Flower constancy in the hoverflies Episyrphus balteatus (Degeer) and Syrphus ribesii (L.) (Syrphidae)

The causes and consequences of flower constancy have been the focus of many studies, but almost all have examined the foraging behavior of bumblebees, honeybees, or butterflies. We test whether constancy occurs in an overlooked group of pollinators, the syrphid flies. Foraging sequences of wild flies of two species, Episyrphus balteatus and Syrphus ribesii, were examined when visiting flowers in seminatural plant communities and in artificial arrays of two color morphs of Lobularia maritima planted at a range of frequencies. Both species exhibited marked floral constancy when foraging in the mixed-plant community. Because all groups of pollinating insect so far examined exhibit constancy at least under some circumstances, we suggest that this is the predominant strategy used by pollinators and that there is probably a common explanation. Neither syrphid species exhibited constancy to different color morphs within a plant species, in contrast to previously published studies of Hymenoptera foraging among polymorphic flowers, which all describe positive frequency-dependent selection. Possible explanations for this discrepancy are discussed. We argue that constancy in these syrphids is unlikely to result from learning constraints on handling ability, currently the most widely accepted explanation for flower constancy, because they forage primarily for pollen which is easily located in most flowers they visit.