Lack of floral constancy by bee fly pollinators: implications for ethological isolation in an African daisy

Although divergent floral preferences of pollinating species is a likely mechanism maintaining integrity of co-occurring floral ecotypes or incipient species, reproductive isolation of floral morphs could also result from temporary floral constancy of shared pollinators. Context-dependent floral preferences have been demonstrated for many pollinating insects, suggesting that this behavior might be widespread. However, it is not yet known whether 2 plant species or morphotypes that share a single pollinator could be reproductively isolated through assortative visitation. We investigated the foraging behavior of the bee fly, Megapalpus capensis, in experimental arrays of distinct floral morphotypes of Gorteria diffusa, a South African daisy which is pollinated almost exclusively by M. capensis. Flies exhibited no intrinsic or context-dependent (according to prevailing morph) preference for floral variants nor do they display floral constancy behavior during visitation bouts. Thus, pollinator isolation through assortative visitation is an unlikely explanation for the maintenance of floral integrity across the narrow contact zones encountered in Gorteria. Despite random visitation patterns on arrays, flies exhibited different behaviors on different floral morphotypes, suggesting intrinsic responses to morphotype-specific floral cues. Our findings suggest that results of previous studies demonstrating labile preferences and constancy in insects such as bees and butterflies cannot necessarily be extrapolated to other important groups of pollinators. In pollinators, such as Megapalpus, which use flowers as arenas for multiple behaviors, including mating, and feed during long stationary bouts, learning constraints on flower handling time, the most widely accepted explanation for floral constancy, is unlikely to drive constant foraging behavior.