Dose-dependent insecticidal activity of wild basil (Ocimum americanum L.) against the cowpea seed beetle (Callosobruchus maculatus Fab.): Evidence for a locally sourced postharvest protectant in semi-arid Kenya

Abstract

Cowpea (Vigna unguiculata (L.) Walp) is a critical grain legume in sub-Saharan Africa, yet postharvest losses inflicted by the cowpea seed beetle (Callosobruchus maculatus Fab.) can reach 100% of stored grain, with weight losses up to 60%. Synthetic insecticides remain the primary control strategy despite documented human health and environmental risks, creating an urgent need for locally available, low-cost botanical alternatives. This study evaluated the dose-dependent insecticidal activity of Ocimum americanum (wild basil) leaf, flower, and whole-plant powders against C. maculatus in stored cowpea seeds, and characterized the concentration–mortality response across five dosage levels. This research was anchored on Botanical Insecticide Theory and Modes of Action, Dose–Response Theory, Phytochemical Variability Theory and Postharvest Loss Theory. Dried and ground plant materials (leaves, flowers, and whole mature plants) were applied to 10 g of disinfested cowpea seeds (variety KVU 27-1) at 0.5, 1.0, 2.0, 4.0, and 8.0 g per jar. Ten pairs of 1–3 day-old adult beetles were introduced per treatment. Controls included untreated seeds and Actellic® Super dust (pirimiphos-methyl). The experiment followed a completely randomized design with four replications. Adult mortality was recorded at 72 hours, and seed weight loss was measured at 90 days. Data were analyzed using generalized linear models with binomial and beta error structures, followed by non-linear concentration–response modelling and effective concentration (EC₅₀/EC₉₀) estimation. Leaf powder at 8.0 g produced 92.5% adult mortality (95% CI: 88.4–95.4%) and reduced seed weight loss to 3.0%. Leaf powder was the most efficacious plant part, followed by whole-plant and flower powders. Non-linear regression revealed a steep sigmoidal mortality response for leaf powder (EC₅₀ = 3.1 g; EC₉₀ = 6.8 g per 10 g seed). Actellic® Super dust achieved 100% mortality at all tested dosages. A strong negative exponential relationship between mortality and weight loss was observed (R² = 0.94). Ocimum americanum leaf powder at 8.0 g per 10 g seed (800 g kg⁻¹) provides effective, low-cost protection against C. maculatus and represents a viable biorational alternative to synthetic insecticides for smallholder farmers in semi-arid Kenya. The steep, predictable dose–response relationship supports the development of standardized botanical formulations for integrated postharvest pest management.