Evaluation of selected medicinal plant extracts for insecticidal properties against phlebotomine sand flies (Diptera: Psychodidae)
Njeru, Ireri Laban
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Phlebotomine sand flies are the vectors of Leishmania species, the causative agents of leishmaniases that are a global health problem and are usually endemic in developing countries affecting predominantly the poor in society. Complementally control measures through use of biodegradable, safe and species selective natural compounds are urgently needed. Plant based compounds are safe in most biosystems. Some synthetic chemical insecticides are toxic, expensive and discriminate use may cause vector resistance. In this study, the aerial parts of Tagetes minuta Linnaeus (Asteraceae), Acalypha fruticosa Forssk (Euphorbiaceae) and Tarchonanthus camphoratus L. (Compositae) currently being used in endemic areas as repellents were extracted using methanol and ethyl acetate prior to being filtered, dried out by rotary evaporation at 30-35°C then dissolved in Dimethylsulphoxide (DMSO) and prepared into 2.5, 5 and l0mg/ml using distilled water. The different concentrations of the extracts were evaluated against Phlebotomus duboscgi while incorporated in filter papers. Other sets of similar concentrations were prepared using 10% sucrose solution and used in feeding technique using cotton wool balls. Pyrethrin incorporated in artificial food was also investigated for insecticidal efficacy using feeding techniques to 10 adult sand flies and 10 larvae and replicated three times. Pyrethrin (0.6 %) was diluted in 10% sucrose solution at 0.08, 0.25 and 0.5 mg/ml and used in the adult feeding bioassays while bioassays on the larvae were conducted using the extracts, powdered plant parts, 0.2% pyrethrin powder and 0.5% EC incorporated in larval food. Mortality was recorded every 24 hours of exposure and the data entered into MS Excel and subjected to computerized probit analysis. The extracts had significant mortality to adult sand flies in both contact and feeding methods but exhibited no larvicidal activity. Extracts of A. fruticosa and T. minuta had significant mortality (P<0.05) to adult sand flies at 48 hours of exposure than those of T. camphoratus. The combination of the extracts yielded weaker insecticidal properties than separate extracts. The feeding techniques had higher LD50 values in all the bioassays at 48 hours of exposure when compared to the contact bioassays using similar concentrations. In the pyrethrin bioassay there was no significant difference between male and female LD50 at 48 hours of exposure. The powdered plant parts were not larvicidal. The two pyrethrin formulations were effective against larvae with a LD50 of 0.1 mg/ml in the third instar larvae groups yielding mortality of about 100% in only 96 hours of exposure. The plant extracts were found to be effective against adult phlebotomine sand flies while pyrethrin products were found to be efficient larvicides when incorporated in artificial sand fly larvae food and are therefore proposed to be harnessed, made into appropriate formulations and used alongside other control strategies.