Sequential expression of multiple herbivore-induced defensive traits confers sustained resistance
J. MUTZ, S. KALISZ
Department of Ecology and Evolutionary Biology, University of Tennessee, 1416 Circle Drive, Knoxville, TN 37996, USA

Plant induced responses to herbivory often involve multiple chemical or structural defense traits, which together increase resistance to future attack. Differences among traits in induction timing, efficacy, or sensitivity to herbivore density could influence the strength or duration of resistance and the overall costs of mounting a response. We used a field experiment to investigate how multiple induced defenses change through time, as a function of herbivore density and at the scale of genetically variable plant populations. We established 70 replicate populations of upland cotton, Gossypium hirsutum, each consisting of ten plants caged in mesh. We added Spodoptera exigua neonates to cages at one of four densities, then measured resistance (via bioassays with naïve caterpillars), and defenses (chemical and structural) at five timepoints. Mean induced resistance increased with herbivore density and was sustained from 4-12 days following caterpillar introduction, then decayed to baseline after 21 days (coincident with caterpillar pupation). In contrast, structural defenses were expressed as consecutive short-lived peaks, with trichome densities greatest at four days and terpenoid gland densities greatest at seven days. Offsets in the timing of peak defense induction may help maintain high resistance through time (i.e., across herbivore development) while minimizing energetic or ecological costs.