Induced responses to herbivory in wild radish: Effects on several herbivores and plant fitness

Induced responses to herbivory are changes in plant quality following initial damage by herbivores. These changes can negatively affect subsequent herbivores. I studied induced responses in annual wild radish plants, Raphanus raphanistrum and R. sativus (Brassicaceae), which included increased density and total number of setose trichomes on newly formed leaves of previously damaged plants compared to undamaged controls. I also characterized the effects of induced responses on the preference and performance of several herbivores and the net consequences for plant performance in the field. Wild radish plants damaged by caterpillars or sprayed with a natural plant response elicitor, jasmonic acid, supported reduced growth of generalist noctuid larvae compared to unmanipulated control plants. Induced responses did not affect the growth of specialist Pieris rapae larvae. In choice and no-choice experiments, induction reduced the feeding by noctuid larvae but did not reduce gross growth efficiency, an indicator of insect growth per unit of plant material consumed. Finally, colonization of plants by leafminers was lower on plants that received previous damage, compared to controls. In field experiments conducted over 2 yr, herbivory by naturally occurring grasshoppers was lower on induced plants than on both unmanipulated controls and leaf damage controls (plants with leaf area removed without the associated induced response). Plant fitness, as estimated by fruit and seed production, was enhanced for induced plants compared to controls. These results demonstrate that induced responses to herbivory can reduce the preference and performance of a variety of herbivores and ultimately increase plant fitness in natural environments. In order for a plant trait to be characterized as a defense, it must increase plant fitness in environments with herbivory. Although much attention has been given to detecting costs of plant defenses, this is one of the first systems for which fitness benefits of induced resistance have been demonstrated.