Synergistic efficacy of Tephrosia vogelii and Azadirachta indica leaf powders: optimizing sustainable admixtures for Sitophilus zeamais management in stored maize

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Stored-maize protection remains a major challenge for smallholder farmers in sub-Saharan Africa, where Sitophilus zeamais ( S. zeamais ) causes substantial postharvest losses in grain quantity and quality. While synthetic pesticides remain a primary management strategy, their high cost, environmental impact, and health risks to smallholder farmers necessitate the evaluation of safer, locally available alternatives. This study evaluated the comparative efficacy of Azadirachta indica and Tephrosia vogelii leaf powders, applied singly and as binary admixtures, in suppressing S. zeamais infestation and grain degradation. The experiment was carried out at the Lilongwe University of Agriculture and Natural Resources (LUANAR) in Malawi. A Completely Randomized Design (CRD) with four replications was employed to evaluate ten treatments on the maize variety DKC80-53. Samples were monitored over a 12-week storage period, and data were analyzed using a Generalized Linear Mixed Model (GLMM). Results revealed significant treatment-time interactions (Tr × T) for most parameters. A clear treatment hierarchy emerged over time: while low-dose single botanicals failed to significantly deviate from the untreated control, binary admixtures exhibited a strong synergistic effect on pest suppression. The 1.0 g Neem + 1.0 g Tephrosia combination proved most effective, reducing grain weight loss and larval emergence by 50.4% and 85.5% respectively, compared to the control. These findings demonstrate that botanical efficacy is strongly dose-dependent, while the duration of protection is time-dependent. Specifically, interaction analysis identified a critical efficacy threshold at week 8, after which a sharp increase in grain weight loss was observed, indicating a decline in the residual bioactivity of the botanical secondary metabolites. This study identifies the 1.0 g/kg binary admixture as a statistically robust alternative for medium-term storage, while highlighting the necessity of re-application strategies beyond the 8-week mark to maintain localized grain security for rural households. Adopting these optimized botanical formulations can enhance household food stability while significantly reducing the economic and health risks associated with synthetic chemical dependency in smallholder farming systems.