Foraging Leaf-Cutting Ants Learn to Reject Vitis vinifera ssp. vinifera Plants that Emit Herbivore-Induced Volatiles
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Leaf-cutting ants (LCAs) are dominant herbivores of the Neotropics, as well as economically important pests. Their foraging ecology and patterns/mechanisms of food selection have received considerable attention. Recently, it has been documented that LCAs exhibit a delayed rejection of previously accepted food plants following treatment with a fungicide that makes the plants unsuitable as substrate for their symbiotic fungus. Here, we investigated whether LCAs similarly reject plants with induced chemical defenses, by combining analysis of volatile emissions with dual-choice bioassays that used LCA subcolonies (Atta sexdens L.). On seven consecutive days, foraging ants were given the choice between leaf disks from untreated control plants and test plants of Vitis vinifera ssp. vinifera L. treated with the phytohormone jasmonic acid (JA) to mimic herbivore attack. Chemical analysis revealed the emission of a characteristic set of herbivore-induced volatile organic compounds (VOC) from JA-induced plants. Dual-choice experiments indicated that workers did not show any preference initially, but that they avoided JA-treated plants from day five onwards. Our finding that A. sexdens foragers learn to avoid VOC-emitting plants, which are likely detrimental to their symbiotic fungus, represents the first evidence for avoidance learning in attine ants toward plants with induced defenses.
KeywordsAtta sexdens Avoidance learning Delayed rejection Food plant selection Herbivore-induced plant volatiles Induced defense hypothesis
We are grateful to Wilhelm Boland from the Max Planck Institute in Jena for providing us access to the volatile analysis equipment, as well as Maritta Kunert and Anja David for help with GC/MS analysis. We also thank our colleagues from the University of Kaiserslautern: Burkhard Buedel (Department of Plant Ecology & Systematics) for financial support, Hans Reichenberger and Beatrix Weber for laboratory assistance, and gardeners (botanical garden) for growing the experimental plants. The stockmen (animal houses, University of Kaiserslautern) and Annette Laudahn (Biocenter, University of Würzburg) provided considerable support in maintaining the laboratory colonies. This study was partially supported by funds from the German Research Foundation (SFB 1047/TP C1)