High incidence of pollen theft in natural populations of a buzz-pollinated plant

Abstract

More than 20,000 angiosperm species possess non-dehiscent anthers that open through small pores at the anther’s tip. These flowers are visited by bees that use vibrations to remove pollen, a phenomenon known as buzz pollination. However, some floral visitors fail to transfer pollen efficiently, either through a mismatch of flower and insect size, or because they are unable to buzz-pollinate. These visitors collect pollen, but provide little or no pollination, behaving as pollen thieves. Although pollen theft is widespread in plants, few studies have quantified the incidence of pollen thieves for buzz-pollinated plants. We use observations of natural populations and floral manipulations of Solanum rostratum (Solanaceae) to investigate the incidence of pollen theft, find morphological and behavioural differences between pollinators and thieves, measure the pollination efficiency of visitors, and characterize the reproductive ecology of this herb. We found that most visitors act as thieves, with <20 % of all bees contacting the stigma. Insect visitors that regularly failed to contact the stigma (illegitimate visitors), included buzzing and non-buzzing bees, were significantly smaller, visited fewer flowers per bout, and stayed longer in each flower than (legitimate) visitors that regularly contact the stigma. Few flowers visited solely by illegitimate visitors set fruit. Our results show that S. rostratum requires insect visitation to set seed and natural populations experience moderate pollen limitation. We conclude that insect size, relative to the flower, is the main determinant of whether a visitor acts as a pollinator or a pollen thief in S. rostratum.