Plant induced responses to herbivory and their effects on insect population dynamics

When plants are exposed to environmental stresses, such as insect herbivores, they change their primary and secondary metabolism. Some of these changes are functional as direct (e.g. toxic and anti-digestive compounds, low nutritional value) and indirect resistance (e.g. increased extrafloral nectar production, herbivore –induced volatile organic compound production) while other metabolic adjustments (e.g. over compensation, reduced secondary metabolite production) can increase plant palatability. As a consequence insects are exposed to a different nutritional and interactional environment when feeding on plants that have previously been damaged by other herbivores of the same or different species. Ecologists and entomologists have long been asking in how far herbivore-induced changes in plant metabolism can affect insect herbivore population dynamics.

Here we present tall goldenrod, Solidago altissima L. (Asteraceae) and one of its most important herbivore species, the chrysomelid beetle Trirhabda virgata LeConte, 1865, as a model to link plant induced responses to herbivore damage with spatial and temporal dynamics of insect herbivores. Specifically, we demonstrate significant effects of plant induced responses on the movement of larval T. virgata throughout the plant population with significant consequences on the distribution of plant tissue damage. Moreover, we hypothesize rapid evolution and/or behavioural and physiological adjustment of beetle behaviour and metabolism to the induction status of the plant population (e.g. beetle/damage density) as major predictors of critical population density and the resulting insect population outbreaks. We demonstrate that plant chemistry, altered by insect attack, is one of the major factors driving insect population dynamics on a spatial and temporal scale.