Do extrafloral nectaries distract ants from visiting flowers? An experimental test of an overlooked hypothesis

When ants visit flowers, they can interfere with plant reproduction. Some plant species possess adaptations, such as specialized floral morphologies, that reduce visitation by unproductive or injurious insects. Kerner (1878) hypothesized that extrafloral nectaries, by distracting ants from foraging at floral nectaries, might also serve as an adaptation to reduce ant visitation to flowers. The viability of this hypothesis rests, in part, on ant foraging behaviour. When presented with extra nectar sources, do fewer ants visit each nectary or does recruitment keep visitation constant? Using artificial plants placed near free-living colonies of two ant species, we varied the presence, absence and quality of extra nectar sources and monitored visitation to primary, invariable nectar sources, representing floral nectaries. Significantly fewer ants of both species visited primary nectaries when extra nectar sources were present. Variation in the quality of the extra nectar sources had no significant effect on the number of ants visiting primary nectaries, but did affect visitation to extra nectar sources by one of the two species. Workers of Forelius sp. 1 strongly preferred sugar solutions with amino acids over sugar solutions alone and significantly greater numbers of Forelius workers visited artificial plants that provided amino acids than plants in other treatments. In contrast, workers of Formica perpilosa showed no significant preference among nectar types and neither the presence nor the quality of extrafloral nectar significantly affected numbers of ants on artificial plants. The results suggest that extrafloral nectaries could benefit plants by reducing the number of ants visiting flowers.