Insecticidal Efficacy and Repellency of Oregano (Lippia palmeri) Essential Oil for Control of Sitophilus zeamais and Prostephanus truncatus in Stored Maize

Maize, Zea mays L., is the most important crop in Mexico. The maize weevil, Sitophilus zeamais Motschulsky, and larger grain borer, Prostephanus truncatus (Horn), cause significant loss to stored maize. The insects are controlled by insecticides that are toxic to humans and pollute the environment. An alternative to control insect damage is essential oil from plants. The aim of this research was to evaluate the effect of oregano (Lippia palmeri S. Watson) oil at different concentrations on repellence, emergence, and mortality of maize weevil and larger grain borer. All maize weevils died at 48 hours, while most (92%) larger grain borer died at 72 hours with 1,000 l/liter of oregano oil. Emergence of treated insects was significantly (p < 0.05) less than the check. The repellency index at 24 hours for larger grain borer was 0.15 at a concentration of 20 l/liter of oregano oil. The repellency index for maize weevil was zero, indicating total repellence at 24 hours. Components of the oil were determined by gas chromatography-mass spectrometry to be thymol (58.9%) and p-cymene (21.8%). Oregano oil concentration and time were important in controlling both insect species.