The spatial aggregation of phytophagous insects driven by the evolution of preference for plant chemicals

Ecologists have shown considerable interest in the spatial patterns of organism distribution and the processes responsible for their formation and maintenance. The phytophagous insects typically use chemicals in plants as host-finding cues. Because nonvolatile chemicals remain near the source, the spatial structure of plant community determines the local distribution of insects. In addition, the plant chemical accumulation due to plant-plant interaction also influences the distribution of insects. In Rumex obtusifolius, for example, the production of phenolics is mediated by conspecific interaction. Rumex plants with high phenolic concentrations are preferred by the leaf beetle Gastrophysa atrocyanea, resulting in its spatial aggregation. Although this preference of beetles for nonvolatile chemicals should be beneficial in finding host plants, there is also a cost in terms of intraspecific competition among the beetles due to aggregation on certain chemical-rich hosts. To investigate the evolutionary significance of preference for nonvolatile chemicals and the ecological consequence of spatial distribution in leaf beetles, we constructed a mathematical model for the joint evolution of two preferences for plant size and chemical condition. In the model, beetles choose a resource based on the size and chemical concentrations of plants and are exposed to resource competition. Host plants accumulate the chemicals when they interact with neighboring conspecifics, and hence the level of chemical accumulation varies depending on the species composition and spatial distribution of the plant community. As a result, beetles became more sensitive to chemicals when the host species was rare and sparsely distributed in the community. The evolution of high chemical preference caused the aggregation of beetles and hence population size declined. We proposed a potential mechanism that underlies aggregated distribution in phytophagous insects, driven by the evolution of chemical preferences in response to plant community structure.