Male bumble bees as pollinators of a late season plant

Female bumble bee workers are the most recognised bumble bee caste to act as pollinators of wild plants. However, male bumble bees can be common on flowers in the late summer and fall, and they differ from worker bees in both morphology and behaviour, so may differ as pollinators for plants. For instance, male bumble bees may retain more pollen than workers, and move pollen longer distances as they may not be central place foragers. We compared the abundance, foraging behaviour and pollination effectiveness of workers and males on a late summer flowering gentian, Gentiana parryi, in a sub-alpine meadow in the Rocky Mountains of Colorado. Throughout the entire blooming season of the gentian, male bees almost always outnumbered female worker bees. Males and workers also differed in their foraging behaviour: males spent a longer time foraging on flowers and on plants than female workers, though visited a similar number of flowers on a plant. Contrary to expectation, a higher proportion of individually marked males were seen again in the same foraging areas than worker bees, indicating that males show higher site fidelity. Despite differences in foraging behaviour, male and worker bees were equivalent in their ability to set seed during a single visit to a flower. Male bumble bees play an important role in the pollination of late blooming Gentiana parryi, as they are abundant on the flowers and are effective pollinators.