Composition and bioactivity of essential oils of Lantana camara L., tephrosia vogelii hook and ocimum americanum L. against major coleopteran pests of stored food grains.

Abstract

Food security and post-harvest handling are greatly undermined by insect pests which cause about 20% post-harvest losses of food grains in subsistence agriculture. Despite the enormous potential for pest control. the indigenous plant-based products have remained largely anexploited. In an attempt to address the insect pest menace in grain storage, a scientific study was conducted to evaluate the contact toxicity (instant and residual). fumigant toxicity (space and grain). repellence (instant and residual) and reproduction inhibition of essential oils extracted from aerial parts of three indigenous plants. Ocimum americanum L., Lantcma camara L. and Tephrosia vogelii Hook, and selected isolated constituents against adult Sitophi/us oryzae L., Rh_\:0perrha dominica F., Tribolium caslaneum (Herbst) and Callosobruchus chinensis‘ L. insects. The study also evaluated the intra-species and inter-plant variations in composition of essential oils and possible phytotoxic effects of selected essential oils on treated seed grains. In the contact toxicity. repellence and reproduction inhibition studies. essential oils and a sorstituent. eugenol. were each evaluated at four rates (0.5. 1.0. 1.5 and 2.0 uL/g grain) with 4-6 replicates per concentration. In the same bioassays. soya oil (10 pL/g grain) and Actellic SEC ~ 1.0 _uL g grain) were included as positive controls with Acetone (0.5 uL/g grain) and untreated grains as negative controls. In the space and grain fumigation studies. four (0. 1. 5 and 10 ul/L air) and five (O. 30, 50. 70 and 100 pl/L air) rates. respectively, with 4-6 replicates per essential oil concentration or constituent were tested against adult stages of three or four test insects. All the laboratory bioassays, arranged in a completely randomized design (CRD). were conducted in a room maintained at 30i2°C, 68i2% RH and 12D: 12D (total darkness). Except for residual tcontact and repellence) studies and chemical compositional analysis. bioactivity of test essential oils and their constituents against adult stages of four coleopteran pests were significantly influenced (P<0.0001- 0.0484) by intra-species and inter-plant variations in chemical composition. concentration applied, time after application, insect species and corresponding factor interactions. With a few exceptions, test essential oils demonstrated clear temporal dose- dependent contact toxicity. repellence and reproduction inhibition against adult stages of test insects and phytotoxicity on treated wheat seeds. Results of space fumigation showed that. irrespective of plant part assayed. O. americarmm, L. camara and T. vogelii essential oils produced LC;0 values of 0.38- 34.68. 0.77- 64.10 and 044- 43.50 p.l/L air. respectively. against adult stages of the four test insects. In the grain fumigation studies with O. arnericanum leaf essential oil. 50 },ll/L, 7 days exposure and 120 h post-fumigation time were enough to obtain 95.4-100% kill of adult S. oryzae and R. dominica and 65.5% mortality of T. casrunez/m. which was comparable to methyl bromides recommended dose of 30-50 g/M3 grain and that of highly active Labiate sp. Oil. ZP51"i (50 ul/L). At the highest concentration (0.2O% v/w) and 120 days of storage, O. americanum and T. vnge/ii fruit essential oils caused 25.0- 40.0 and 33.0- 51.0% inhibition of wheat seed germination with corresponding EC5O values of 6.22- 0.37 and 2.10- 0.19% v/w, respectively. Results of GC-MS analysis showed marked intra-species and inter- plant variations in which eugenol (49.2%) and germacrene D (19.22%) were dominant in leaves of O. americanum and T. vogelii whereas E-caryophyllene (15.5%) in the fruits of L. camara implying the test plants were eugenol, germacrene D and E-caryophyllene chemotypes, respectively. The potential of essential oils and their volatile constituetnts as alternative contact insecticides, fumigants and protectants of durable agricultural products and scientific implications are discussed.