Individual-level differences in generalist caterpillar responses to a plant-plant cue

1. Plant-plant communication has been found to affect interactions between herbivores and plants in several model systems. In these systems, herbivore-induced volatile chemical cues are emitted and perceived by other plants (receivers), which subsequently change their defensive phenotypes. Most studies have focused on how the effects of volatile cues affect plant damage, whereas herbivore performance has rarely been examined. 2. In this study, it is shown that plant-plant communication between willows reduced the growth rate, feeding rate, and conversion efficiency of some individuals but not others of a generalist caterpillar, Orgyia vetusta. 3. Using a paired, no-choice trial design, there was substantial variation between caterpillar individuals in their response to willows that had been induced with a volatile plant-plant cue. This variation was explained by feeding parameters of the individual herbivores. Individuals behaved similarly when fed induced and non-induced willow leaves. Specifically, growth rates of caterpillars that grew rapidly on non-induced willow leaves were negatively affected by plant-plant cues, but growth rates of caterpillars that grew slowly on non-induced willow leaves were not affected by the responses to volatiles from neighbouring willows. 4. Induction by volatile plant-plant cues reduced the growth rates of those individual herbivores that caused the greatest damage to willow, but had little effect on weak growers.