Interactions between insect herbivores and a plant architectural dimorphism

1 Plants possess many traits that influence their resistance to insect herbivores and thus can shape the structure of herbivore assemblages. While plant chemistry and structural defences have received the most attention, plant architecture may also be important. We examined patterns and consequences of insect herbivory for two distinct architectural morphs (prostrate or erect) of the perennial shrub, Baccharis pilularis (Asteraceae, coyote bush) in coastal California, USA. 2 For B. pilularis, differences in plant height, branch architecture and leaf size persisted in a common garden, consistent with previous evidence suggesting a genetic basis for dimorphism in plant architecture. 3 In both naturally occurring plants and in a common garden experiment, the composition of the herbivore assemblage varied strongly with plant architectural morph. Prostrate plants attracted higher densities of a gall-forming midge (Rhopalomyia californica, Diptera), whereas erect plants supported more moth galls (Gnorimoschema baccharisella, Lepidoptera) and experienced greater folivory. Furthermore, architectural traits were correlated with herbivory levels both across and within architectural morphs. 4 Prostrate plants had greater reproductive output (flowers or seeds) than erect plants in the presence of herbivores. However, under experimental reductions of herbivory spanning 40 months, the morphs had similar reproductive output, demonstrating that herbivores have the potential to act as agents of selection on the dimorphism in plant architecture. 5 These results confirm that intraspecific variation in plant traits can shape the composition of herbivore assemblages. Furthermore, in this system, herbivores alter the performance of the architectural morphs, suggesting a dynamic system of feedbacks between the population genetic and community levels.