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Mechanisms of plant resistance: Plant architecture alters herbivory on a coastal shrub.
Rudgers, Jennifer*,1, 1 Indiana University, Bloomington, IN
ABSTRACT- Plants possess many traits that influence their resistance to herbivores. Traditionally, traits such as plant secondary metabolites and physical deterrents (thorns, trichomes) have received the most attention in both natural and agricultural systems, with less consideration given to characteristics of plant architecture. Plant architecture may have particularly important effects on gall-forming insects, which benefit by causing plants to reallocate resources. Architectural traits that reduce the impact of herbivores on plant fitness are expected to experience positive selection; these traits may work directly against herbivores or indirectly by modifying the effectiveness of herbivores' enemies, such as predators or parasitoids. Ultimately, differential herbivory may contribute to the maintenance of architectural polymorphisms. The perennial shrub, Baccharis pilularis (Asteraceae, coyote bush), grows as two distinct architectural forms in coastal California, USA: an upright form (1-4 m tall) and a prostrate form (0.1-0.2 m). Differences in height, branching architecture, and leaf size persisted in a common garden comprised of multiple clones of the two forms, suggesting these architectural traits are genetically based. In a four-year field experiment, I showed that plant architecture affected resistance to two gall-forming herbivore species. The upright form supported higher densities of lepidopteran stem-galls (Gnorimoschema baccharisella), whereas the prostrate form received more bud galls formed by midges (Rhopalomyia californica, Diptera). Furthermore, for the midge, architecture did not alter the effectiveness of parasitoids, which cause the majority of mortality in the field. Experimental reductions and additions of galls over four years demonstrated that the prostrate form always had greater reproductive success than the upright form, regardless of the level of herbivore damage. Thus, although plant architecture strongly influenced the distribution and abundance of galling insects, mechanisms other than differential herbivory must explain the maintenance of the architectural polymorphism in Baccharis.
Key words: Baccharis, polymorphism, galling insects, parasitoids