HOST-PLANT MEDIATION OF INSECT MUTUALISMS - VARIABLE OUTCOMES IN HERBIVORE-ANT INTERACTIONS

I present and evaluate the hypothesis that host plants indirectly affect the fitness of many homopterans and lepidopterans by mediating the outcome of their interactions with ants. Two conditions must be met to support this contention. First, the attractiveness of herbivores to ants must vary predictably with intra- and/or interspecific changes in host-plant quality. Such variation will occur if a) the chemical composition and/or quantity of ant attractants (excretions and secretions) produced by herbivores varies with changes in host quality and b) ants preferentially tend those herbivores that produce the most nutritionally rewarding attractants. Second, the number of tending ants must have a significant effect on the fitness of herbivores. While there is abundant evidence to support these individual conditions, few studies have examined both conditions for a single herbivore-ant association. I outline a series of testable predictions based on the host-mediation hypothesis. 1) The effects of host plants will vary with the degree to which herbivores depend on ants. 2) Hosts will affect the ability of herbivores to compete for ant mutualists. 3) Tended herbivores will exhibit host-selection behaviors that increase their ability to attract ants. And most importantly, 4) variation in host quality will predict fluctuations in the strength and sign of herbivore-ant interactions. I present a graphical model to illustrate 1) the ways in which plant-induced variation in the value of attractants to ant colonies can be used to predict switchpoints in the behavior of ants and 2) the effects of distance to servicing ant nests and nutritional status of these colonies on the predictions.