Insect herbivore stoichiometry: the relative importance of host plants and ant mutualists

1. Mutualistic associations can vary over spatial and ecological gradients. For herbivorous insects that engage in mutualisms with ants, plant quality can be a particularly important source of variation, because of the upward transfer of nutrients from plants to herbivores to ants. 2. A previous study demonstrated that mutualistic ants, Formica obscuripes, exert a top-down effect on the carbon and nitrogen concentrations (stoichiometry) in an herbivorous membracid, Publilia modesta. We characterised the consequences of mutualism for carbon and nitrogen stoichiometry between the same species pair, yet on an alternative, geographically-distinct host plant. 3. We found no top-down effect of ants on the carbon or nitrogen in the herbivore, but a strong, bottom-up effect of individual plants on membracid nitrogen concentration. 4. These results suggests that spatial heterogeneity in host plant traits, and ultimately the diet breadth of herbivore mutualists, may be important factors mediating stoichiometric patterns in mutualistic associations.