Determination of chemical composition and insecticidal potential of leaf essential oil of Mentha piperita L.

Pulverised fresh Mentha piperita leaves was subjected to hydrodistillation for three hours in a Clevenger-type apparatus. The oil obtained was characterised using Gas Chromatography-Mass Spectrometry (GC-MS) and also evaluated for insecticidal activity on Callosobruchus maculatus (bean weevils) and Sitophilus zeamais (maize weevils). The oil afforded a yield of 0.78% (w/w). A total of 39 compounds were identified from their mass spectra representing 98.9% of the oil. The classes of compounds identified revealed predominance of oxygenated monoterpenoids (72.4%), hydrogenated monoterpenoids accounted for 6.0%, hydrogenated sesquiterpenoids and oxygenated sesquiterpenoids constituted 12.2 and 4.0% respectively. Other non-terpenic compounds detected in the oil accounted for 4.3% of the total oil. The most abundant compound was menthol (40.4%), other major compounds identified were menthone (12.3%), menthofuran (9.2%), 1, 8-cineole (5.6%) and limonene (5.5%). The percentage mortality of between 68-94.5% and 65-90.2% were recorded for bean weevils and maize weevils respectively after exposure to 10µl essential oil vapour in fumigation chambers for 6, 12, 18 and 24 hours. The insecticidal activity of the oil compared favourably with that of standard insecticide and so can be used to formulate an alternative insecticide from botanical source.