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Trophic interactions – between plants,herbivores,their natural enemies and symbionts – drive the structure of ecological communities and,in agroecosytems are keys to managing the losses from pest species.Since plants are sessile they have evolved sophisticated defence mechanisms to compensate for the fact they are unable to flee attacking herbivores.Defences range from thorns to toxins and in many cases are inducible,turned-on only when the plant is in danger,thereby avoiding the need to invest in costly defences when unnecessary.