Physical interaction between floral specialist bees Ptilothrix bombiformis (Cresson) (Hymenoptera: Apidae) enhances pollination of hibiscus (section Trionum: Malvaceae)

Authors: Sampson, Blair; Pounders Jr, Cecil; Werle, Christopher; Mallette, Trevor; Larsen, Drew; Chatelain, Lauren; Lee, Karmen

Submitted to: Journal of Pollination Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/15/2016
Publication Date: 2/18/2016
Citation: Sampson, B.J., Pounders Jr, C.T., Werle, C.T., Mallette, T., Larsen, D., Chatelain, L.M., Lee, K.C. 2016. Physical interaction between floral specialist bees Ptilothrix bombiformis (Cresson) (Hymenoptera: Apidae) enhances pollination of hibiscus (section Trionum: Malvaceae). Journal of Pollination Ecology. 18(2):7-12.

Interpretive Summary: Bees that rely on a few related species of floral hosts for food are thought to be the most efficient pollinators of these plant species. However, their efficiencies may be no greater than those of less specialized generalist bees. The mallow bee, Ptilothrix bombiformis, is just such a species. It may not be the most effecient pollinator on a per bee basis, but its sheer abundance at host blooms makes it a highly effective one. What was most interesting was pollination efficiency for mallow bees periodically increased 30% – 1000% with changes in host flower shape and with aggressive bee behavior in flowers. Interestingly, Ptilothrix bees increase rates of pollen transfer and host seed set when they fight amongst themselves inside Hibiscus flowers. Tussles between bees represent a new class of pollinator interactions that contribute to the pollination of flowering plants.

Technical Abstract: Specialist bees, those species with narrow dietary niches, rely on a few related species of floral hosts for food. Accordingly, specialists are thought of as being more efficient pollinators than are generalists. There is growing evidence, however, that this is not true in all cases. For example, we found that only 3% of floral visits by large floral specialist bees Ptilothrix bombiformis (Cresson) resulted in bees brushing against stigmas of Hibiscus hosts. However, as low as pollination efficiency seems for this bee, it can periodically increase 30% – 1000% with changes in floral morphology and pollinator behavior. Field observations of Ptilothrix and pollinator exclusion experiments show that the percentage of contact visits, those visits transferring =10 pollen grains onto host stigmas, and pollen deposition, the mean pollen grains deposited, can increase 30% for this bee species as host petals close during the day. Pollen deposition rates increased an astounding 300% to 1000% as Ptilothrix tussled around inside host blooms. Tussling bees often grapple with, lunge at, and bite other conspecifics, which causes them to tumble around inside flowers. Such fights between two bees constituted only 5% of the visitation bouts, yet accounted for 20% of contact visits. Interestingly, Ptilothrix bees increase rates of pollen transfer and host seed set when they interrupt one another’s foraging at Hibiscus flowers. Tussles between bees represent a higher-order pollinator interaction that could be common in wild bee populations with assertive individuals. In addition to a diversity of bee species, a diversity of complementary and conflicting bee behaviors may also contribute to the pollination of flowering plants.