Associational resistance and associational susceptibility: specialist herbivores show contrasting responses to tree stand diversification

Abstract

Heterospecific neighbors may reduce damage to a focal plant by lowering specialist herbivore loads (associational resistance hypothesis), or enhance damage by increasing generalist herbivore loads (associational susceptibility hypothesis). We tested the associational effects of tree diversity on herbivory patterns of the tropical focal tree Tabebuia rosea in an experimental plantation setup, which contained tree monocultures and mixed stands. We found higher herbivore damage to T. rosea at higher tree diversity, indicating that T. rosea did not benefit from associational resistance but rather experienced associational susceptibility. The specific consideration of the two dominant insect herbivore species of T. rosea, the specialist chrysomelid Walterianella inscripta and the specialist pyralid Eulepte gastralis, facilitated understanding of the detected damage patterns. Tree diversity exerted opposite effects on tree infestation by the two herbivores. These findings point to resource concentration effects for the chrysomelid beetle (favored by tree monoculture) and to resource dilution effects for the pyralid caterpillar (favored by tree mixture) as underlying mechanisms of herbivore distribution. A strong contribution of the pyralid to overall damage patterns in diversified stands suggests that associational susceptibility may not necessarily be related to higher abundances of generalist herbivores but may also result from specialized herbivores affected by resource dilution effects. Thus, the identity and biology of herbivore species has to be taken into account when attempting to predict damage patterns in forest ecosystems.

Growth, herbivore distribution, and herbivore damage of timber trees in a tropical silvopastoral reforestation system

“…Furthermore, W. inscripta tended to respond positively to tree height, suggesting that resource apparency might affect infestation by this herbivore. These indications support the view that tree infestation by W. inscripta is positively affected by resource concentration (Plath et al 2012), and they suggest that silvopastoral planting designs can serve as a strategy to reduce damage by these herbivores.…”

“…These findings support the observation that the key herbivores and resulting damage were not responding to the presence of companion trees but rather to the general planting design of the silvopastoral reforestation system, namely to the low density of timber host trees. Compared to a nearby highdensity timber plantation at the same study site (Plath et al 2012), the abundance of the lepidopteran key herbivore E. gastralis was markedly higher in the silvopastoral reforestation system (0.6-0.9 individuals/leaf) than in the high-density plantations (0.2-0.5 individuals/leaf). This higher abundance of E. gastralis on host trees growing at low density in the silvopastoral reforestation system strongly supports the assumption that E. gastralis follows a resource-dilution strategy to avoid enemies or competition (Plath et al 2012), and it rendered T. rosea timber trees in the silvopastoral reforestation system susceptible to damage by E. gastralis (76-85 % contribution to total leaf damage).…”

“…Compared to a nearby highdensity timber plantation at the same study site (Plath et al 2012), the abundance of the lepidopteran key herbivore E. gastralis was markedly higher in the silvopastoral reforestation system (0.6-0.9 individuals/leaf) than in the high-density plantations (0.2-0.5 individuals/leaf). This higher abundance of E. gastralis on host trees growing at low density in the silvopastoral reforestation system strongly supports the assumption that E. gastralis follows a resource-dilution strategy to avoid enemies or competition (Plath et al 2012), and it rendered T. rosea timber trees in the silvopastoral reforestation system susceptible to damage by E. gastralis (76-85 % contribution to total leaf damage). In contrast to E. gastralis, abundance of the coleopteran key herbivore W. inscripta was much lower in the silvopastoral reforestation system (0.06 to 0.12 individuals/ leaf) than in the high-density plantations (0.2 to 0.4 individuals/leaf).…”

“…Larvae of Eulepte gastralis Guenée (Lepidoptera: Pyralidae) and adults of Walterianella inscripta Jacoby (Coleoptera: Chrysomelidae) are the two most relevant herbivores of T. rosea in high-density reforestation plots at the same study site (Plath et al 2012). To assess their specific abundance and main activity period in the studied silvopastoral reforestation system, insect counts were conducted for all timber trees in the first two study years on a biweekly basis from April of year 1 to April of year 2, except from end of December of year 1 to mid-February of year 2 when T. rosea was seasonally leafless.…”

“…In year 4, we additionally quantified the leaf damage caused by the key herbivores E. gastralis and W. inscripta. Symptoms of damage by either key herbivore species E. gastralis and W. inscripta were assigned according to Plath et al (2012). Large-scale skeletonization, including continuously missing and necrotic areas, was characterized as damage by E. gastralis larvae, whereas small holes in the leaf were characterized as damage by W. inscripta.…”