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

Induced responses to herbivory are changes in plant quality following initi al damage by herbivores. These changes can negatively affect subsequent her bivores. I studied induced responses in annual wild radish plants, Raphanus raphanistrum and R. sativus (Brassicaceae), which included increased densi ty and total number of setose trichomes on newly formed leaves of previousl y damaged plants compared to undamaged controls. I also characterized the e ffects of induced responses on the preference and performance of several he rbivores 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 no ctuid larvae compared to unmanipulated control plants. Induced responses di d not affect the growth of specialist Pieris rapae larvae. In choice and no -choice experiments, induction reduced the feeding by noctuid larvae but di d not reduce gross growth efficiency, an indicator of insect growth per uni t of plant material consumed. Finally, colonization of plants by leafminers was lower on plants that received previous damage, compared to controls. I n field experiments conducted over 2 yr, herbivory by naturally occurring g rasshoppers was lower on induced plants than on both unmanipulated controls and leaf damage controls (plants with leaf area removed without the associ ated induced response). Plant fitness, as estimated by fruit and seed produ ction, was enhanced for induced plants compared to controls. These results demonstrate that induced responses to herbivory can reduce the preference a nd performance of a variety of herbivores and ultimately increase plant fit ness in natural environments. In order for a plant trait to be characterize d as a defense, it must increase plant fitness in environments with herbivo ry. Although much attention has been given to detecting costs of plant defe nses, this is one of the first systems for which fitness benefits of induce d resistance have been demonstrated.