Why does the flower of Lonicera japonica open at dusk?

Abstract
We investigated contributions of diurnal pollinators and a nocturnal pollinator to pollen transfer in Lonicera japonica (Caprifoliaceae), whose flowers have traits typical of the hawkmoth-pollination syndrome. Diurnal bees, Tetralonia nipponensis and Lasioglossum sp., delivered more pollen grains than the nocturnal hawkmoth Theretra japonica per visit, suggesting that these bees were also effective pollinators. However, these bees removed over 10 times more pollen at one visit than the hawkmoth. Because of the higher pollen consumption by these bees, anthesis at dusk is considered to be optimal for Lonicera japonica to maximize overall pollen transfer under the visitation of both nocturnal and diurnal pollinators. Tetralonia japonica dispersed color dye farther than the other pollinators, which suggests that the nocturnal pollinator contributes more to cross-pollination than the diurnal pollinators.Key words: Lonicera japonica, hawkmoth-pollinated flowers, pollinator efficiency, pollen removal, pollination syndromes, timing of anthesis.