Conspicuous extra-floral nectaries are inducible in Vicia faba

Mutualistic interactions are dynamic associations that vary depending on the costs and benefits to each of the interacting parties. Phenotypic plasticity in mutualistic interactions allows organisms to produce rewards to attract mutualists when the benefits of their presence outweigh the costs of producing the rewards. In ant–plant defensive mutualisms, defences are indirect as plants produce extra‐floral nectaries (EFN) to attract predatory ants to deter herbivores. Here we demonstrate that in broad bean, Vicia faba, the overall number of EFNs on a plant increases dramatically following leaf damage. In two damage treatments, removal of: (1) one‐third of one leaf in a single leaf pair or (2) one‐third of both halves of a single leaf pair, resulted in a 59 and 106% increase in the number of EFNs on the plants, respectively, over 1 week. We suggest that the increased production of visually conspicuous EFNs is an adaptive inducible response, to attract predatory arthropods when risk of herbivory increases.