Plant resistance and predators influence the density dependence of herbivore survival and distribution of herbivory.

Abstract

Plant resistance and predators can influence density-dependent survivorship and growth of herbivores, and their damage to plants. Although the independent effects of plant resistance and predators on herbivores and herbivory are well known, little is known about their interactive and density-dependent effects on herbivores and the amount and distribution of damage on plants. These relationships are important for understanding how herbivore and plant populations influence each other. We used a laboratory density-manipulation experiment to determine how plant resistance (three treatments: jasmonate-insensitive, unmanipulated wild type, and jasmonate-sprayed wild-type plants) and predation (two treatments: predator or no predator) affect the survivorship and growth of an herbivore, as well as per capita damage and the distribution of damage on plants. We found evidence that the density dependence of herbivore survivorship was influenced by predators and an interactive effect of plant resistance and predation. Herbivore growth was reduced by higher plant resistance but was not density-dependent nor affected by predation. Per capita plant damage was reduced by plant resistance, predation, and herbivore density. The within-plant distribution of damage became more even with increasing herbivore density but was not affected by predation or the independent effect of plant resistance. The distribution of damage was also affected by an interaction between plant resistance and herbivore density; damage became less aggregated with density across all plant resistance treatments, but the decrease was strongest for the jasmonate-insensitive plants. These results show that predators influence herbivore density dependence, and that plant resistance can affect the impact of predators on herbivores. Though plant resistance, predation, and herbivore density all reduced per capita herbivore damage to plants, only herbivore density and plant resistance affected the distribution of damage. Distributions of herbivory can influence plant success; documenting patterns of herbivory is an under-appreciated avenue for integrating effects of plant resistance, predators, and herbivore density on plant-herbivore interactions.