Plant volatiles mediate various interactions with animals. While volatiles emitted by vegetative plant parts effectively function as herbivore deterrents or as attractants for their enemies, flower volatiles are traditionally regarded as pollinator attracting signals. A huge body of detailed investigations has focused on floral adaptations like color, shape, nutritious rewards and scents on effective pollination. However, besides mutualists, non-pollinating flower visitors from several taxa are potentially interested in exploiting nectar, pollen and other floral tissues. The negative impact of such antagonists on plant reproduction may even exceed the benefits from mutualistic services. Since pollination is viewed as perhaps the classic example of mutualistic coevolution, antagonists have been largely ignored in the interpretation of floral traits. Flowers provide valuable resources for both pollinators and antagonists, and they need to synchronously attract pollinators and repel antagonists. Ants represent important floral antagonists that are detrimental for the plants’ reproduction since they exploit nectar without pollination service in return and thereby displace pollinators. A modified Pettersson four-arm olfactometer was used to test the hypothesis that floral scents effectively repel ants from visiting the flowers.
Results/Conclusions
The effect of floral scents on ants provided an unequivocal explanation for the distribution of Formica rufibarbis ants among nectar-bearing flowers in situ: flower scents of unvisited flowers were repellent in the olfactometer, while visited flowers were not. Furthermore, a broad spectrum of flowers (32 species) and individual floral scent compounds were screened for their effect on different ant species. More than two thirds of the floral bouquets tested were repellent against one or two ant species. Ant repellence was confirmed for individual terpenoids commonly found in floral scents. Moreover, food choice experiments with generalistic, flower feeding herbivores (bush crickets) revealed deterrent effects of floral scent compounds. A meta-analysis of 32 publications confirmed that complex floral scents provide a key solution for the conflicting tasks of synchronously attracting pollinators and repelling antagonists. These results suggest a scenario that fundamentally differs from the prevailing opinion on the ecological function and evolution of floral scents: they may function as allomones against enemies and not solely as synomones to attract mutualists.
