Delayed chemical defense: timely expulsion of herbivores can reduce competition with neighboring plants

Time delays in plant responses to insect herbivory are thought to be the principal disadvantage of induced over constitutive defenses, suggesting that there should be strong selection for rapid responses. However, observed time delays between the onset of herbivory and defense induction vary considerably among plants. We postulate that strong competition with conspecifics is an important co-determinant of the cost-benefit balance for induced responses. There may be a benefit to the plant to delay mounting a full defense response until the herbivore larvae are mobile enough to leave, and large enough to cause severe damage to neighboring plants. Thus, delayed responses could reduce the competitive pressure on the focal plant. To explore this idea, we developed an individual-based model using data from wild tobacco, Nicotiana attenuata, and its specialized herbivore, Manduca sexta. Chemical defense was assumed to be costly in terms of reduced plant growth. We used a genetic algorithm with...