Nocturnal pollination by the carpenter bee xylocopa tenuiscapa (Apidae) and the effect of floral display on fruit set of heterophragma quadriloculare (Bignoniaceae) in India

Abstract

We provide the first data on nocturnal pollination by bees. Heterophragma quadriloculare is a self-incompatible hermaphroditic tree solely pollinated at night by the carpenter bee Xylocopa (Mesotrichia) tenuiscapa Westwood, whose pollinating flights were lunar insensitive and unaffected by low nighttime temperatures (2-14°C). The number of carpenter bees visiting a tree per minute and the number of flowers visited per foraging bout were positively related to the size of the floral display. We found that floral display size was negatively related to fruit set but positively related to absolute fruit numbers. In pollen carryover experiments, cross-pollen did not travel far in sequences of non-emasculated flowers (up to the 4th flower), but traveled farther in emasculated flower sequences (up to the 15th flower) as evidenced by fruit set in these sequences. The relatively poor fruit set on trees with many open flowers may have resulted from deposition of self-pollen on touch-sensitive stigmas, which close on first contact. Because large-girth trees tend to have large floral displays, trees may increasingly realize a higher percentage of fitness through male rather than female function as they age. The steady-state flowering phenology of H. quadriloculare may limit reduction in the female function compared to the cornucopia strategy of many other members of the Bignoniaceae and may also provide a reliable resource for X. tenuiscapa.