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

The causes and consequences of flower constancy have been thefocus of many studies, but almost all have examined the foragingbehavior of bumblebees, honeybees, or butterflies. We test whetherconstancy occurs in an overlooked group of pollinators, thesyrphid flies. Foraging sequences of wild flies of two species,Episyrphus balteatus and Syrphus ribesii were examined whenvisiting flowers in seminatural plant communities and in artificialarrays of two color morphs of Lobularia maritima planted ata range of frequencies. Both species exhibited marked floralconstancy when foraging in the mixed-plant community. Becauseall groups of pollinating insect so far examined exhibit constancyat least under some circumstances, we suggest that this is thepredominant strategy used by pollinators and that there is probablya common explanation. Neither syrphid species exhibited constancyto different color morphs within a plant species, in contrastto previously published studies of Hymenoptera foraging amongpolymorphic flowers, which all describe positive frequency-dependentselection. Possible explanations for this discrepancy are discussed.We argue that constancy in these syrphids is unlikely to resultfrom learning constraints on handling ability, currently themost widely accepted explanation for flower constancy, becausethey forage primarily for pollen which is easily located inmostflowers they visit.