‘The Three Sisters’ (maize/bean/squash) polyculture promotes the direct and indirect defences of maize against herbivores

It is well-known that increasing crop biodiversity plays an essential role in achieving ecologically based pest management (EBPM); however, the mechanism is poorly understood. The Three Sisters (Maize/Bean/Squash polyculture [MBS]) is a traditional mixed cropping strategy that has been practiced for thousands of years in Mesoamerica; however, mechanistic evidence for its resistance to herbivores is still lacking. In the present study, using both greenhouse and field experiments, we investigated the effects of MBS on the direct and indirect defences of maize ( Zea mays ) against Asian corn borer ( Ostrinia furnacalis ) and fall armyworm ( Spodoptera frugiperda ) caterpillars. MBS significantly improved the direct chemical defences of maize, especially against O. furnacalis , by increasing the content of primary metabolites, secondary metabolites (Benzoxazinoids), and phytohormones ((±)-Jasmonic acid-isoleucine, Jasmonic acid) in maize leaves, thus significantly reducing their survival and relative growth rates. MBS significantly improved the indirect chemical defences of maize by varying the species and content of volatile organic compounds (such as a mixture of nonanal, 1-dodecanol and 2-butyl-1-octanol), seemingly resulting in the avoidance of MBS plants by both herbivores in the Y-tube olfactometer tests. Field experiments further demonstrated that MBS significantly reduced damage to maize leaves by herbivores, enhanced maize yields, and increased the abundance of natural enemies. Therefore, we concluded that MBS improves the resistance of maize to major pests, and can be used as an effective and eco-friendly crop protection strategy in agroecosystems.