Wasp pollination and pollinator filtering by dense hairs at the floral tube entrance in Marsdenia tinctoria (Apocynaceae)

The physical filtering of pollinators is an important factor influencing pollination effectiveness. This study explored the potential functions of dense hairs that completely obstruct the entrance of floral tube in Marsdenia tinctoria by characterizing the flowers of this species, as well as its pollinators and their behavior. The corolla was white upon blooming in the morning, then turned yellow at night, and the flower finally dropped by the third morning. The hairs tended to be disheveled in yellow-petaled flowers. Pollination success increased with floral age. Direct observations of flowers in natural M. tinctoria populations over a period of 32 h recorded 126 visitors, of which 70% were wasps. We observed pollinia attached to the mouthparts of wasps, carpenter bees, and honeybees, but not to those of butterflies, moths, flies, or ants. Detailed examination of insect mouthparts and floral morphology indicated that insect visitors that acted as pollinators had mouthparts longer than the floral tubes, equipped with hairs to which pollinia could attach. The mouthparts of potter wasps were often covered with pollinaria, carrying on average 30-75 pollinia. The dense floral hairs were penetrated by large-bodied visitors, and blocked smaller visitors. Disturbance of these floral hairs allowed smaller insects to access nectar, suggesting that the hairs function in preventing nectar theft by smaller insects. This study presents the first case of wasp pollination in the genus Marsdenia and provides insights into the potential function of its dense floral hairs, a synapomorphy of this genus, in filtering floral visitors.