Visual signalling of nectar-offering flowers and specific morphological traits favour robust bee pollinators in the mass-flowering tree Handroanthus impetiginosus (Bignoniaceae)

Abstract

Mass flowering is a widespread blooming strategy among Neotropical trees that has been frequently suggested to increase geitonogamous pollination. We investigated the pollination ecology of the mass-flowering tree Handroanthus impetiginosus, addressing its breeding system, the role in pollination of different visitors, the impact of nectar robbers on fruit set and the function of colour changes in nectar guides. This xenogamous species is mainly pollinated by Centris and Euglossa bees (Apidae) seeking nectar, which are known to fly long distances. The flowers favour these bees by having: (1) a closed entrance in newly opened flowers which provides access only to strong bees capable of deforming the flower tube; and (2) a nectar chamber that is accessible only to long-tongued bees. Only first-day flowers with yellow nectar guides produce nectar. Pollinators prefer these flowers over second- and third-day flowers with orange and red nectar guides, respectively. Nectar robbers damage two-thirds of the flowers and this robbing activity decreases fruit set by half. We attribute the low fruit set of H.impetiginosus to the intense nectar robbing and hypothesize that visual signalling of nectar presence in newly opened (receptive) flowers reduces geitonogamy by minimizing bee visits to unrewarding (non-receptive) flowers.