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.
The abusive and repeated use of synthetic chemical insecticides has proven to be harmful to human health and the viability of the cotton production system in Ivory Coast, so it is imperative to find alternatives. . Thus, the objective of this study was to study the chemical composition and biological activity of essential oils of Lippia multiflora (Verbenaceae) and Eucalyptus globulus (Myrtaceae) and to evaluate their insecticidal potential in the laboratory on three main pests of cotton. After essential oils extraction, their chemical composition was determined. Also, antioxidant activity and cholinesterase inhibitor of essential oils were evaluated. After that, different concentrations of the two essential oils were prepared and applied by contact on groups of insects constituted by ten. The essential oil of L. multiflora was the most toxic for the three pests tested. Indeed, the lethal concentrations (LC50) were 1.74 %, 1.39 and 7.20 %, respectively, on Pectinophora gossypiella, Thaumatotibia leucotreta and Helicoverpa armigera. In contrast, the values obtained with E. globulus essential oil were nine to two times greater (16.05 %, 10.23 % and 16.32 %, respectively on these pests). With respect to the chemical composition of the essential oils, E. globulus essential oil was the richest in oxygenated monoterpenes (65 %) with 1,8‐cineole or eucalyptol as the majority compound (61.6 %). The essential oil of L. multiflora was distinguished by a lower proportion of oxygenated monoterpenes (44.3 %), but it contained more terpene elements (24 vs. 15 for the essential oil of E. globulus). The essential oils of L. multiflora and E. globulus also showed significant inhibition of acetyl (2.13 and 2.16 mg galantamine equivalent (GALAE)/g, respectively) and butyryl cholinesterase (4.03 and 3.61 mg GALAE, respectively). L. multiflora was differentiated by its good inactivation of tyrosinases (163.46 versus 58.95 mg kojic acid equivalent (KAE)/g in E. globulus). Better antioxidant activity was observed with L. multiflora essential oil relative to DPPH (7.05±0.34 mg trolox equivalent (TE)/g). Biopesticides based on L. multiflora essential oil could be developed for the phytosanitary protection of cotton plant.
In the context of climate change, the Ivorian cotton industry is facing with the loss of sensitivity of pests (Helicoverpa armigera) and the appearance of new so‐called emerging insects. Faced with this situation, cotton producers tend to use insecticide products in high doses, in excess of the norm. However, the misuse of chemical products poses many health risks. Therefore, to limit the use of chemicals, aqueous extracts of local plants with insecticidal properties were examined in the laboratory and in the field. Four local plant species were selected [Anacardium occidentale (Anarcardier); Azadirachta indica (Neem); Hyptis suaveolens (Hyptis) and Tephrosia vogelii (Tephrosia)]. After determining the chemical profiles of the four extracts by high performance liquid chromatography (HPLC)‐mass spectrometry, their inhibitory activities were assessed in cholinesterase and tyrosinase. The sensitivity of Helicoverpa armigera larvae was evaluated by ingesting the aqueous extracts at several concentrations ranging from 2% to 64% in an artificial nutrient substrate. Then, the mortality rates of the larvae during 72 h were evaluated and the lethal concentrations were determined. The results of chemical analyses (HPLC) showed that the richest aqueous extract in phytochemicals with 54 elements detected was that of cashew (A. occidentale). T. vogelii, A. indica and H. suaveolens presented 44, 45, and 39 chemical compounds, respectively. In addition, the total phenolic content was higher in A. occidentale (110.67 mg gallic acid equivalents/g) followed by A. indica (42.43 mg gallic acid equivalents/g). The highest antioxidant ability was observed with the aqueous extract of cashew (A. occidentale). Anti‐enzymatic activities such as acetylcholinesterase, butyrylcholinesterase and tyrosinase inhibition were most pronounced in A. occidentale (2.35 ± 0.02 mg galanthamine equivalent/g, 3.77 ± 0.01 mg galanthamine equivalent/g and 71.28 ± 0.07 mg kojic acid equivalent/g, respectively). The most toxic aqueous extract for H. armigera larvae was that of cashew with a lethal concentration LC50 = 11.68%. Moreover, the principal component analysis performed showed that the insecticidal activity is strongly correlated with the antioxidant and enzymatic activities of the aqueous extracts. Then, the hierarchical ascending classification showed cashew as the best plant. For the sustainability of cotton production, it would be necessary to limit the use of chemical‐synthetic insecticides through the use of plant extracts, especially from cashew leaves.
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