I used a host-visitor modeling framework to examine the interaction between the treehopper Publilia concava and ants in the genus Formica. In particular, I tested the functional relationship between ant tending, the spatial distribution of treehoppers, and treehopper density. The per-capita density of ants at each host plant was a decreasing function of treehopper density, distance from the ant nest and the neighborhood density of treehoppers. Treehopper survivorship was proportional to the per-capita density of ants and the duration of ant tending. Consequently, treehoppers in low-density aggregations on isolated host plants near the nest received maximum benefit from ant tending. Treehoppers tended by the ant Formica integra were abandoned as the summer progressed, although many of these treehoppers were re-colonized by other species of ants. While F. integra ultimately abandoned all treehoppers, treehoppers on host plants with fewer initial ants were abandoned first. Results from the present study are consistent with previous findings suggesting that patterns of density-dependent benefit for homopterans are a function of the recruitment response of ants. Additionally, results suggest a tradeoff between maximizing the persistence or probability of ant-tending and minimizing competition for ants when tended. In general, host-visitor models of mutualism may provide a theoretical framework for understanding conditional outcomes in ant-homopteran, and other host-visitor mutualisms.