Introduction

Biological control of plant pests involves the use of natural enemies to control arthropods that in turn are enemies of the plant. Classic theory on biocontrol focused on density-dependent interactions between natural enemies and plant pests (Hassell 1978), but largely ignored a role for the plant other than directly affecting the growth rate of the plant pest (i.e., resulting from nutritional quality and direct anti-herbivore defence of the plant). That the plant’s role goes far beyond setting the growth rate of the herbivore was realized much later. Prompted by a seminal review by Price et al. (1980), an avalanche of research now shows the multifarious ways in which plants influence the effectiveness of natural enemies against plant pests and the vulnerability of these pest organisms to their enemies. Such effects on the herbivore’s predators may arise from constitutive investment by the plant, or from versatile investments induced by herbivory. Thus, herbivore-induced changes in the state of the plant may directly and indirectly, i.e., via predators, affect herbivores (Dicke and Sabelis 1988; Turlings et al. 1995; Karban and Baldwin 1997). To the best of our knowledge, predators do not change the state of plants, but they can induce changes in behaviour and life history of herbivores, thereby indirectly affecting the plant (Schmitz et al. 2004). Moreover, herbivorous arthropods may foresee predation risk and avoid predator-occupied plants in response to chemical alarm from herbivores under attack (Janssen et al. 1998; Pallini et al. 1999; Nomikou et al. 2003; Lee et al. 2011). Taken together, tritrophic systems of arthropods on plants are replete with trait changes induced by organisms at adjacent trophic levels and with effects cascading across all trophic levels. Such indirect effects become ever more complex in multispecies food webs because they arise whenever there are more than two species (Werner and Peacor 2003). One example is competition between plant pest species, which not only directly compete for food or by interference, but may also compete indirectly via the plant’s defence response and via responses of shared and non-shared predators.