Chemical Composition and Spectrum of Insecticidal Activity of the Essential Oils of Ocimum gratissimum L. and Cymbopogon citratus Stapf on the Main Insects of the Cotton Entomofauna in Côte d'Ivoire

Among the alternatives to environmentally toxic and socio-economically unacceptable chemical pesticides, essential oils from Ocimum gratissimum and Cymbopogon citratus were tested on the main pests and beneficial insects of the cotton plant in Côte d′Ivoire. After extraction and chemical analysis of the essential oils, field trials were carried out using a Fisher block system with three treatment repetitions where their effects compared with those of a registered synthetic insecticide (IBIS A 52 EC). Foliar applications of the products were carried out in accordance with the cotton plant protection extension programme in Côte d′Ivoire from the 45th to the 115th day after plant emergence, with one application every fortnight. Twenty-three and forty compounds representing about 96 and 99 % of the oil composition of O. gratissimum and C. citratus respectively were elucidated. The most abundant compounds were p-cymene and thymol (O. gratissimum) and myrcene, neral and geranial (C. citratus). The essential oil of O. gratissimum at concentrations of 2 and 5 % showed insecticidal activity on all pests (biting-sucking and carpophagous), except the phyllophagous Syllepte derogata. C. citratus, at a low concentration (1 %), was particularly toxic to whiteflies (Bemisia tabaci), however, it favoured the action of beneficial insects, specifically black ants and ladybirds in the cotton plots, unlike the chemical product. EO of O. gratissimum (1.60 and 4.62 mg GALAE/g, respectively) and C. citratus (2.26 and 2.78 mg GALAE, respectively) exhibited also significant acetyl and butyryl cholinesterase inhibitors. Insecticide formulations based on the essential oils of O. gratissimum and C. citratus offer favourable prospects for their use in cotton cultivation as an alternative to chemical pesticides.     

Effects of Ocimum sanctum hexane extract on survival and development of Dysdercus koenigii Fabricius (Heteroptera: Pyrrhocoriedae)

Abstract

The effects of hexane extract of Ocimum sanctum leaves on the survival, longevity, growth and development of the Dysdercus koenigii Fabricius were investigated. Newly emerged fifth instar nymphs were exposed to nine concentrations ranging from 0.00625% to 10% of the extract for 24?h by “dry film residual method.” The results indicated that the extract has adverse effect on various life activities studied. The effects were dose related; exposure to a dose of 5% or more causes high mortality. Normal adults were not produced from the nymphs treated with extract concentrations of 1.25% or more. The treated nymphs exhibited developmental malformation. The gas chromatography and mass spectroscopy analysis of the extract revealed presence of chemical compounds having insecticidal and juvenile hormone (JH) mimicking activities. Some of the compounds act as precursors of JH biosynthetic pathways. Therefore, these compounds may individually or synergistically can be used in management of D. koenigii and other insect pests.