Pollination Ecology of Cimicifuga arizonica (Ranunculaceae)

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Abstract
Cimicifuga arizonica is pollinated by workers of three bumblebee species. The flowering peak coincides with maximal colony size of the bees. The bees preferentially alight on the upper half of the row of flowers, run over the flowers, and collect pollen by pressing their undersides against the stamens. Duration of the visit is similar for all species. Bombus occidentalis regularly used buzzing to make almost dehiscent anthers burst, thus gaining access to large amounts of pollen. Pyrobombus huntii buzzed at one locality only. While Separatobombus morrisoni usually specializes and collects pollen exclusively, B. occidentalis seems to depend on Aquilegia chrysantha as a nectar source. Analyses of coribculae showed that Cimicifuga was the major pollen source, mixed with small amounts of Geranium, Vicia, and Ipomopsis. Differences in nectar and pollen sources were recorded among the bees. An exceptionally high pollen/ovule ratio is explained as a compensatory allocation into the only floral reward for pollen-robbing visitors.