Just spines—mechanical defense of malvaceous pollen against collection by corbiculate bees

Abstract

Bee-pollinated plants face the dilemma that bees do not only transport pollen grains between flowers as pollen vectors but also collect large amounts of pollen that is withdrawn from pollination. Here, we show that pollen of the common hollyhock, Alcea rosea, is mechanically protected against collection by corbiculate bees. In a laboratory setup, bumblebees did not collect the large, sticky, and spinose pollen grains of A. rosea from artificial flowers unless following manipulation of the spines or the pollenkitt. Following removal of the sticky pollenkitt or bending the spines by vortexing the bumblebees readily collected the pollen. Our results show that the pollen of A. rosea is not bitter-tasting or toxic, but mechanically protected by the spines against being collected by corbiculate bees. Light microscopic (LM)-micrographs indicate that the long spines of malvaceous pollen grains are not covered by pollenkitt suggesting a new mechanism of mechanical defense against collection: pollenkitt-free spines and lipophilic pollenkitt prevent compacting pollen grains into pollen storage organs of corbiculate bees. The experimental data fit to observations at flowers of A. rosea and other malvaceous plants that honeybees and bumblebees are densely dusted with pollen but discard the pollen while packing it into their corbiculae.