All bees, no flies for Spalding’s catchfly: Bumble bee pollination of a threatened plant

Background/Question/Methods

Silene spaldingii (Spalding’s catchfly) is a threatened wildflower of the Caryophyllaceae family. Insect-mediated cross-pollination is critical to maintaining viable populations of this species, as fitness of plants decreases drastically when they are denied insect pollination services. Little is known of the primary pollinators of S. spaldingii, particularly in the Zumwalt Prairie of northeastern Oregon, which contains a high density of S. spaldingii isolated from other populations. The objectives of our study were to 1) quantify visits of potential pollinators to S. spaldingii, 2) compare the relative abundance of visitors to S. spaldingii to the available pool of pollinators on the prairie, 3) determine whether the density of S. spaldingii at the patch scale was associated with higher per capita visitation rates, as predicted by the resource concentration hypothesis, and 4) investigate the potential importance of interspecific competition among flowering plant species for pollinators. To address these objectives, we conducted an observational study of pollinator visits to S. spaldingii during peak blooming season at 30 plots, and characterized the overall native bee community in the prairie using blue vane traps. We also measured the density of S. spaldingii and the blooming plant species richness and abundance within each plot.

Results/Conclusions

We observed 78 potential pollinators visits to S. spaldingii flowers. All visitations were by two species of bumble bees, Bombus fervidus and Bombus appositus, that were relatively uncommon on the prairie as a whole.  Bumble bees showed a high degree of host fidelity, frequently visiting multiple S. spaldingii plants in a row, even when other blooming plants were present. The number of visits per catchfly plant increased with catchfly density at the patch scale, supporting the resource concentration hypothesis. We found no evidence that other blooming species act as competitors for catchfly pollinators. These results indicate that S. spaldingii in the Zumwalt Prairie relies on a relatively narrow pool of pollinators, that these pollinators may preferentially visit S. spaldingii over other blooming plants, and that more dense patches of S. spaldingii may increase the efficiency of pollination.