BIOEFFICACY OF ORGANIC EXTRACTS OF FISH POISON BUSH (GNIDIA GLAUCA, FRESEN) AGAINST COWPEA WEAVIL (CALLOSOBRUCHUS MACULATUS, FABRICIUS)
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Date
2018-11
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Abstract
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, DCMand 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.