Bioefficacy of Organic Extracts of Fish Poison Bush (Gnidia Glauca, Fresen) Against Cowpea Weavil (Callosobruchus Maculatus, Fabricius)
Wilhelmy, Marion Jebet
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Cowpea weevil (Callosobruchus maculatus) is a major pests of stored cowpea in the tropical region of the world. In Kenya, the damage caused by C. maculatus impacts negatively on its economic and nutritional values and contributes to food insecurity. The widely adopted use of chemical pesticides is marred with health and environmental hazards. Global concern on synthetic chemicals has led to heightened restrictions and limitations on their use. This, therefore, has prompted the search for alternatives to synthetic pesticides. New studies are focusing on the use of botanicals as a novel approach to the management of pests. Gnidia glauca has been exploited by local people in control of post-harvest pests. However, no scientific research has been undertaken to evaluate its potential anti-insect properties. In this study, four organic leaf extracts of G. glauca (methanol, ethyl acetate, DCM and blend) were evaluated for contact toxicity, oviposition deterrence, inhibition of progeny emergence and repellency against cowpea weevil. The plant leaves were collected from Embu County, Kenya. The samples were prepared, extracted and investigation carried out under ambient laboratory conditions. The experimental design entailed five test concentrations (2g/100ml, 4g/100m, 6g/100ml, 8g/100ml, and 10g/100ml) of each extract, the untreated control, the solvent control and the positive control-Actellic. Each bioassay had four replications. Adult weevils (1-3 days old) were exposed to the extracts and mortality was monitored daily for the first four days. Subsequently, oviposition deterrence was assessed on the 15th day while inhibition of progeny emergence was evaluated on the 49th-day post-treatment. Extract repellency was assessed for the first 6 hours after treatment. Screening for plants phytochemicals was conducted using the standard recommended procedures. The results of this study revealed all G. glauca extracts, to a varied extent, induced mortality on C. maculatus. Mortality was concentration and exposure time dependent. Highest mortality of 89.74% was recorded with 10g/100ml ethyl acetate extract 96 hours post-treatment. The extracts significantly deterred oviposition with the 10g/100ml concentration of ethyl acetate, DCM and blend statistically (p>0.05) comparable to the activity of synthetic pesticide. All the extracts were found effective in inhibition of progeny emergence. Ethyl acetate extract at the test dose of 10g/100ml demonstrated the highest inhibition of 99.3% while the least inhibition of 9.03% was exhibited by 2g/100ml methanol extract. G. glauca extracts proved to be attractant of C. maculatus rather than repellant, none of the extract concentration attained repellency greater than 50%. Results also showed that the extracts had tannins, phenols, flavonoids, terpenoids, saponins, alkaloids, cardiac glycosides and steroids which have been associated with insect control properties. It was therefore concluded that the plant extracts, possess bioactivities against Callosobruchus maculatus on the tested parameters of contact toxicity, oviposition deterrence, inhibition of progeny emergence and repellency. Hence the studied extracts can further be purified and developed into the plant-derived bio-pesticides to control C. maculatus.