Treehoppers (Publilia concava) are homopteran insects that feed on the phloem sap of tall goldenrod (Solidago altisima) and excrete excess sugar as honeydew. Several genera of ants (Myrmica, Formica, Camponotus, etc.) form mutualistic associations with treehoppers in which ants consume honeydew and protect treehoppers from predation. Because treehopper densities are aggregated at the scale of goldenrod stems, and because the benefit of mutualism depends on density, aggregation dynamics play an important role in this mutualism. However, the processes that generate this aggregation are not well understood. To address this, individually marked treehoppers were tracked to evaluate proximate mechanisms influencing movement decisions. In addition, competing models of treehopper movement were evaluated based on the distribution of treehoppers resulting from confined populations in netted enclosures. Results/Conclusions

Analysis of the field experiment showed that treehoppers were less likely to move if they were ant-tended (odds ratio 0.109, 95% CI: 0.049–0.226). These results support a role for ant presence in determining movement behavior, and thus aggregation. To evaluate whether treehopper response to ant presence was sufficient to explain observed patterns of aggregation, individual-based models were designed to embody competing hypotheses for the processes driving aggregation. These models were stochastic and lacked closed-form solutions; an optimized Gillespie algorithm was written for exact stochastic simulation. Using empirical maximum likelihood methods, the models were confronted with stem occupation data from the enclosures. A null model was rejected and the most strongly supported model had ant discovery as the initiator of treehopper aggregation.