Enforced herbivory on Canavalia ensiformis and Tithonia diversifolia and its effects on leaf-cutting ants, Atta cephalotes

Herbivory by leaf-cutting ants can lead to serious problems for agricultural and forest systems in the Neotropics. Chemical insecticides are the major, if not only, control tactic, but the efficiency of this approach is questionable. Moreover, continued reliance on insecticidal control leads to environmental damage and risks to human and animal health. Developing alternative management tactics for leaf-cutting ants is thereby a justifiable and sensible need. Recent findings suggest that the Mexican sunflower, Tithonia diversifolia and the jackbean, Canavalia ensiformis, have detrimental effects on leaf-cutting ant populations under laboratory and field conditions. It is unclear, however, whether those effects involve the development and growth of the ant's symbiotic fungus, the ants or both. To address this question, an experiment was conducted on 18 artificial Atta cephalotes colonies maintained with three feeding treatments: a control (mango, Mangifera indica), T. diversifolia and C. ensiformis. Six repetitions of each treatment were assigned to the colonies in a completely randomized design and evaluated over a period of 2 months under laboratory conditions. Both T. diversifolia and C. ensiformis negatively altered growth of the symbiotic fungus Leucoagaricus gongilophorus and thereby affected the survival of A. cephalotes. The greatest reduction in fungus size (up to 100%) and the highest ant mortality (83.3%) were observed in colonies fed on T. diversifolia, but not until 45 days after the start of the experiment. In colonies fed on C. ensiformis, the greatest reduction in fungus size (95.3%) and the highest ant mortality (66.6%) were reached at 57 days. During the course of the study, no negative effects were observed in the control colonies, which displayed a consistently healthy symbiotic fungus and low rate of ant mortality (16.6%). Based on these results, plants of both T. diversifolia and C. ensiformis offer promise for the biological control of A. cephalotes populations.