Drought stress drives intraspecific choice of food plants by Atta leaf-cutting ants

Leaf-cutting ants (LCA) are polyphagous and dominant herbivores throughout the Neotropics that carefully select plant individuals or plant parts to feed their symbiotic fungus. Although many species-specific leaf traits have been identified as criteria for the choice of food plants, the factors driving intraspecific herbivory patterns in LCA are less well studied. Herein, we evaluate whether or not drought-stressed native plants are a preferred food source using free-living colonies of two leaf-cutting ants, Atta sexdens L. (Hymenoptera: Formicidae: Attini), in combination with five plant species, Ocotea glomerata Nees (Lauraceae), Lecythis lurida S. A. Mori (Lecythidaceae), Miconia prasina DC (Melastomataceae), Tovomita brevistaminea Engl. (Clusiaceae), and Tapirira guianensis Aubl. (Anacardiaceae), and Atta cephalotes L., in combination with two plant species, O. glomerata and Licania tomentosa Benth. (Chrysobalanaceae). In dual-choice bioassays, ants removed about three times more leaf area from drought-stressed plants compared to control plants. Both leaf-cutting ant species consistently preferred drought-stressed plants for all species tested, except T. guianensis. The mean acceptability index - expressing the preference for one of two options on a scale of 0 to 1 - of drought-stressed plants ranged from 0.65 to 0.86 across plant species, and the preference did not differ significantly among the tested plant species. Our results suggest that selection of drought-stressed individuals is a general feature of food plant choice by leaf-cutting ants irrespective of ant or plant species. As human-modified forest assemblages across the Neotropics are increasingly prone to drought stress, the documented preference of Atta for drought-stressed plants may have tangible ecological implications.