Behaviour of pollinator insects within inflorescences of Scrophularia species from Iberian Peninsula

Several hypotheses have been proposed to explain the pattern of pollinator visits to vertical inflorescences. These range from a response to a pattern of resources to merely instinctive behaviour. In dichogamous plants, such behaviour has been associated with promoting outcrossing and avoiding geitonogamy. We here analyse behaviour of the principal pollinator groups in five protogynous species of Scrophularia with different flower sizes (S. sambucifolia, S. grandiflora, S. lyrata, S. scorodonia and S. canina), and the distribution of sexual phases along the inflorescences. The results in all cases show that pollinators follow a pattern of ascending visits accompanied by movements between flowers of the same whorl (horizontal movements). The relative frequency of these horizontal movements depends on the flower size, with a higher frequency in species with large flowers. In vertical movements of the three more common pollinator groups to several plant species (bumblebees, wasps and small bees), the behaviour was essentially independent of flower size, with bumblebees having the highest ratio of ascents to descents. Behaviour of the pollinators, together with the absence of a definite pattern of distribution of the sexual phases along the inflorescence, implies that geitonogamy is not avoided in any of the Scrophularia species studied.