Induced plant defence and the evolution of counter-defences in herbivores

We examine how induced plant defences affect the evolution of resistance in herbivores (i.e. the ability to overcome plant defences) compared with constitutive defence strategies. Since resistance of herbivores may evolve as a result of major monogenic and/or quantitative (polygenic or gene amplification) genetic sources, and the selective pressure imposed by plant defences affects the rate of evolution of each genetic source of resistance, we incorporate both into a model of herbivore evolution. We combine Mendelian single-locus and quantitative genetic models with a logistic population growth model based on an empirical plant–herbivore system. Induced defences may delay the evolution of both quantitative and major gene resistance and thus depress herbivore population size for more than 50 herbivore generations longer than constitutive defences. This increased longevity in the effectiveness of plant defence is associated with the production of substantially less plant defence over the long term, hence maximizing the benefit to cost ratio from the plant’s perspective.