Comparison of defence buzzes in hoverflies and buzz-pollinating bees

Bees and many flies, particularly hoverflies (Syrphidae), have evolved a diverse range of mechanisms to gather pollen from a wide variety of flowering plants. Bees and hoverflies use protein-rich pollen as a food resource to mature reproductive organs and eggs and, in bees, to feed their larvae. A particularly striking pollen-collecting behaviour involves the production of thoracic vibrations to dislodge pollen from flowers. Vibratile pollen collection is widespread in bees (>11,600 species) but extremely rare in flies (~1 species of hoverfly). Why the use of floral vibrations to collect pollen is so rare among flies is currently unknown. A hypothesis proposed to explain why flies do not engage in vibratile or buzz pollination is that they are unable to reach the vibration amplitude required to expel pollen from anthers. Here we document, for the first time, the mechanical properties of non-flight thoracic vibrations produced by hoverflies and compare them to the vibrations produced by buzz-pollinating bees under similar contexts (defence buzzes). We analysed ~4,000 vibrations produced by nearly 300 individuals representing 20 species of hoverflies and 22 bee taxa, recorded using a miniature piezoelectric accelerometer. We characterised both frequency and acceleration amplitude components of non-flight thoracic vibrations and their relationship to insect size. Our results show that, after accounting for size, buzz-pollinating bees and hoverflies produce vibrations with similar acceleration. We show experimentally that the acceleration amplitude produced by some hoverflies is sufficient to elicit pollen release from buzz-pollinated flowers (Solanum dulcamara and S. rostratum). Our study does not support the hypothesis that the dearth of buzz-pollinating flies is caused by their inability to produce vibrations of sufficient amplitude. We discuss alternative hypotheses to explain why most flies do not engage in buzz pollination and suggest that the lack of buzz-pollinating flies might be best explained through their life history.