Investigating Molecular Features of the Push-Pull System – An A-maize-ing Application of Mass Spectrometry

The maize-desmodium push-pull system, developed in the 1990s, is a promising mixed cropping strategy using Desmodium spp. as a repellent intercrop alongside a border grass for biological pest control in sub-Saharan Africa. However, the molecular mechanisms behind its effectiveness remain debated to this day. To add to the current understanding of the (bio-)chemical basis of the push-pull effect, we conducted a large-scale sampling campaign in farmer fields to analyse volatile emissions and non-volatile metabolites. We developed an on-site sample extraction procedure for mass spectrometry-based metabolomics, which we employed in Kenya, Uganda, and Rwanda where maize leaf tissue samples were collected from 74 farmer fields. This allowed us to identify two benzoxazinoid glycosides with higher abundance in push-pull fields compared to conventional agricultural fields and both of those compounds are known to play a role in plant defence against insect herbivores. Additionally, we found that the volatile profile in push-pull fields is significantly influenced by emissions from the border grasses, whereas the desmodium intercrop releases fewer volatile compounds. These findings suggest that the molecular mechanism of the push-pull system differs slightly from previous theories, with a greater emphasis on insect pest defence linked to border grass emissions and a potential innate resistance of maize plants due to the increased presence of anti-herbivore metabolites.