Mutualism, mutually beneficial interactions between species, is a fundamental category of between-species interaction. The treehopper Publilia concava feeds on tall goldenrod (Solidago altissima) in the northeastern United States, and participates in a mutualistic relationship with ants (primarily Myrmica and Formica spp.). Ants protect treehoppers from predation and facililate their phloem feeding. In return, the ants consume honeydew, a carbohydrate-rich byproduct of treehopper feeding. Theoretical studies have suggested that in protection-mutualisms such as this one, removal of top-down control by ants is destabilizing and may cause cycles in the plant and treehopper population. Exploratory analyses testing this hypothesis with results from this system suggested that, instead, treehoppers avoid ovipositing on plants with a history of herbivory, implying a strategy to deal with reductions in host quality. In this study, we explicitly tested the hypotheses that treehoppers avoid or suffer reduced performance on plants previously fed on by treehoppers, and that mutualism exacerbates this effect. We used a two-way factorial design manipulating ant and treehopper presence on goldenrod plants in one year, and assessing oviposition and survivorship on those same plants in the subsequent year, replicated over two years.
Results/Conclusions
Female treehoppers laid significantly more egg masses on plants that had not experienced herbivory the previous year, regardless of whether those previous treehoppers were tended by ants. In contrast, a prior history of herbivory had no impact on future treehopper performance. Survival rates of treehopper nymphs were not significantly affected by whether treehoppers had fed on that plant the previous year or not. Finally, evidence was mixed on the degree to which prior herbivory reduces plant quality. These results show that treehoppers avoid previous sites of herbivory, and suggest that these host plants might be reduced in quality. Further research is needed to better understand the decoupling of oviposition choices by females with an apparent lack of evidence that this host-choice behavior ultimately affects nymph survival.