Phytometabolite characterisation and bioefficacy of croton dichogamus and Warburgia ugandensis extracts against Callosobruchus maculatus in stored Cajanus cajan grains
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Date
2025-05
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Kenyatta University
Abstract
Pigeon pea bruchids, Callosobruchus maculatus has been a major biotic factor of stored pigeon pea, Canjanus cajan (L.) Huth grains whose damage can lead to food insecurity. Conventional use of synthetic insecticides is expensive and arguably associated with many side effects. Therefore, there is need to develop not only safe but also effective botanical pesticides as an alternative. The current study was designed to evaluate bioefficacy and phytometabolite characterization of dichloromethane (DCM) leaf extracts of Croton dichogamus and Warburgia ugandensis against C. maculatus in stored grains Canjanus cajan. Fresh leaves of C. dichogamus were harvested from Kangonde, Machakos County, Kenya, while as W. ugandensis leaves were collected from Lower Yatta, Kitui County, Kenya. The extraction was carried out using DCM solvent. The plant leaf extracts were prepared at concentrations of 25, 50, 75 and 100 % for bioassays. Pigeon pea bruchids, C. maculatus of a single strain were subjected to free-choice tests with pigeon pea grains treated with the DCM leaf extracts and controls. Untreated grains and solvent treated grains served as the normal and negative controls respectfully, whereas grains treated with Actellic 50ECTM served as the positive control. The extracts were assayed for their insecticidal abilities through fumigant and direct-contact toxicityinduced mortalities of C. maculatus exposed to treated grains of C. cajan for the period of 96 hours. The repellency properties of these extracts on bruchids were evaluated using the area preference method. Crude acetlycholinesterase enzyme (AChE) was extracted from adult bruchids and the Ellman method was employed to determine inhibitory action of the C. dichogamus and W. ugandensis DCM leaf extracts on AChE activity. The effects of C. dichogamus and W. ugandensis DCM leaf extracts in terms of adult bruchids reduction/percentage mortality (PM), percent corrected bruchids mortality (Pr), percent repellency, repellency index (IR) and AChE inhibitory effects was determined. Phytometabolites analyses of the leaf extracts were done using Chromotography-Mass Spectrometry (GC-MS). The DCM extracts of the two plants caused significant death of C. maculatus on C. cajan grains upon 96 hours of fumigation. Corrected bruchids mortality (Pr) rating of DCM extracts of C. dichogamus was between 61.84 and 97.37 %. On the other hand, DCM extracts of W. ugandensis had a bruchids killing potential that ranged from 82.37 to 100 %. The extracts also manifested potent repellency effects of repellency indices (IR) between 0.06 and 0.53 in C. dichogamus while in W. ugandensis recorded between 0.08 and 0.49 on adult bruchids. Moreover, the studied organic leaf extracts of C. dichogamus and W. ugandensis showed potent in vitro AChE inhibitory effects of up to 84.58 % and 86.55% at a concentration of 17.5mg/ml in C. dichogamus and W. ugandensis respectively which was significantly comparable with effects of synthetic insecticide. The GC-MS analysis showed that C. dichogamus extracts contained 75 phytocompounds while W. ugandensis revealed 58 phytocompunds. Further, the extracts of C. dichogamus and W. ugandensis showed several terpenoids, fatty acids and their derivatives, vitamins, phytosterols and phenols that have previously been associated with insecticidal and anti-AChE effects. In conclusion, the present study showed significant fumigant toxicity effects of C. dichogamus and W. ugandensis DCM extracts on C. maculatus. Further, it was evident that the extracts contain several insecticidal and anti-AChE phytometabolites. This study provides significant information for further study on the plants as bio-resource of novel biopesticide against C. maculatus in stored grains of C. cajan. Nevertheless, there is need for fractionation of specific phytometabolites in the extracts associated with the confirmed properties (insecticidal and anti-AChE).
Description
A thesis submitted in fulfilment of the requirements for award of the degree of Doctor of Philosophy (Biotechnology) of the School of Pure and Applied Sciences of Kenyatta University
May, 2025
Supervisors
Prof. Mathew Piero Ngugi
Dr. Stephen Maina Gitahi