Why honeybees are poor pollinators of a mass‐flowering plant: Experimental support for the low pollen quality hypothesis

Abstract

Premise Honeybees dominate the flower‐visitor assemblages of many plant species, yet their efficiency in terms of the quality of pollen delivered to stigmas is largely unknown. We investigated why honeybees are poor pollinators of Aloe ferox, a self‐incompatible succulent treelet with large numbers of flowers. Honeybees are very frequent visitors to flowers of this species, yet contribute very little to seed production. Methods We assessed pollen loads on honeybees, studied their visitation behavior, selectively excluded birds from plants to determine direct effects of bees on pollen deposition, seed set, and ovule abortion, and used a novel “split‐pollinator” method to test whether honeybees deposit mainly low‐quality self pollen. For the latter, we captured honeybees, and with their existing pollen loads, used them to either pollinate virgin flowers on the plant on which they were caught or to pollinate virgin flowers on different plants. Results Honeybees cumulatively deposit as much pollen on stigmas as do birds, but our experiments showed that the pollen deposited by honeybees is mostly low‐quality self pollen that leads to substantial ovule discounting and depressed seed set. Conclusions Lack of movement among A. ferox plants during individual honeybee foraging bouts is the most likely explanation for their deposition of low‐quality self pollen on stigmas. The “split‐pollinator” method is a simple and cost‐effective technique to test the quality of pollination.

Floral scent emission is the highest at the second night of anthesis in Lonicera japonica (Caprifoliaceae)

“…A comparison between nocturnal hawkmoths and diurnal bees of pollen removal and deposition in L. japonica by single visits showed that pollination effectiveness of hawkmoths was over 10-fold higher than that of bees . Our observations showed that honeybees usually behaved as pollen thieves (see also Diller et al, 2022) given that they were observed trying to collect pollen even from the undehisced anthers. Longitudinally dehiscing anthers in L. japonica with mass pollen presentation (Fig.…”

The economy of pollen dispersal in flowering plants

“…An important caveat of transfer efficiency as a measure of pollen economy is that it weights conspecific pollen on stigmas equally, even though pollinators can vary in the quality of pollen that they deposit on stigmas [83]. For the majority of the approximately 37% of angiosperm species that are self-incompatible [84], self-pollination contributes no more to siring success than pollen lost during transport; whereas for species that are self-compatible or have lateacting self-incompatibility, self-pollination can be associated with detrimental effects on ovule fertilization and seed development [85][86][87], reducing both female and male success. Similar qualifications apply to biparental inbreeding.…”

Characterizing each step of pollination in Phlox drummondii reveals that a single butterfly species predominates in the pollinator assemblage

“…The final step of the pollination process is fertilization. Vectors may visit flowers of a different plant species or flowers on the same self‐incompatible plant, resulting in pollen transfer without fertilization (Diller et al, 2022). If floral visitors deposit an insufficient number of pollen grains to elicit fertilization, pollination does not occur (Németh and Smith‐Huerta, 2003; Koski et al, 2018).…”