Small but spicy? Small ants repel herbivores fast of a Mexican endemic shrub

Ant–plant mutualisms are widespread, with over 4000 angiosperm species worldwide bearing extra-floral nectaries where sugar is secreted in exchange for protection against herbivores. Turnera velutina, an endemic Mexican shrub, has facultative mutualist interactions with different ant species, and these interactions have variable effects on its fitness. Specifically, the presence of Camponotus ants have been linked to the highest T. velutina’s fruit production, in comparison with the presence of other patrolling ant species. Whether this outcome is because Camponotus ants are the most aggressive, the largest, the fastest at locating herbivores, and/or more gregarious with many workers patrolling/attacking herbivores, has not yet been formally investigated. In this study, we assessed the defensive behavior and repelling capacity of six ant species against larvae of Euptoieta hegesia (Lepidoptera), the main herbivore of T. velutina in the studied population. Contrary to our expectations, Camponotus ants were not the most effective defenders, and they were only better than Cephalotes ants, a genus that has been found to be opportunistic in other plant systems. Surprisingly, the two smallest ants, Dorymyrmex bicolor and Brachymyrmex sp., were the best defenders and the fastest at repelling herbivores, partly because many workers simultaneously attacked the larvae. We discuss how different ants may impact various aspects of a species’ fitness, highlighting the dynamic and complex outcomes of ant–plant interactions, as a function of species identity and their impacts on herbivores.