The effects of flower, floral display, and reward sizes on bumblebee foraging behavior when pollen is the reward and plants are dichogamous

Authors: Brunet, Johanne; THAIRU, MARGARET - University Of Wisconsin; Henss, Jillian; LINK, R - University Of Wisconsin; KLUEVER, J - University Of Wisconsin

Submitted to: International Journal of Plant Sciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/28/2015
Publication Date: 10/13/2015
Publication URL: http://handle.nal.usda.gov/10113/61699
Citation: Brunet, J., Thairu, M.W., Henss, J.M., Link, R.I., Kluever, J.A. 2015. The effects of flower, floral display, and reward sizes on bumblebee foraging behavior when pollen is the reward and plants are dichogamous. International Journal of Plant Science. 176(9):811-819. doi: 10.1086/683339.

Interpretive Summary: Pollinators are known to prefer some floral traits which, in turn, influence their foraging behavior. In this study, we examine the preference of bumble bees for flower size, spur length, pollen reward and floral display size in the Rocky Mountain columbine, Aquilegia coerulea. This plant species has two major pollinators, bumble bees and hawkmoths. While bumble bees actively forage for pollen because they cannot reach the nectar produced at the bottom of the spur, hawkmoths forage for nectar and only indirectly move pollen from flower to flower during this process. Variation in flower size, spur length and floral display size is common both within and among populations of this plant species and some specific floral traits can be associated with the presence and abundance of pollinators. For example, populations with longer spurs are associated with the presence of the hawkmoth Sphinx vashti (Strecker). It is important to understand the floral traits that influence pollinator foraging behavior because pollinator foraging decisions will influence pollinator movement, gene flow and the foraging range of a pollinator. Because pollinators move pollen within and among populations they are responsible for the movement of genes or gene flow via pollen. Pollinator foraging decisions affect the foraging range of a pollinator which in turn influences land management practices for pollinator conservation. The current study examines some of the characteristics of a plant that influence bumble bee choices while foraging. A better understanding of pollinator foraging behavior provides useful information to scientists and government officials interested in gene movement via pollen and in managing landscapes to improve pollinator conservation.

Technical Abstract: Insect-pollinated plants have developed showy flowers and floral displays that attract pollinators. Pollinators, in turn, show preferences for specific floral traits and their foraging behavior is influenced by floral traits. In this study, we examined the preference of bumble bees for flower size, spur length, pollen reward and floral display size in the Rocky Mountain columbine, Aquilegia coerulea. Dual choice assays were used to examine the preference of bumble bees for flower size, spur length and pollen reward while experimental arrays helped determine the preference of bees for floral display size and the proportion of flowers visited per inflorescence. Bumble bees preferred larger floral displays and visited a smaller proportion of the flowers on larger inflorescences. Variation in pollen reward among flowers did not explain the smaller proportion of flowers visited on larger inflorescences. Bumble bees did not show a preference for larger flowers, greater pollen reward or shorter or longer spurs. In A. coerulea, floral display size played a major role in attracting bumble bees. We therefore expect bumble bees to select for larger floral displays which will increase geitonogamy levels. Since A. coerulea suffers from strong inbreeding depression, there will be a balance between the increase in floral display resulting from pollinator attraction and the decrease resulting from increased geitonogamy levels. In addition, the lack of potential selection by bumble bees on flower size and spur length could help maintain the variation in these floral traits in A. coerulea populations.