Effect of drying regime on the chemical constituents of Plectranthus glandulosus leaf powder and its efficacy against Callosobruchus maculatus and Sitophilus zeamais

Abstract The objective of this study was to determine the chemical constituents and insecticidal efficacy of Plectranthus glandulosus Hook (Lamiaceae) leaf powders dried in shade or sunlight against Callosubruchus maculatus and Sitophilus zeamais. Leaf powder extracts were submitted to stir bar sorptive extraction (SBSE) and volatile compounds analyzed by GC and GC-MS. Fifty components were identified in both drying methods. α-pinene, limonene, 1,8-cineole, terpine and ocimene known for their insecticidal activity were found at higher levels in the shade-dried leaves while camphor, linalool and thymol were higher in the sun-dried leaves. Fenchone, β-pinene and eugenol were found in the same proportion, irrespective of drying regime. The drying regime had no effect (t = 0.34; P > 0.05) on the mortality caused by the leaf powders to C. maculatus. Within seven days of exposure, S. zeamais were more susceptible (t = -1.29; P < 0.001) to the powder from the sun-dried (LC50 = 14.04 g/kg) leaves compared to that from the shade-dried leaves (LC50 = 34.51 g/kg). Powders from the sun-dried leaves of P. glandulosus stand as a good candidate for protecting maize against the infestation of S. zeamais during storage.