Pollination in Jacaranda rugosa (Bignoniaceae): euglossine pollinators, nectar robbers and low fruit set.

Nectar robbers access floral nectar in illegitimate flower visits without, in general, performing a pollination service. Nevertheless, their effect on fruit set can be indirectly positive if the nectar removal causes an incremental increase in the frequency of legitimate flower visits of effective pollinators, especially in obligate outcrossers. We studied pollination and the effect of nectar robbers on the reproductive fitness of Jacaranda rugosa, an endemic shrub of the National Park of Catimbau, in the Caatinga of Pernambuco, Brazil. Xenogamous J. rugosa flowers continuously produced nectar during the day at a rate of 1 mul.h(-1). Female and male Euglossa melanotricha were the main pollinators. Early morning flower visits substantially contributed to fruit set because stigmas with open lobes were almost absent in the afternoon. Ninety-nine per cent of the flowers showed damage caused by nectar robbers. Artificial addition of sugar water prolonged the duration of flower visits of legitimate flower visitors. Removal of nectar, simulating the impact of nectar robbers, resulted in shorter flower visits of euglossine bees. While flower visits of nectar-robbing carpenter bees (Xylocopa frontalis, X. grisescens, X. ordinaria) produced only a longitudinal slit in the corolla tube in the region of the nectar chamber, worker bees of Trigona spinipes damaged the gynoecium in 92% of the flowers. This explains the outstandingly low fruit set (1.5%) of J. rugosa in the National Park of Catimbau.