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In this study, we evaluated the effects of the secretory activity of nectaries on multitrophic interactions involving ants and herbivores foraging on individuals of Prosopis laevigata (Fabaceae) located in the arid Tehuacan-Cuicatlan valley, Mexico. We observed that the secretory activity of nectaries of Prosopis laevigata appeared to follow the predictions of the ’Optimal Defense Theory’. Specifically, our results suggest that due to high temperature and low humidity during the day, nectaries were more active at night, precisely when herbivores and ants were more abundant. Moreover, protective ants were always more abundant in the apical zones of the branches where youngest leaves are most susceptible to attack by herbivores. We hypothesized that Prosopis laevigata could be optimizing nectar secretion in order to allocate defenses to maximize their individual fitness. Our study contributes to our understanding of how Optimal Defense Theory could shape multitrophic interactions between ants, plants, and herbivores mediated by secretory activity of nectaries in arid environments. (C) 2014 Elsevier Ltd. All rights reserved.