While the concept of competition for mutualists has attracted considerable attention during the past century, few studies have provided direct experimental evidence documenting its occurrence. This absence is particularly evident for homopteran-ant associations. The proposed research is designed to experimentally evaluate the hypothesis that neighboring homopteran aggregations intraspecifically compete for the services of an ant mutualist. Manipulative experiments have previously demonstrated that the membracid Publilia modesta (Homoptera) benefits from being tended by the ant Formica occulta in most years. A series of experiments will evaluate the following hypotheses: (1) ants are a limited resource for membracids, (2) the number of ants tending membracids, and not just the presence of ants, has a significant impact on membracid fitness, (3) intra- and interspecific variation in host-plant quality influences the chemical composition of membracid honeydew and therefore the ability of membracids to compete for ants, and (4) tending ants benefit membracids by reducing the impact of predatory spiders (Pellenes sp.). Such experimental studies will provide needed insight into the dynamics of mutualistic interaction in spatially-variable environments.
