“Autumn Legionary Caterpillar.” Behind this warlike name lies a formidable crop pest, which attacks more than 80 plant species, including maize, rice, sorghum, cotton and market gardening.
A species from the tropical and subtropical regions of the Americas, this brown caterpillar was first detected in West Africa in early 2016. The butterfly (adult stage) can move more than 100 km overnight, which could explain its rapid progress: the species is now present in about 20 countries, from Senegal to South Africa and Kenya, including Togo, the DRC and Malawi.
This new threat is in addition to the many other pests that affect the productivity of agriculture on the continent. Thus, from 2003 to 2005, West Africa faced a spectacular locust invasion. Its control cost more than $400 million, and the swarms of these insects caused an estimated $2.5 billion in crop losses.
Fungi, bacteria, viruses, nematode worms, mites, insects… many aggressors attack the crops of African farmers. While their damage is not always as spectacular as that of locusts, its impact on agricultural yields is no less real. Each year, economic losses due to pests amount to millions of dollars, threatening the food security of tens of thousands of people.
More and more pesticides
To fight this voracious competition, farmers use pesticides. In West Africa, their imports increased by 19% per year during the 1990s, even though agricultural production increased by only 2.5% per year over the same period. The amount allocated to them represents up to 2% of total GDP, or even 6% if we consider only the agricultural share.
Beyond their cost, pesticides have a significant impact on human health, wildlife (terrestrial and aquatic) and the environment (soil and water pollution, etc.). Pesticides are often of poor quality or illegal, due to the low level of control of sales and distribution channels, and are generally poorly used, as farmers have little training in their use and little knowledge of pest biology.
Indeed, farmers and their families are frequently exposed to toxic products at high concentrations, some of which are banned in Europe (such as DDT or lindane). And ultimately, consumers may also be exposed to harmful pesticide residues.
Plants as an alternative to chemistry
While the use of synthetic products is a trend that does not seem likely to reverse in the near future, long-term strategies are being put in place to limit their use. In addition to training farmers, various alternative pest control methods are currently being tested: safety nets, biological control, diversified crop systems, etc.
Among these approaches, which are a priori more environmentally friendly, is the use of plants with pesticidal properties. Traditionally used in Africa, plants with phytosanitary effect are used in various ways: production of extracts applied to the leaves of the crops to be protected, use as whole plants or essential oils to protect stored foodstuffs, combined cultivation in the fields…
Several studies have assessed the potential of pesticidal plants traditionally used in West Africa. Their conclusions are interesting: many plants have a real effect on crop stressors.
This is particularly the case for neem (Azadirachta indica), a tree native to India whose leaves and seeds have insecticidal, antifungal and deworming properties. The application of neem extracts to tomato crops can reduce the severity of fungal infections, limit the hatching of lepidopteran eggs, or modify the fertility or behaviour of certain insects. Caterpillar or aphid populations are lower on plots treated in this way than on others.
Other plants produce similar results, such as spicy substances from Guinea pepper (Xylopia aethiopica), black mustard (Sinapsis nigra) or tobacco (Nicotiana tabacum). On plots treated with these extracts, populations of beetles, whiteflies or thrips decrease in size from 61 to 78%. Eradication is not complete, but yields are equivalent to those obtained on plots treated with synthetic insecticides. Moreover, these effects can be improved by mixing extracts of various pesticide plant species.
Finally, some species, such as those belonging to the genus Ocimum (which contains basil in particular), have the advantage of having not only pesticidal but also medicinal properties, and can also be consumed as leafy vegetables or spices. This versatility greatly enhances their interest.
A limited use
Pesticidal plants do not eliminate all pests, but keep their populations below the pest threshold, while offering many advantages over synthetic pesticides. Generally less dangerous to health, pesticide plant extracts decompose rapidly in the environment, limiting the risk of environmental pollution and improving the health quality of the products grown. Cultivated in association, pesticide plants make it possible to maintain the balance between crop pests and associated auxiliary animals.
The use of these natural products can in some cases increase yields, with a cost-benefit ratio similar to that of synthetic pesticides. However, to achieve their full potential, there are still several barriers to overcome.
The first concerns their acceptance by farmers, who consider that their use is too restrictive (time required to produce extracts, number of treatments required, specificity of extracts limiting, variability of results…). Their effects on crop auxiliaries (e. g. ladybirds) are not well known. Their cost is also problematic, because for the time being, when these products are intended to be marketed, they are manufactured in small quantities by small production units.
Finally, the latter cannot be part of the assessment procedures required by the (rare) existing regulatory frameworks. The latter, which are very heavy, are indeed the same as those required for synthetic pesticides, and small production structures cannot comply with them. This constraint, in particular, limits the commercial viability of pesticide plants.
Beyond these intrinsic and structural problems, the development of plant pesticides is also hampered by the availability and dissemination of knowledge.
Although the use of pesticide plants is an ancestral practice, the associated knowledge is fragmented and dispersed within communities. The identification of this knowledge, which has already been undertaken in some English-speaking African countries, has yet to be carried out in French-speaking countries.
This is the objective of the “Knomana” project (Knowledge management on pesticide plants in Africa). Initiated as part of “Glofoods”, a programme jointly run by INRA and CIRAD, it aims to identify, through the literature, pesticide plants, their uses, their modes of action, the organisms they are likely to target, their collateral effects (on non-target organisms for example), etc.
The knowledge collected will be formalized through a knowledge base and then disseminated. Although plant pesticides are far from replacing synthetic pesticides in field crops, they could nevertheless constitute a viable alternative in market gardening production on small areas. Today, farms of less than 15 ha generate nearly 80% of the food consumed in sub-Saharan Africa.