Male bumble bees are important pollinators of a late-blooming plant

Differences in the pollinator performance of flower visitor sexes are rarely considered. In bumble bees, males differ from workers in morphology and behaviour in ways that may affect their contribution to pollination. We compared the abundance, foraging behaviour, and pollen transfer ability of worker and male bumble bees on late-blooming Gentiana parryi (Gentianaceae) in subalpine meadows of the Colorado Rocky Mountains. Male bees mostly outnumbered female workers throughout blooming of the gentian. Males and workers foraged similarly, though individual males were more faithful to small foraging areas than workers. During single flower visits, males and workers caused similar levels of pollen deposition and seed production, yet female bees left fewer pollen grains in anthers to be transferred to other stigmas in the plant population. Overall, male bumble bees are common and capable pollinators of G. parryi and in some years and sites could be more important than workers. Male bumble bees may be important but unrecognized pollinators of other late-season plant species, and animal sexes may differ in their pollinator performance in other systems.