Spatial and Temporal Dynamics of a Fungal Pathogen Promote Pattern Formation in a Tropical Agroecosystem

Recent studies have shown that the spatial pattern of nests of an arboreal ant, Azteca instabilis (Hymenoptera: Formicidae), in a tropical coffee agroecosystem may emerge through self-organization. The proposed self-organization process involves both local expansion and density-dependent mortality of the ant colonies. We explored a possible mechanism for the density-dependent mortality involving the entomopathogenic fungus Lecanicillium lecanii. L. lecanii attacks a scale insect, Coccus viridis (Coccidae, Hemiptera), which is tended by A. instabilis in a mutualistic association. By attacking C. viridis, L. lecanii may have an indirect, negative effect on ant colony survival. To explore this hypothesis, we conducted investigations into the spatial and temporal distributions of L. lecanii. We measured incidence and severity at 4 spatial scales: (1) throughout a 45 hectare study plot; (2) in two 40 X 50 meter plots; (3) on coffee bushes within 4 m of two ant nests; and (3) on individual branches in a single coffee bush. The plot-level censuses did not reveal a clear spatial pattern, but the finer scale surveys show distinct patterns in the spread of infection over time. We also developed a simple cellular automata model of the coupled ant nest-L. lecanii system which is able to produce spatial patterns qualitatively and quantitatively similar to that found in the field. The accumulated evidence suggests that L. lecanii may very well be responsible for the density-dependent control thought necessary for spatial pattern formation of ant nests in this system.