High incidence of pollen theft in natural populations of a buzz - 1 pollinated plant 2 3

13 More than 20,000 angiosperm species possess non-dehiscent anthers that open through small pores 14 at the anther's tip. These flowers are visited by bees that use vibrations to remove pollen, a 15 phenomenon known as buzz-pollination. However, some floral visitors fail to transfer pollen 16 efficiently, either through a mismatch of flower and insect size, or because they are unable to 17 buzz-pollinate. These visitors collect pollen, but provide little or no pollination, behaving as pollen 18 thieves. Although pollen theft is widespread in plants, few studies have quantified the incidence of 19 pollen thieves for buzz-pollinated plants. We use observations of natural populations and floral 20 manipulations of Solanum rostratum (Solanaceae) to investigate the incidence of pollen theft, find 21 morphological and behavioural differences between pollinators and thieves, measure the 22 pollination efficiency of visitors, and characterise the reproductive ecology of this herb. We found 23 that most visitors act as thieves, with <20% of all bees contacting the stigma. Insect visitors that 24 regularly failed to contact the stigma (illegitimate visitors), included buzzing and non-buzzing 25 bees, were significantly smaller, visited fewer flowers per bout and stayed longer in each flower 26 than (legitimate) visitors that regularly contact the stigma. Few flowers visited solely by 27 illegitimate visitors set fruit. Our results show that S. rostratum requires insect visitation to set 28 seed and natural populations experience moderate pollen limitation. We conclude that insect size, 29 relative to the flower, is the main determinant of whether a visitor acts as a pollinator or a pollen 30 thief in S. rostratum.