitness outcomes by comparing the pollens collected and transported by individual bees in nature. Specifically, we compare the relative abundance of pollen taxa in individual bee foragers’ scopal structures (where bees actively collect and store pollen to feed to larvae) with the relative abundance of pollen taxa on the rest of the bee's body (which is more likely to be transferred to stigmas). Our pollen data come from 312 individual female bees of three species, which we collected foraging on forest spring ephemerals at three national parks in Indiana and Michigan, USA.

Results/Conclusions

Using light microscopy, we identified 40 pollen taxa in scopal and body pollen samples. In preliminary analyses, 43% of individual bee specimens carried pollen taxa in their scopae that were not represented on the rest of their bodies, indicating that at least some plant species whose pollen bees intentionally collect are unlikely to receive pollen transport on all foraging trips. To assess which pollen taxa were systematically under- or overrepresented on bees’ bodies compared to their relative abundance in bees’ scopae, we ran Wilcoxon signed rank tests for twelve highly-collected pollens. For most plants, the difference between relative abundance in scopae vs. bodies varied widely among individual foragers, but averaged around zero, suggesting that, on average, bee foragers’ transport of pollen for these plants reflects their scopal collection of their pollen. For some plants, though, bees’ collection of its pollen was a less reliable indicator of pollen transport. Two pollen taxa were systematically underrepresented on bees’ bodies, and three were systematically overrepresented. This study will be the first published comparison of scopal and body pollens on wild bees for multiple plant species simultaneously. As such, it will provide insight into the trade-off between bees’ pollen consumption and transport in nature.