Larvicidal potency of plectranthus barbatus, Andr. Labiatae on some culicine and anopheline mosquitoes

Abstract

For a long time, plectranthus barbatus Andr. (Labiatae), a dicotyledonous shrub has been used successfully by the Luo people in Eastern Uganda as a mosquito repellent by placing it’s branches in windows. This act, implies that P. barbatus has mosquitocidal properties, which were investigated in this study with a major objective of finding out whether or not this plan contained bioactive compounds against mosquitoes. Samples of the plant were collected from Tororo District and separated into floral, leaf and stem portions. Following methods described by Odyek et al. (1989), cold extraction was carried out using ether, methanol and water. Crude extracts obtained, were tested for their toxicity against shrimp, mosquito larvae and adults, using bench to bioassay guided screenings, recommended for dose-effect experiments. Results obtained were analysed using Finney’s subprogram and three factor Analysis of Variance which was recommended in reviews cited by Simpson et al (1960). Examination of results obtained from the shrimp assay indicated significant bioactivity within the leaf and stem methanolic crude extracts. However, only ether crude extracts were found significantly bioactive against mosquito larvae. Floral and leaf ether crude extracts exhibited the highest activity against all mosquito larvae (LSD value 12.73; 10.75, p=0.05, 2df) unlike the stem extract (LSD value 6.78, p=0.05, 2df). Larval response varied among species with higher mortalities in Anopheles and Culex (LSD values respectively = 11.87; 11.22, p=0.05, 2df) unlike Aedes (LSD value 7.159, p=0.05, 2df). LD50 values obtained also agreed with the above results confirming that floral and leaf crude extracts of P.borbatus are significantly toxic to Culex, Aedes and Anopheles mosquito larvae unlike the stem extracts. All the ether crude extracts obtained did not exhibit toxic or repellent properties to Culex, Aedes and Anopheles adult mosquitoes. This contradicated with the indigenous knowledge and use of the plant by the Luo people in Eat Uganda, as a mosquito repellent. It was thus deduced that P.barbatus contains useful larvicidal coumpounds which could be isolated and identified.