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Growing the wrong Tephrosia

A plant used by farmers in Africa to control pest insects occurs in two varieties. Only one is effective, and many farmers are growing the wrong type.

Tephrosia vogelii

Synthetic pesticides can be problematic in African farming systems due to environmental persistence, exposure to hazardous chemicals, cost and adulteration (dilution) by unscrupulous traders. Pesticidal plants provide the poorest farmers with a low-cost alternative that is environmentally benign and less toxic.

Working with University of Greenwich’s Natural Resources Institute and Mzuzu University in Malawi, Kew has been optimising the use of pesticidal plants by determining mechanisms of activity and understanding better the behaviour and variability of the target pests.

Tephrosia vogelii is a popular pesticidal plant in Africa, although surveys in Malawi revealed it was not always effective. This has been explained by the identification of two phenotypes that differ chemically. One contains insecticidal rotenoids, whereas the other does not and is ineffective at controlling insects.

Approximately 25% of farmers were found to be using the ineffective plant material in an attempt to control pests. This information has helped optimise processing and application of Tephrosia. Farmers typically use water to extract the active chemicals, but rotenoids are only sparingly soluble in water. Adding liquid soap – a cheap and easily available additive – greatly improves extraction efficiency and increases efficacy by acting as a spreading and sticking agent.

Item from Prof. Philip Stevenson (Natural Product Chemist, RBG Kew/NRI)

Article references:

Belmain, S.R., Amoah, B.A., Nyirenda, S.P., Kamanula, J.F. & Stevenson, P.C. (2012). Highly variable insect control efficacy of Tephrosia vogelii chemotypes. J. Agric. Food Chem. 60: 10055−10063.

Stevenson, P.C., Kite G.C., Lewis G.P., Nyrienda S.P., Forest F., Belmain, S.R, Sileshi, G. & Veitch, N.C. (2012). Distinct chemotypes of Tephrosia vogelii and implications for their use in pest control and soil enrichment. Phytochemistry 78: 135-146.

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