24 April 2017
Using pesticidal plants for crop protection
Phil Stevenson, from Kew's Jodrell Laboratory, reports on how small holder farmers in Africa use wild plants to control pests and how his new projects are helping optimise their use and reduce pressure on wild habitats
While we worry about excessive salt and sugar in our breakfast cereals and debate whether red wine is good for us one week and bad for us the next, it’s easy to forget how much we take our food for granted.
Our worries generally revolve around convenience, discounts and choice – we have lots of choice and, apart from the occasional ready meals scandal, the selection available to us is mostly of good quality food. However, this is not the case for millions of people across the world, who lead a subsistence lifestyle and who grow their food on somewhere in the region of 500 million smallholder farms. These provide employment for around one in four people on the planet and produce 80% of all food in developing countries, so are critical for global health.
The majority of farming in sub-Saharan Africa (SSA) is carried out on small holdings, often of less than 1 hectare - smaller than the average farm in East Anglia under the rule of Edward 1 (c1280) (Eastwood et al., 2004). Size limits what farmers can produce, which is a great burden when most are entirely reliant for their families’ livelihood on this small patch of ground. Add to this the many food production constraints faced by small holders, such as inadequate water, insect pests and diseases as well as poor quality soil and it’s clear that this is precarious existence.
However, with appropriate new technologies farmers can be helped to produce enough food for their families and potentially to produce excess from which the small profits can make a huge difference to their wealth. According to the International Fund for Agricultural Development (IFAD) an increase of just 1% in agricultural per-capita Gross Domestic Product (GDP) would reduce the poverty gap five times more than a similar increase in GDP in any other sector, and would particularly target the poorest people. So helping smallholders to produce more food can help alleviate poverty.
Over the past few years, Kew in collaboration with the Natural Resources Institute (University of Greenwich) and its partners in Tanzania, Malawi, Zimbabwe, Zambia and Kenya, have been helping famers fight back against insect pests through exploring the potential to utilise the pesticidal properties of plants. Insects are ever present in field crops and in storage and can have a devastating impact on production. While commercial insecticides are used widely, their availability is unreliable in remote rural areas. They are frequently adulterated (diluted to ineffective concentrations by unscrupulous traders), outdated (owing to their toxicity to people and the environment), and ineffective owing to rapid evolution of pesticide resistance.
Health and safety
Health and safety is also a serious issue. Insecticides are typically applied with no protective clothing, there is no mechanism to ensure food safety for consumers, and little concern for the chronic effects of exposure. The environmental impact for wildlife, crop pollinators and natural enemies is also severe and insecticides are costly. However, some form of pest control is necessary – without it crop damage is guaranteed. For example, nearly US$4 billion of the staple food maize is lost annually in Africa to insects.
The alternative we are developing and promoting with our network of African scientists and hundreds of African farmers is the use of plants to control pests. Many plants can protect themselves against insects by producing their own chemical defences that are toxic or repellent. We have demonstrated, for example, that sweet potato, Ipomoea batatas (Convolvulaceae), an important staple food in Uganda, produces natural defences in this way. We are currently trying to find ways of optimising natural defences in this particular crop, so that farmers needn’t use pesticides at all (Anyanga et al., 2013). Other crops, however, that have been bred so intensely for yield that they have lost their natural defence mechanisms, need help to defend themselves. We are therefore looking to wild plant species to provide an effective alternative to pesticides to help farmers secure the food they grow.
We have recently evaluated several pesticidal and repellent plants including goat weed, Ageratum conyzoides (Asteraceae), Siam weed, Chromolaena odorata (Asteraceae), Cinderella weed, Synedrella nodiflora (Asteraceae), chili pepper, Capsicum annuum (syn. C. frutescens; Solanaceae), cassia, Senna sophera (syn. Cassia sophera; Leguminosae), physic nut, Jatropha curcas(Euphorbiaceae), castor oil plant, Ricinus communis (Euphorbiaceae), and basil, Ocimum gratissimum (Lamiaceae). Most of these are as effective at controlling pests as the synthetic products, but are much less harmful to the beneficial insects such as the ladybirds, spiders and wasps that feed on the pests and help to control aphids and caterpillar pests (Amoabeng et al., 2013). We’ve even found that aqueous fruit extracts of Strychnos spinosa (Loganiaceae) and Solanum incanum (Solanaceae) can control disease-spreading ticks of livestock (Madzimure et al., 2013), and we are currently working with farmers to help optimise how the plant materials are applied.
Cheap to grow
Another advantage of these so called ‘pesticidal plants’ is that they are cheap to grow (much cheaper than buying pesticides; Amoabeng et al., 2014) and can be harvested from neighbouring agricultural ecosystems (Grzywacz et al., 2014). The use of plants in this way is also effective because they are trusted by farmers, environmentally benign, safer than synthetic pesticides and are difficult to adulterate when produced or harvested by farmers themselves. All in all, plants are a compelling alternative to synthetic pesticides.
As a result of our recent progress (see Adappt link below), Kew and the Natural Resources Institute have been awarded two new funded initiatives by the European Union’s Africa, Caribbean and Pacific Science and Technology Program and the McKnight Foundation’s Collaborative Crop Research Programme to help take these success stories forward across eastern and southern Africa. Our new focus will be on the cultivation and propagation of the most effective pesticidal plant species, to reduce pressure on natural populations and ensure supply. We will also improve the way farmers apply the materials and look into the commercial potential of some species, to help community based farming groups produce their own pest control products and help them create additional wealth by selling to other farmers. This will need support from relevant African governments and will rely on policy interventions, which our work will also address (Sola et al., 2014).
- Amoabeng, B. W., Gurr, G. M. Gitau, C. W., Nicol, H.I., Munyakazi, L. & Stevenson, P. C. (2013). Tri-trophic insecticidal effects of African plants against cabbage pests. PLoS One. 8(10): e78651. Available online
- Amoabeng, B. W., Gurr, G. M. Gitau, C. W. & Stevenson, P. C. (2014). Cost: benefit analysis of botanical insecticide use in cabbage: implications for smallholder farmers in developing countries. Crop Protection 57: 71-76. Available online
- Anyanga, O.M., Muyinza, H., Hall, D.R., Porter, E., Farman, D.I. Talwana, H., Mwanga, R.O.M. & Stevenson, P.C. (2013). Resistance to the weevils Cylas puncticollis and Cylas brunneus conferred by sweetpotato root surface compounds. Journal of Agricultural and Food Chemistry 61: 8141-8147. Available online
- Eastwood, R., Lipton, M., Newell, A. 2004. Farm Size. Paper prepared for Volume III of the Handbook of Agricultural Economics. Available to download
- Grzywacz, D., Stevenson, P.C., Belmain, S.R. & Wilson, K. (2014). The Use of Indigenous Ecological Resources for Pest Control in Africa. Food Security 6: 71-86. Available online
- Madzimure, J., Nyahangare, E.T., Hove, T., Hamudikuwanda, H., Belmain, S.R. Stevenson, P.C. & Mvumi, B.M. (2013). Efficacy of Strychnos spinosa (Lam.) and Solanum incanum L. aqueous fruit extracts against cattle ticks. Tropical Animal Health and Production 45: 1341–1347. Available online
- Sola, P., Mvumi, B.M., Nyirenda, S.P.M., Ogendo, J.O., Mponda, O., Andan, F.P.H., Kamanula, J.F., Belmain S.R. and Stevenson. P.C. (2014). Botanical pesticide production, trade and regulatory mechanisms in sub-Saharan Africa: making a case for plant-based pesticidal products. Food Security, (in press).