Intercrop-mediated inducibility affects direct defenses and plant resistance but not indirect defenses in maize

Functional intercropping aims to beneficially associate two or more plant species or varieties, simultaneously increasing plant diversity and the provisioning of ecosystem services. Although the benefits of diversified cropping systems are widespread and well documented, the underlying mechanisms of increased pest resistance and the relative contributions of different modes of plant defense remain unclear. Plant chemistry can mediate resistance to herbivores through toxic or antidigestive modes of action (direct defenses) or by providing host finding cues that recruit natural enemies that predate on herbivore populations (indirect defenses). Both direct and indirect defense can be elevated in response to previous herbivore damage leading to induced resistance. Here we address the question of how intercropping with four companion plants (alfalfa, bean, Desmodium, and red clover) affects the constitutive and induced expression of plant direct/indirect defenses and resulting herbivore resistance. We found that defensive plant secondary metabolite production of focal maize plants varies with both, previous herbivore damage (induction treatment) and the presence of an intercrop species. Intercropping - specifically with Desmodium - alters the expression of plant chemical defenses and increases plant resistance in 1) no-choice bioassays by reducing larval performance and 2) the incidence of damaged leaves at the field-scale experiment. Thereby some intercrop species do not only directly affect maize plant secondary metabolism but also alter how defensive metabolites are expressed in response to herbivory (intercrop-mediated induced responses). In contrast to direct resistance, the expression of indirect resistance did not vary with intercropping or herbivory suggesting that under realistic field conditions, direct defenses are more reliable as pest control mechanisms than chemical information-mediated indirect defenses. However, within-plant spatial separation of predation pressure suggests a role of vegetation structure in the efficiency of biocontrol. We present evidence that 'intercrop-mediated induced responses' is an integrated ecological mechanism determining the outcome of associational resistance (or susceptibility) and conclude that intercrop-mediated alterations of constitutive and herbivory-induced secondary metabolite production mediate increased associational resistance in diversified maize systems.