Toxicity and repellent effects of leaf extracts of four Kenyan plants against nymphs and adults of Rhipicephalus appendiculatus (Neuman 1901)

Abstract

Ticks are important arthropod vectors of diseases of man and farm animals in Africa and in spite of the efforts to control them using synthetic acaricides, they continue to be amongst the leading vectors of disease-causing agents. Unfortunately, over-reliance on synthetic acaricides for tick control has led to problems including acaricide resistance, environmental pollution, and acaricide residues in products destined for human consumption. These problems highlight the need for alternative tick control methods that are environment friendly and yet effective. In the current study, the repellency and toxic effects of essential oils extracted from four Kenyan plants namely;Tagetes minuta L.(Asteraceae), Fuerstia. africana T.C.E. Friers (Lamiaceae), Tephrosia vogelii Hook. f. (Leguminosae) and Sphaeranthus ukambensis against the ixodid tick Rhipicephalus appendiculatu were determined. Essential oils were extracted by steam distillation from fresh plants using Clevenger apparatus. Host seeking nymphs (2-3 weeks old)and adult ticks were used and fumigant toxicity bioassay was done using four concentrations (0.1µ/ml, 0.01 µ/ml, 0.001 µ/ml, and 0.0001µ/ml) of the essential oils, each in four replicates and with ten ticks per replicate. In the control experiment, ticks were exposed to n- hexane. Mortality was recorded every 48 hours for 2 weeks. In repellency bioassay, a Y tube olfactometer and same concentrations as above were used. Treated paper discs were placed in one arm and control (discs treated with hexane) in the other. Ticks were introduced at the main arm and allowed to make choices on the direction of movement. Ticks on the two arms were counted after 30 minutes. The essential oils showed dose dependant mortality and repellency values. The highest repellency value for R. appendiculatus adults was observed with T. vogelii (93.8% at 0.01µ/ml) while for the nymphs, both S. ukambensis and T. vogelii recorded the highest repellency values of 100% and 95% respectively both at 0.1µ/ml concentration. Essential oils from Fuerstia. africana recorded the highest mortality values for nymph (97.8% at 0.1µ/ml) and adult (53.2 % 0.1µ/ml) Rhipicephalus appendiculatus . The present study has demonstrated that Tephrosia vogelii, Tagetes minuta and Sphaeranthus ukambensis have repellent and toxic effects on Rhipicephalus appendiculatus and therefore can