Induced Plant Defences against Insect Grazing: Fact or Artefact?

Recent evidence for the ecological effects of damage-induced changes in leaves is reviewed and a framework proposed for the types of evidence needed to separate coincidental effects of wound repair from an evolved defence response. The ecological evidence presented is: the overdispersion of grazing damage within and between leaves at the end of the season; the induction of unpalatability and chemical changes following damage; negative associations between insect grazers on plants; grazer foraging behaviour. The framework of types of evidence which help to distinguish wound-repair processes from defence is: the induced chemical substances are not always directly involved in repair; chemical changes may be induced in tissues remote from the site of damage; the speed and magnitude of the changes are sufficient to affect grazer behaviour or fitness; plant species exhibiting induced changes show other general ecological similarities.