COS 29-4
Timing matters: Seasonality of defensive induction and herbivore community assembly in Asclepias eriocarpa
Plants attacked by herbivores can display either induced resistance or susceptibility to future herbivory. The amount of future herbivory experienced, and thus the effectiveness of defensive induction, depends not only on the plants allocation toward defense, but also the presence and preferences of future herbivores. In many perennial plants, defensive traits, herbivore abundance, and herbivore preference can change dramatically within the growing season. Despite high variation in herbivory between seasons, little research has focused on the seasonality of defensive induction and subsequent community assembly. Our study investigates changes in future herbivory, plant defensive traits, and herbivore community assembly in response to simulated damage at 6 times of the growing season in Asclepias eriocarpa (Apocynaceae). Each month, from April-September, we applied simulated herbivore damage to a subset of experimental A. eriocarpa stems at the UC Hastings Reserve, CA. During each of 6 monthly visits, we measured plant defensive traits, herbivore damage, and herbivore community assembly, on both previously damaged and unmanipulated stems, to test the seasonality of defensive induction.
Results/Conclusions
We found that A. eriocarpa increased in leaf toughness while decreasing in density of trichomes through the growing season, as leaves expanded and toughened. Latex exudation peaked midseason, and declined during the long post-flowering senescent period. Abundance of a specialist herbivore, Chrysochus cobaltinus, showed a rapid increase as plants grew and simultaneously increased their defenses.
Induction treatments occurring early in the growing season resulted in higher future herbivory, suggesting induced susceptibility may occur in this species, but only in response to early season damage. These results demonstrate that both plant and herbivore traits determining the intensity of herbivory vary greatly across seasons and that the timing of herbivore attack is important in determining future plant and herbivore fitness.