Associational resistance in the milpa: herbivore-induced maize volatiles enhance extrafloral nectar-mediated defenses in common bean via shared parasitoids

Abstract

• Mixed cropping systems typically provide better natural pest control compared with monocultures, although the success varies depending on the crop and cultivar combinations. Understanding trait interactions that confer associational resistance (AR) to companion plants is key to optimizing these benefits. The Mesoamerican milpa system, known for its pest resistance, provides a model for studying these interactions.
• We tested two hypotheses to investigate whether access to extrafloral nectar (EFN) produced by Phaseolus vulgaris (common bean) can protect companion Zea mays (maize): (1) access to EFN enhances the survival and performance of a parasitoid wasp, leading to increased parasitism of fall armyworm (FAW) caterpillars on accompanying maize and reduced herbivory, and (2) bean plants can detect maize herbivore-induced plant volatiles (HIPVs) and respond by increasing EFN secretion.
• Controlled experiments demonstrated that wasps with access to EFN from bean plants lived longer, had higher fecundity, and parasitized more caterpillars on companion maize, thereby reducing herbivore damage. Additionally, caterpillar-damaged maize primed EFN secretion in companion bean plants via HIPVs.
• Our findings reveal a potentially important AR mechanism in the milpa, contributing to its reputed pest resistance. This understanding could inform the design of sustainable mixed cropping systems that enhance natural pest control.