Preliminary phytochemical analysis and insecticidal activity of ethanolic extracts of four tropical plants (Vernonia amygdalina, Sida acuta, Ocimum gratissimum and Telfaria occidentalis) against beans weevil (Acanthscelides obtectus)

Abstract

Ethanolic extracts of the leaves of Ocimum gratissimum, Sida acuta, Telfaria occidentalis and Vernonia amygdalina were screened for secondary metabolite constituents and insecticidal activity against beans weevil (Acanthscelides obtectus). Phytochemical screening of the extracts revealed the presence of alkaloids, flavonoids, saponins, steroids, tannins, phlobatannins and terpenoids in the plants investigated. Phlobatannins and terpenoids were found to be absent in ethanol extract of T. occidentalis (leaf), saponins were also absent in both O. gratissimum and S. acuta while steroidal compounds were found to be absent inT. occidentalis and V. amygdalina (leaf). The extracts of O. gratissimum, S. acuta,T. occidentalis and V. amygdalina of different concentrations were also investigated for their insecticidal activity against A. obtectus. Average mortality indicated that the extracts caused significant mortality on the target insects. The bioassay has indicated that the toxic effect of the extracts was proportional to the concentration and higher concentration has stronger effect. The observed overall mean mortality also increased with increase in time intervals after treatment. The overall mean mortality at 0.25, 0.50, 0.75, 1.00 and 1.50 h after treatment (HAT) indicated that 4% solution of the extracts of V. amygdalina, S. acuta, O. gratissimum and T. occidentalis showed the highest mortality of 33.60, 31.47, 28.80 and 15.20, respectively in A. obtectus at 1.50 h after treatment. It could be inferred from the study that extract from V. amygdalina leaf could cause the highest significant mortality. Key words: Phytochemical, Ocimum gratissimum, Sida acuta, Telfaria occidentalis, Vernonia amygdalina.