The final part of the research by Guèye examined the essential oils released by certain aromatic plants. “We have known for a number of years that these oils possess interesting properties for the control of pests, working either via simple contact or through inhalation by pests. This type of use is more familiar to farmers, even though they have conflicting opinions as to the efficacy of the oils: do their effects last for a month, five months or a year? And against which insects are they particularly effective? Questions of this nature are all the more vital as in hot countries like ours, evaporation occurs very quickly.” Guèye took a special interest in two species of the Lamiaceae plant family: Hyptis Suaveolens and, in particular, Hyptis Spicigera, the essential oils of which are widely used by farmers for their biocidal properties. Indeed, during the construction of storage granaries, farmers from some regions are used to incorporating one of these plants into the construction of the granary itself, in order to act as an insect repellent.

By comparing the content of the essential oils of these plants with the data obtained in other countries (particularly in Cameroon), Guèye has highlighted the importance of two points. First, there is the necessity for the farmer to harvest Hyptis when the repellent power of the plant is at its peak in December and January. This requirement is already well known in the field and widely implemented in practice. Second, what is the most efficient method to spread the plant’s essential oils around the granary? “The repellent effect is at its highest when the plant is carefully mixed in with the harvested crop in a ‘sandwich’ form. This involves the cobs of maize being placed on a layer of the plant and this arrangement being repeated in layers, which are then piled on top of each other. There is another possible variant of this: completely wrapping the cobs within an enveloping mass made up of the plants. This type of arrangement is little known and not practised in all regions. Promoting this practice where it is unknown could prove very useful.”

Discoveries such as those described here regarding the natural biocidal properties of plants could have consequences that extend well beyond Senegal. Boscia, for example, is found from the Atlantic coast as far as Sudan, Burkina Faso, Niger and Egypt. The widely spread shrub is available across the cited countries, and does not need to be transported over long distances.  “With the use of pesticides, farmers who were facing destruction of their stocks began to recover most of their harvests.  In order to persuade them to abandon this practice and to give them a greater awareness of more natural methods of crop protection, it is vital to describe and quantify the importance of these factors in relation to plants and their composite parts. Once these factors are known, they can be used as a complementary/alternative treatment to traditional agricultural knowledge, which is more widespread than is sometimes thought. What counts is complementarity: constantly combining experimentation in the laboratory with field observations that are directly rooted in local realities.” This is certainly a long-term task, but one that is also proving to be fascinating.