Mosquitocidal activity against Anopheles arabiensis of plants used as mosquito repellents in South Africa.

Abstract

Ten plant species that are used as mosquito repellent in South Africa, Aloe ferox (leaves), Atalaya alata (leaves), Balanites maughamii (bark), Clausena anisata (leaves), Croton menyaarthii (leaves), Lippia javanica (leaves), Melia azedarach (leaves), Olax dissitiflora (bark), Sclerocarya birrea (seeds) and Trichilia emitica (seeds) were screened for adulticidal, larvicidal and repellent activities against Anopheles arabiensis, a potent malaria vector in South Africa. The plant extracts were screened following the WHO standard methods with slight modifications. The plant materials were extracted separately with ethanol (EtOH) and dichloromethane (DCM). All the extracts showed adulticidal activity. The highest activity was observed in the DCM extract of A. ferox leaves with an EC50 value of 4.92 mg/ml. With regards to larvicidal screening, all the DCM extracts showed larvicidal activity, while only five EtOH extracts showed activity. The highest larvicidal activity was found in the DCM extract of O. dissitiflora bark with an EC50 value of 25.24 μg/ml. All the investigated plants showed no repellent activity. Due to its good larvicidal activity, O. dissitiflora was evaluated for antibacterial, antifungal and antiplasmodial activities. The antibacterial activity was evaluated against two Gram-positive (Bacillus subtilis and Staphylococcus aureus) and two Gram-negative (Escherichia coli and Klebsiella pneumoniae) bacteria using the micro-dilution assay. The micro-dilution assay was also used to evaluate the antifungal activity of O. dissitiflora against Candida albicans. The antiplasmodial activity was evaluated against a chloroquine-sensitive strain of Plasmodium falciparum (D10) using the parasite lactate dehydrogenase assay. Both DCM and EtOH extracts showed good antibacterial activity against all four tested strains with MIC values less than 1 mg/ml. They also showed good antifungal activity with MIC values less than 1 mg/ml. Both DCM and EtOH extracts showed a moderate antiplasmodial activity, with IC50 values of 15.6 and 45 μg/ml, respectively. Good larvicidal activity observed in the DCM extract of O. dissitiflora bark prompted an attempt to isolate active compounds. Two active compounds were isolated from O. dissitiflora bark, ximeninic acid and a mixture of two closely related compounds (exocarpic acid and octadec-9,11-diynoic acid). The mixture of exocarpic acid and octadec-9,11-diynoic acid exhibited the highest larvicidal activity with an EC50 value of 17.31 μg/ml compared to ximeninic acid which had an EC50 value of 62.17 μg/ml. The results of the present study showed that the bark of O. dissitiflora and leaves of A. ferox may have potential to be used as larvicides and adulticides against An. arabiensis mosquitoes, respectively. This study also indicated that the bark of O. dissitiflora may have potential to be used as an antibacterial, antifungal and antimalarial agent.