Screening of selected ethnomedicinal plants from South Africa for larvicidal activity against the mosquito Anopheles arabiensis.

BACKGROUND: This study was initiated to establish whether any South African ethnomedicinal plants (indigenous or exotic), that have been reported to be used traditionally to repel or kill mosquitoes, exhibit effective mosquito larvicidal properties. METHODS: Extracts of a selection of plant taxa sourced in South Africa were tested for larvicidal properties in an applicable assay. Thirty 3rd instar Anopheles arabiensis larvae were exposed to various extract types (dichloromethane, dichloromethane/methanol) (1:1), methanol and purified water) of each species investigated. Mortality was evaluated relative to the positive control Temephos (Mostop; Agrivo), an effective emulsifiable concentrate larvicide. RESULTS: Preliminary screening of crude extracts revealed substantial variation in toxicity with 24 of the 381 samples displaying 100% larval mortality within the seven day exposure period. Four of the high activity plants were selected and subjected to bioassay guided fractionation. The results of the testing of the fractions generated identified one fraction of the plant, Toddalia asiatica as being very potent against the An. arabiensis larvae. CONCLUSION: The present study has successfully identified a plant with superior larvicidal activity at both the crude and semi pure fractions generated through bio-assay guided fractionation. These results have initiated further research into isolating the active compound and developing a malaria vector control tool.

Screening for adulticidal bioactivity of South African plants against Anopheles arabiensis.

BACKGROUND: This study was conducted to evaluate whether a selection of South African ethnomedicinal plants included in this study displayed insecticidal properties when screened against adult stages of the mosquito. METHODS: 381 crude extracts of 80 plant taxa in 42 families were sprayed onto ceramic tiles and screened using the cone bio-assay method for insecticide efficacy testing. Blood-fed, female Anopheles arabiensis mosquitoes were exposed to the treated tiles for a period of sixty minutes. Mosquito mortality was monitored for twenty-four hours. RESULTS: Of all the extracts analysed, the highest activity was observed in Ptaeroxylon obliquum (Ptaeroxylaceae) and Pittosporum viridiflorum (Pittosporaceae), a single extract from each, exhibiting more than 50% mortality. A large proportion (81.63%) of the extracts tested displayed low levels of mosquitocidal activity. The remainder of the extracts (17.85%) exhibited no bioactivity (0% mortality). CONCLUSIONS: The screening results have shown that in accordance with WHO standards, none of the crude extracts tested had exhibited greater than 60% mortality against the adult stages of the malaria vector Anopheles arabiensis.

Evaluation of selected South African ethnomedicinal plants as mosquito repellents against the Anopheles arabiensis mosquito in a rodent model.

BACKGROUND: This study was initiated to establish whether any South African ethnomedicinal plants (indigenous or exotic), that have been reported to be used traditionally to repel or kill mosquitoes, exhibit effective mosquito repellent properties. METHODS: Extracts of a selection of South African taxa were tested for repellency properties in an applicable mosquito feeding-probing assay using unfed female Anopheles arabiensis. RESULTS: Although a water extract of the roots of Chenopodium opulifolium was found to be 97% as effective as DEET after 2 mins, time lag studies revealed a substantial reduction in efficacy (to 30%) within two hours. CONCLUSIONS: None of the plant extracts investigated exhibited residual repellencies >60% after three hours.