Induced Defense in White Oak: Effects on Herbivores and Consequences for the Plant

Abstract

Many plants respond to herbivory by decreasing the nutritional quality of their foliage and increasing production of allelochemicals. These induced responses are defensive if they reduce future tissue loss or disperse future damage and, as a result, increase fitness relative to conspecifics with a weaker induced response. We assessed three predicted effects of early-season damage in white oak (Quercus alba L.): (1) early-season damage elicits both systemic and localized responses in leaf quality; (2) mid- and late-season herbivores respond to changes in leaf quality both among and within trees; and (3) plants with high early-season damage suffer less subsequent tissue loss than plants with low early-season damage. In a field experiment, we manipulated levels of early-season damage in white oak saplings and monitored subsequent changes in leaf quality, herbivore distributions, and plant damage levels at two spatial scales: (1) between experimental and control trees differing in overall level of early-season damage, and (2) between damaged and undamaged leaves within individual trees. One-third of the plants had herbivores removed at 2–3 d intervals throughout the early-season, one-third had damage from natural densities of herbivores plus additional damage from hole-punching, and one-third were attacked by natural densities of herbivores and served as controls. Early-season treatments influenced leaf quality both among and within trees. We observed differences in protein-binding capacity (PBC) among treatments, indicating a systemic response to early-season damage for this trait. The treatment with the most early-season damage (control) had higher average protein-binding capacity than the treatment with the lowest level of early-season damage (herbivore-removal treatment). Within trees, damaged leaves (>10% leaf area missing) had higher protein-binding capacity and lower nitrogen content than intact leaves (≤10% leaf area missing), suggesting localized induction to herbivory for these traits. Despite differences in protein-binding capacity among treatments, herbivore densities did not differ among treatments at any time following the early season. In addition, species composition of herbivores did not differ among treatments. However, within individual trees, herbivore densities were higher on intact leaves than on damaged leaves. This finding is consistent with patterns of leaf quality: intact leaves were higher quality (i.e., more nitrogen and lower PBC) and had higher herbivore densities than damaged leaves. Because herbivores appeared to avoid previously damaged leaves, induction may have led to dispersed damage in this study. Finally, saplings that suffered the highest level of early-season damage (controls) had less subsequent damage than saplings with the lowest amount of early-season damage (herbivore-removal). This result supports the hypothesis that leaf quality changes following insect herbivory reduce subsequent damage to individual plants.