The spatial distribution of nests of an arboreal-nesting ant, Azteca instabilis, in a coffee agroecosystem in Mexico has been shown to play an important role in maintaining natural pest control. Determining the factors influencing the generation of this spatial distribution, therefore, is crucial for understanding how ecosystem services are maintained in this system. The low-density, clumped distribution of nests is thought to arise through a self-organization process involving local expansion of nest clusters and density-dependent mortality of colonies. The density-dependent mortality may be driven by local epizootics of the entomopathogenic fungus Lecanicillium lecanii, which attacks the mutualistic partner of the ants, the green coffee scale Coccus viridis. In this study, we artificially induced epizootics of L. lecanii by spraying a suspension of spores and monitored the change in foraging behavior of A. instabilis. We tested the hypothesis that A. instabilis colonies maintain their original foraging behavior despite an epizootic, i.e., they do not adapt, resulting in reduced intake of the carbohydrate-rich honeydew provided by the scale insects. Results consistent with this hypothesis would provide additional support for the hypothesis that L. lecanii is the agent responsible for the density-dependent mortality of ant colonies.
A. instabilis foraging activity decreased significantly at two of four experimental sites compared to a control site. In the third site, there was a decrease in activity, but it was not significant. At the fourth site, there was a non-significant increase in activity. There was a significant correlation between the change in foraging activity and the availability of shade trees, which may provide alternative carbohydrate resources via extrafloral nectaries or other scale insects. The availability of alternative resources was estimated using a shade tree resource index that was a function of the number and proximity of shade trees. These results suggest that the degree to which an A. instabilis colony is able to adapt to a local epizootic of L. lecanii depends on accessibility of neighboring shade trees. If shade trees are abundant and nearby, A. instabilis can respond to an epizootic by abandoning the affected coffee bushes and shifting its foraging activity to the shade trees. In the absence of the foraging refugia provided by shade trees, the colony is forced to continue tending the scale insect populations decimated by the epizootic, leading to a reduction in food intake and possibly the eventual death of the colony.