A Toxicological study of Millettia Usaramensis Stem Bark extract on Aedes Aegypti (mosquito), Schistocerca Gregaria (desert locust) And Mus Musculus (Mouse)
Some of the many insects that proliferate in tropical environments due to conduciveness of its weather conditions are crop pests both in the field and in storage. Others transmit diseases and affect the health of both man and livestock. Synthetic insecticides have been used to control them but these have shown pest resistance, they bioaccumulate and are non biodegradable. This has led to a search for alternative insecticides and plant insecticides are a promising source. This study investigated the potential of Millettia usaramensis subspecies usaramensis stem bark extract as a plant insecticide and assessed its toxicological effect on mammals. Shadow dried and powdered M usaramensis stem bark was extracted using dichloromethanel methanol at the ratio of 1: 1 (v/v). The crude stem bark extract was tested for larvicidal activity against the 4th instar Aedes aegypti mosquito larvae. Probit analysis of the results showed a 48 hour activity with a median dose of 50.82 mg/L. The crude extract administered to the locust Schistocerca gregaria elicited insecticidal activity as indicated by LDso values of 445.65Jlg/g through injection at 48 hours, 569.77 ug/g through topical treatment at 72 hours and 504.69 ug/g through oral treatment at 144 hours post exposure. It also showed an anti-feedant activity of EDso 660.71 ug/ml. Pure compounds (+)-12a- epimilletosin, (+)-usararotenoid-A and deguelin were tested in Aedes aegypti 4th instar larvae and LCso activities of 2037 mg/L, 4.27 mg/L and 2.63 mg/L respectively were observed at 48 hours post exposure .. The effect of post-treatment temperature on the insecticidal activity of the crude extract was investigated in S. gregaria. Increase in temperature significantly increased toxicity of the extract by decreasing LDso from 913.65 ug/g at 25°C to 323.59 ug/g at 40°C in a 48 hour post-treatment exposure period. No significant toxicity of the extract was observed in the mouse Mus musculus showing that the extract is safe to non-target mammals. The results therefore indicate that M usaramensis is a potential source of botanical insecticide(s). Key words: Millettia usaramensis, plant insecticide, Aedes aegypti, Schistocerca gregaria, Mus musculus, toxicological study .