Pesticidal Plants in African Agriculture: Local Uses and Global Perspectives

In 2001 in this publication we drew attention to the wealth of potential in pesticidal plants in Africa. We reported from a regional focus on West Africa where we had been undertaking research identifying new plant sources of pesticides, verifying their efficacy and considering how we could apply our scientific knowledge to improving the way farmers used them. Here, 15 years on, we consider how this research and development domain has changed, what are the success stories and priorities for Africa and where is this sector heading. Plants cannot move so must defend themselves against herbivores where they stand. We occasionally encounter these defensive traits when we are stung by nettles or pricked by a cactus. However, most plants depend on non-physical chemical mechanisms of defence which are often subtle but highly effective survival strategies. Farmers across the world have for centuries exploited these biological activities and used plants that have particularly potent defence chemicals to control pests in stored food or in the field. In fact, the earliest commercial pesticides were of plant origin such as rotenone and nicotine but the dawn of industrial organic chemistry provided a suite of highly effective pest control agents that left plant chemistry far behind. Yet, as the negative environmental impacts of synthetic chemicals began to emerge in the 1960s and 70s, the pendulum swung back towards more natural pest management strategies during the 1980s and 90s. Scientists were again full of optimism that plants might provide the next generation of effective but environmentally benign pesticides. A huge amount of time and resource was invested to study plants for their potential as pesticides. The better studied was the Neem tree (Azadirachta indica) which produces tetratnortriterpenoid feeding repellents and development inhibitors including zadirachtin A and several analogues. While some plant products have had limited commercial success globally these have largely been restricted to Neem (triterpenoids), Derris (rotenoids), Pyrethrum (pyrethrins) and various mixtures of essential oil products (e.g. 1, 8 cineole, a-pinene, verbenone). The optimism of the 80s and 90s unfortunately did not deliver a revolution in more natural pest control products, and a recent assessment of biological pesticides of any kind suggests they constitute less than 1% of all pest control products, with pesticidal plants being a small fraction of this.