The roles of leaf defense and nutrients in plant-herbivore interactions: A study in the respone to varying herbivore intensity
Reciprocal evolution between herbivores and their host plants has given rise to considerable variety of plant defenses and herbivore counter defenses. Through numerous adaptations, herbivore specificity has risen, and may be trivial to the effectiveness of the host plant defenses against specialist, but not generalist, herbivores. Few studies have examined how plant defense mechanisms are affected by varying degrees of herbivory, or if there are threshold amounts of herbivory above which plant defenses are no longer beneficial. With this in mind, I conducted a field experiment using Chrysochus auratus and its host plant Apocynum cannabinum as model organisms. Stems of A.cannabinum were caged with varying C.auratus beetle densities ranging from 0,2,4 beetles. All stems were assessed before and after caging, with replicates of 10 per each beetle density. Data were collected in pursuit to assess the effects of varying herbivory on plant defense and nutritional quality through the assessment of latex exudation, cardenolide (chemical defense), water, and nitrogen concentration traits. Data were also collected to assess the effects of varied plant defense from different levels of herbivory on C.auratus reproduction by measuring female egg capsule production.
There was a significant effect of herbivore density on plant defenses and nutritional value. I found that with higher beetle densities, defense against folivores (measured as latex exudation) was reduced from 1.28mg exudate to 0.643mg in plants exposed to higher beetle densities. Leaf nutritional value (measured as nitrogen and water concentration) was reduced from 1.97% nitrogen to 1.40% in plants exposed to higher beetle densities. There was no significant effect of previous beetle herbivore density on beetle egg capsule production. My results suggest that, in a coevolved plant-herbivore system, later arriving herbivores would likely benefit from reduced plant defenses but also suffer from lowered nutritional value resulting from earlier herbivory.