EFFECT OF ESSENTIAL OILS FROM SOME TROPICAL PLANTS ON THE OXIDATIVE AND CHOLINERGIC ENZYMES OF CALLOSOBRUCHUS

Abstract

The present study was undertaken to investigate the insecticidal activities of essential oils from Piper guineense, Ocimum gratissimum, Ocimum basilicum, Citrus sinensis and Syzygium aromaticum and their mode of action against the major post-harvest insect pest of cowpea, Callosobruchus maculatus (weevils). The preparation of the essential oils was carried out by hydrostillation in an all glass Clevenger apparatus. The fumigant toxicity of the essential oils was carried out on weevils. The effects of the essential oils on the activities of acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) in vitro were determined. Then C. maculatus were exposed to 25 μl and 50 μl of the essential oils and the activities of superoxide dismutase, catalase, glutathione-s-transferase and AChE were determined in vivo. The ability of the essential oils to induce reactive oxygen species production and lipid peroxidation were determined in vivo. Then safety evaluation of the essential oils was carried out in rats. Gas chromatography−mass spectrometry (GC-MS) analysis of the essential oils was also carried out. The essential oils were shown to possess fumigant toxicity against C. maculatus when exposed to varying (12.5- 150 μl) doses of the oils. The fumigant effect of the essential oils increased with the increasing dose and time (24- 72 hours). The results also showed that the essential oils inhibited AChE and BChE activities in vitro, in a concentration-dependent manner (32.82- 93.85 μl/mol). The GC-MS analysis revealed that the five essential oils had many active compounds ranging from terpenes, benzene derivatives, hydrocarbons etc. Elemicin (25.53%) was the most abundant in P. guineense, Limonene (76.14%) in C. sinensis, Terpinene-4-ol (29.40%) in O. gratissimum, Eugenol 35.55% in O. basilicum and Methyl hexanoate 22.12% in S. aromaticum.The activity of the superoxide dismutase (SOD) enzyme increased at lower dosages of the oil (25 μl) and reduced at higher dosage of the oil (50 μl). There was significant (p < 0.05) decrease in glutathione S-transferase (GST), Catalase and AChE activities in vivo in weevils treated with essential oils when compared with control which were not exposed to essential oils and the decrease was dose-dependent. There was significant (p < 0.05) increase in reactive oxygen species production and lipid peroxidation in vivo when compared to control and the increase was dose-dependent. In the toxicology assay using rat, there were no mortality or change in weight observed in the rats at all doses (50, 150 and 300 μl/kg) of the essential oils. Also there was no significant change in the plasma liver marker enzymes, plasma renal marker enzymes and tissue malondialdehyde levels of rat administered 50, 150 and 300 μl/kg of essential oils when compared to the control. From this study the essential oils from P. guineense, O. gratissimum, O. basilicum, C. sinensis and S. aromaticum are effective in control of weevils and may be safe for human consumption at tested dosage. Ocimum basilicum essential oil had the best insecticidal property.