Evaluation of the Knockdown and Insecticidal Potentials of the Essential Oils of Plectranthus amboinicus and Eryngium foetidum against Culex quinquefasciatus

Abstract

Culex quinquefasicatus is an important vector of lymphatic filariasis and also a well-known nuisance mosquito. The insect’s resistance to synthetic insecticides has made its management extremely challenging. This study evaluated the knockdown and insecticidal effects of essential oils of Plectranthtus amboinicus and Eryngium foetidum against Culex quinquefasicatus. The oils were obtained by hydro-distillation and were each separately serially diluted in 15% tween -80 solution to obtain five concentrations (31.25, 62.5, 125, 250 and 500 μl/ml). Each concentration had four replicates. The control (1ml of 15% tween -80 solution) was also replicated four times. Whatman No. 1 filter paper was separately impregnated with different concentrations of the oils and placed inside the WHO insecticide susceptibility test tubes. Twenty adult mosquitoes were exposed to each oil concentration and control to ascertain the knockdown effect at intervals of 5 for 30 while the insecticidal effect was observed for 60 minutes at intervals of 10 minutes. Complete knockdown was observed in the highest concentration (500μl/ml) for P. amboinicus and E. foetidum after 20 and 30 minutes respectively. Both oils showed increased mortality as concentration increased. After 60 minutes exposure period, P. amboinicus oil recorded 100% mortality against Cu. quinquefasicatus with all test concentrations while 100% mortality was only recorded against the insect after 60 minutes exposure to the highest concentration of E. foetidum oil. There was no knockdown or mortality recorded in the control experiment. Plectranthus amboinicus oil was more potent than E. foetidum oil. This was judged by the 30 minute KD50 value of 12.793 μl/ml and 303.883µl/ml for E. foetidum and P. amboinicus respectively. Also, the LC50 value for P. amboinicus was 67.019 μl/ml while that of E. foetidum was 643.663 μl/ml, supporting a higher susceptibility of Culex quinquefasicatus to the oil of P. amboinicus. Both plant oils exhibited activity against Culex quinquefasicatus but P. amboinicus oil produced a more appreciable potency and should be incorporated into the production of potent natural insecticides against Culex quinquefasicatus.