Comparative insecticidal properties of Basotho medicinal plants against Culex quinquefasciatus (Diptera: Culicidae) mosquitoes from the Eastern Free State Province of South Africa
Culex quinquefasciatus is a vector of human and animal disease causing pathogens that are of socioeconomic problem in developing countries. The mosquitoes have developed resistance against synthetic insecticides, hence the search for natural botanical insecticides. The present study was aimed at investigating the secondary plant metabolites, cytotoxicity, larvicidal, pupicidal and insecticidal potential of Artemisia absinthium, Artemisia afra, Cosmos bipinnatus, Foeniculum vulgare, Mentha longifolia and Tagetes minuta against C. quinquefasciatus. The leaf extracts contained mostly saponins, alkaloids, terpenoids, steroids and flavonoids. Plant aqueous and ethanol extracts exhibited cytotoxic effects for T. minuta (LC50 = 0.10 mg/ml; LC50 = 3.16 mg/ml), A. absinthium (LC50 = 2.89 mg/ml), C. bipinnatus (LC50 = 5.66 mg/ml; LC50 = 4.81 mg/ml), and A. afra (LC50 = 5.39 mg/ml) against brine shrimp nauplii. Ethanolic extract mortality and concentration doses had was significant difference (F5,5 = 13.69; P < 001) towards nauplii mortality. Most larvicidal bioactivities were observed in ethanolic and hexane extracts for F. vulgare (LC50 = 0.10 mg/ml; LC50 = 1.03 mg/ml), M. longifolia (LC50 = 1.05 mg/ml; LC50 = 0.10 mg/ml), T. minuta (LC50 = 1.17 mg/ml; LC50 = 1.01 mg/ml) and A. afra (LC50 = 1.02 mg/ml; LC50 = 1.14 mg/ml), and while larvae mortality and extract concentrations showed significant difference (F5,5 = 9.95; P < 0.01). Pupicidal bioactivity was displayed by both ethanolic and hexane extracts of A. afra (LC50 = 1.10 mg/ml; LC50 = 1.04 mg/ml), T. minuta (LC50 = 1.11 mg/ml; LC50 = 1.12 mg/ml), C. bipinnatus (LC50 = 1.14 mg/ml; LC50 = 1.16 mg/ml) and M. longifolia (LC50 = 1.13 mg/ml; LC50 = 1.21 mg/ml). The extract concentration level were directly proportional to pupa mortality percentage with M. longifolia (R2 = 0.85) and A. afra (R2 = 0.74). The aqueous extracts had no fatal effect on larvae and pupa at all the concentrations tested. The rate of knock-down was highest for M. longifolia (KD50 = 4.91 min-1) followed by F. vulgare (KD50 = 9.87 min-1), T. minuta (KD50 = 12.39 min-1), and A. afra (KD50 = 19.02 min-1). The insecticidal activity was greater in M. longifolia (LD99 = 0.25 g) followed by F. vulgare (LD99 = 0.25 g), T. minuta (LD99 = 0.25 g) and A. afra (LD99 = 0.25 g). The insecticidal mortality ratio between evaluated plants had significant difference (F5,4 = 283.11; P < 0.01). In this study, ethanolic and aqueous extracts had more cytotoxic activity against A. salina nauplii than the hexane extracts, whereas, ethanolic and hexane extracts exhibited stronger larvicidal and pupicidal activities than the aqueous extracts. The selected Basotho medicinal plants possessed convincing insecticidal, pupicidal and larvicidal activities and therefore can be recommended for mosquito control at Kroonstad as well as in nearby communities of the eastern Free State Province.