Conflicting interests of plants and the natural enemies of herbivores

In the literature there is a persistent temptation to accept at face value the hypothesis that plants manipulate the natural enemies of their herbivores. This hypothesis has received a lot of attention both theoretically and in reductionist laboratory experiments, but evidence that plants actually benefit from the active attraction of natural enemies in the field is virtually absent. Especially for parasitoids the validity of this 'evolutionary enlistment hypothesis' should be questioned. In this paper we discuss conditions that may select for active attraction by plants of the third trophic level and formulate a number of questions that should be answered before we can accept this hypothesis.