Specificity of induced resistance in wild radish: causes and consequences for two specialist and two generalist caterpillars

Inducible plant resistance against herbivores is becoming a paradigm of pla nt-herbivore ecology. Fundamental to understanding induced resistance and i ts evolutionary ecology is specificity of "induction" and "effects". Specif icity in the induction of resistance refers to whether plant damage by vari ous herbivores causes the same response in plants. Specificity in the effec ts of induced resistance refers to whether induction has the same consequen ces (i.e., reduced preference or performance) for various herbivores. I exa mined both specificity of induction and effect employing four lepidopteran herbivores and wild radish plants, a system for which fitness benefits and costs of induction have been documented for the plant. Variation in the spe cificity of induction and effects of induced plant resistance was found; ho wever, this variation was not associated with diet specialization in the he rbivores (i.e., specialists vs generalists). Induction caused by Plutella ( specialist) and Spodoptera (generalist) resulted in general resistance to a ll of the herbivores, induction caused by Pieris (specialist) induced resis tance only to Spodoptera (generalist) and Pieris, and plant damage by Trich oplusia (generalist) failed to induce resistance and reduce the performance of any of the herbivores. To the contrary, plants damaged by Trichoplusia supported enhanced growth of subsequently feeding Trichoplusia compared to uninduced controls. These results add a novel level of complexity to intera ctions between plants and leaf chewing caterpillars. Within the same guild of feeders, some herbivores cause strong induced resistance, no induced res istance, or induced susceptibility. Similarly, caterpillar species were var iable in the level to which induced resistance affected their performance. Such interactions limit the possibility of pairwise coevolution between pla nts and herbivores, and suggest that coevolution can only be diffuse.