Abstract
Many plant species rely on female bumble bee workers for pollen transfer. However, male bumble bees, which differ both behaviourally and morphologically from female workers, also visit many species of flowering plants and may transfer pollen differently. Males can outnumber workers on some plants, particularly those that flower late in the season. In laboratory experiments, we compared the movement patterns of male bees and female workers on an artificial flower array. We also compared the pollen transfer efficiency of males and workers foraging on Brassica rapa flowers. Males travelled between patches of flowers more often than workers, which may be an effective method for reducing geitonogamy in plants. Males also had lower foraging rates, longer flower handling time, and transferred more pollen from one B. rapa flower to the next than workers did. These caste-based differences in pollinating behaviour suggest that, under certain circumstances and on a per-visit basis, male bumble bees may be better pollen vectors than female foragers. Furthermore, our results emphasize the need to avoid species-wide generalizations of pollinator effectiveness.
Highlights
Foraging female bees are noted for their industrious focus on collecting nectar and pollen for their brood
If a difference in pollen transfer between workers and males is primarily due to a difference in flower handling time, including caste as a fixed effect in the model should not significantly improve the model fit
Our study shows clear differences between male and worker B. impatiens foraging behaviour and pollen transfer
Summary
Foraging female bees are noted for their industrious focus on collecting nectar and pollen for their brood. In contrast, are thought to focus on mate-finding, collecting no pollen and occasionally foraging for nectar only to fuel their own activities. Most studies of bumble bee pollination concern workers (e.g., Willmer et al 1994; Thomson & Goodell 2001; Ne'eman et al 2006). This focus is justified, in part, because peak worker production coincides with peak flowering of many plant species. Male bumble bees appear in the late summer and early autumn (Kearns & Thomson 2001) and may be numerically important visitors for later blooming flowers. Male bumble bees are behaviourally and morphologically distinct from workers in ways that are likely to affect pollen transfer patterns and effectiveness
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