Persistence and Comparative Pesticidal Potentials of Some Constituents of Lippia adoensis (Hochst. ex Walp.) (Lamiales: Verbenaceae) Essential Oil against Three Life Stages of Callosobruchus maculatus (Fab.) (Coleoptera: Bruchidae)

Over the past decades, the development of an alternative and eco-friendly pest control strategies has become a public concern for the sake of mankind. Plant essential oils are complex mixtures of volatile organic compounds, which play indispensable roles in the environment, for the plant itself, as well as for humans. Aims: The objectives of this study were (i) to identify and report the volatile organic compounds of Original Research Article Akami et al.; BBJ, 13(4): 1-16, 2016; Article no.BBJ.26087 2 Lippia adoensis (Hochst. Ex Walp.) (Lamiales: verbenaceae) essential oil (EO, herein after) and to compare the bioactivity of its four major compounds with the crude EO at relatively low dosages on some fitness parameters of Callosobruchus maculatus (F.) (Coleptera: Bruchidae), (ii) to test the persistency of each compounds over time, and (iii) to evaluate the type of interaction of the major constituents of the EO (whether antagonistic, synergistic or additive) in order to project the biological interaction of the whole compounds. Study Design: Young leaves of L. adoensis were randomly collected in Mbe locality (NgaoundereCameroon) for EO extraction which was then analysed by the means of Gas Chromatography (GCFID) coupled with Mass Spectrometry (GC-MS). Bioassays consisted of three shelves treatments for adult mortality, eggs laying, larval development and progeny production. A complete randomized design (CRD) with five replications was set for each treatment. Places and Duration of Study: Laboratory of Biology of the Faculty of Science, University of Ngaoundere (Cameroon) and Pest Control laboratory of Professor Niu Changying HZAU, Wuhan (China) from February 2014 to January 2016. Methodology: Essential oil was extracted with a Clevencher-type apparatus, hydrodistilled and airdried. Gas Chromatography-Flame Ionization Detector (GC-FID) and Gas Chromatography coupled with Mass Spectrometry (GC-MS) were carried out to analyze the constituents. The toxicity of crude EO and its four major constituents was evaluated at 0.5, 5, 10, 20 and 40 μL/g. Ten glass jars (volume 800mL) containing 50 g of cowpea seeds were prepared. After treatment, ten couples of C. maculatus aged 1-7days were randomly selected and separately introduced in each glass jar and kept at 22.72±1.06°C, 83.73±1.28% RH. Control ja rs were treated only with pure acetone. Each treatment was replicated five times. The exposure lasted for six days post treatment. Data on adult’s mortality, eggs laying, larvae and progeny production were assessed and monitored. Results: The GC–MS analysis allowed the identification of 43 volatile components representing 93.54% of total oil. The major components were Thymol (22.01%), Thymol-acetate (15.21%), paracymene (13.85%) and Triacetin (9.131%). The crude EO suppressed adults, completely inhibited eggs laying and adult emergence at 5 and 10 L/g after 24 h, respectively. Complete adults suppression was observed with thymol and the mixture at 20 L/g. Para-cymene and Triacetin caused complete adult mortality at 40 L/g but did not inhibited eggs laying and progeny production. EO and the mixture have been the most potent and persistent with a higher persistency throughout the experimentation. The synergism ratios (SR) were all higher than 1, thereby suggesting a significant (P = .05) synergistic interaction of major constituents although lower than that of the crude EO. Data on behaviours at death suggests that this EO may be neurotoxic probably through the blocking of the cholinergic receptors through reversible inhibition of acetylcholinesterase. Conclusion: The results obtained from this study revealed a significant (P = .05) insecticidal bioefficacy of EO extracted from young leaves of L. adoensis from Cameroon. This insecticidal properties are due to its richness in chemical constituents (43 in total) that sustained its persistency during the treatment. L. adoensis could therefore be a suitable topical agent to control C. maculatus infestations and could be of value for commercial formulations.