Specificity of herbivore-induced plant defences.

In addition to induced direct defence, plants can defend themselves indirectly by improving the effectiveness of enemies of herbivores. Plants can respond to arthropod herbivory with the induction of a blend of volatiles that attracts predators and/or parasitoids of herbivores. Carnivorous arthropods can discriminate between infested plants and mechanically wounded plants, and between plants infested by different herbivore species. The volatile blends emitted by different plant species infested by the same herbivore species show large qualitative differences, whereas blends emitted by plants of the same species, but infested by different herbivore species are mostly qualitatively similar with quantitative variation. Carnivores can discriminate between blends that differ qualitatively and/or quantitatively. However, it remains unknown what differences in blends are used by carnivorous arthropods in this discrimination. Signal transduction pathways involved in the induction of direct and indirect defence seem to overlap. Direct and indirect defence may interfere with each other's effectiveness. For application of direct and indirect defence in agriculture, it is important to compare the relative importance of these two defence types in the same plant species.