Insecticidal Activity of Powder and Essential Oils of Vepris heterophylla (Rutaceae) and Syzygium aromaticum (Mytaceae) Towards Callosobruchus maculatus F. walp (Coleoptera: Bruchidae) on Post-Harvest Vigna unguiculata (Fabaceae) in the Far-North Region of Cameroon

The bioinsecticidal effect of powders and essential oils of S. aromaticum (L.) flower buds and V. heterophylla (Engl.) leaves against adult cowpea weevil C. maculatus was studied. Powders were tested by direct contact only while essential oils were tested by direct contact and indirect contact (inhalation and repellency). In 500 mL glass jars, the individual and combined powders were applied to 100 g of cowpea seeds at 0.5 g, 1 g, 1.5 g and 2 g doses for both leaves and flower buds of V. heterophylla and S. aromaticum respectively. The essential oils were also applied individually and after equilibrium combination on 50 g of cowpea at 4 µL/mL, 8 µL/mL, 12 µL/mL, 16 µL/mL. All the infestations were then achieved by adding 20 bruchids of 48 h old. The essential oil obtained was purified and analyzed with Gas Chromatography and Mass Spectrometry (GC-MS). 72 h after the individual treatment, 100% mortality was obtained at 2 g dose for V. heterophylla and 0.5 g dose for S. aromaticum compared to the control jars (1%). The combined powders were found to be more effective with 100% of mortality at a dose 1 g after 48 h of exposure. For essential oils, 100% and 90% of mortality were obtained respectively at 8 µL/mL dose for S. aromaticum (160 µL/kg) and 16 µL/mL dose for V. heterophylla (320 µL/kg) 72 h after treatment. In addition, the essential oil of S. aromaticum showed relatively higher repellant properties with an average repellency percentage of PR = 89.37% than that of V. heterophylla (PR = 70.62%). 100% of mortality was induced after inhalation of essential oils of S. aromaticum and V. heterophylla at doses 10 µL and 40 µL respectively after 72 h of exposure time. Results obtained from the GC-MS showed that the major components in the essential oil of S. aromaticum were eugenol (83.40%) while that of V. heterophylla were Spathulenol (23.66%), Beta-Caryophyllene oxide (16.46%) and 6-Isopropenyl-4 (16.30%). The powders and essential oils of the tested plants applied individually or after equilibrium combination showed interesting prospects for controlling Callosobruchus maculatus and effectively preserving cowpea seeds in storage warehouses.