Giving the background to her pet research product, Karigidi said “weevils that attack beans and maize in storage can destroy as much as 40 per cent of these crops, causing huge loss to farmers and traders.”Farmers and food crop traders who are mostly uneducated, she added, indiscriminately apply insecticides to these crops in storage to minimise or eliminate losses as much as possible.
That practice has serious health and environmental implications, so said the researcher while speaking with The Guardian. “So, as a biochemist working on medicinal plants, I decided to investigate and develop a plant-based bio-pesticide to get rid of insect pests that attack food crops in storage, particularly beans and maize weevils using lemongrass leaves.” She said lemongrass is used locally for the treatment of mild fever, malaria and is also used in green tea products because of its medicinal properties. It has a long record of extensive therapeutic applications in traditional medicine in many countries across the globe.
“Apart from its medicinal properties, some people believe that growing the plant around their home can prevent the invasion of snakes and other reptiles. This belief motivated my team members and I to investigate the insecticidal ability of the plant against weevils,” she explained.
The researchers obtained the distillate of lemongrass by a distillation process and treated weevil-infested beans with different concentrations of the distillate. Prior to this, they reared beans weevils to breed them in large quantities for use. “The result was interesting, as we recorded the death of adult weevils within a short while. I thought of strengthening this effect by combining other botanicals like orange distillate for example, to produce a cocktail effect. The bio-pesticide formulated from this yielded 100 per cent weevil mortality within one hour of exposure,” she disclosed.
In another experiment, the researcher infested clean bean seeds that were without eggs with weevils and allowed the females lay eggs on the seeds. Female weevils began to lay eggs within 12 to 24 hours. The seeds containing visible weevil eggs were divided into groups and treated with different concentrations of the formulated distillate and then incubated in the dark for eight hours at room temperature to monitor adult emergence.Compared to the control group that was untreated, the treated groups showed no emergence of larva or adult weevils and no reduction in the weight of treated seeds.
The biochemist said the formulated bio-pesticide led to the mortality of both weevils and eggs, adding that further studies were done to obtain the most effective dosage.
Dosage and method of application
Based on the bio-efficacy of the bio-pesticide formulation carried out by the Nigerian Stored Products Research Institute (NSPRI), Ibadan, Nigeria, 6 ml of the product can effectively control 100g of weevil infested maize within 30 minutes of exposure. They recorded 100 per cent mortality of maize weevils in 30 minutes. At a lower dosage of 3 ml per 100g maize, 100 per cent weevil mortality was achieved after 24 hours of exposure.
For beans weevil control, 30 ml will get rid of all weevils in 3kg of beans within 30 to 60 minutes of exposure.However, the findings showed that preservation rather than control is the better approach for the use of the bio-pesticide, as it requires smaller quantity of the product to prevent weevil infestation on clean seeds and grains than already infested ones.
“So, immediate spraying of sorted harvested beans or maize grains with little quantity of the product and allowing to air dry for five to 10 minutes before bagging will effectively prevent weevil attacks,” the scientist said.
Application of the bio-pesticide, both for control and preventive purposes against weevil attack, can preserve the treated food item for up to a year and six months without the need for re-application. Re-application is often needed in the case of the currently used synthetic pesticides to preserve for this length of time.
Another advantage, she mentioned, is the well published medicinal properties of the plant, including anti-inflammatory, anti-bacterial, anti-fungal and antioxidant properties, which are also beneficial to humans.
The scientist said there was no recorded health implication for the consumption of lemongrass-treated food items, although there may be individual allergy to the lemongrass smell of the product.
Mode of action
The insecticidal and ovicidal effects of the bio-pesticide occur through inhalation and contact. The terpenes and terpenoids, which are plant secondary metabolites present in the plant formulation, cause symptoms that suggest a neurotoxic mode of action on treated beans and maize weevils. On prevention of weevil infestation, the lemony smell of the product repels weevils, thereby preventing their attack and the possibility of laying eggs.
Product barrier and its commercialisation
The chief developer of the product said the present barrier to the commercialisation of the product, which has been patented under the Trademarks, Patents and Designs Registry of the Federal Ministry of Industry, Trade and Investment of the Federal Republic of Nigeria, is the cost price.
She estimated that selling price is relatively higher when produced at the small-scale level, but upon licensing to big manufacturers, production cost would be reduced when material extraction is done large-scale. That way she said, the product could be made available to farmers, traders and households at affordable and competitive rates, especially for organic products.
Mojisola Karigidi was a 2017 Aspen New Voices fellow and a 2016 fellow of the Mandela Washington fellowship programme. She was selected as a 2014 fellow of the African Women in Agricultural Research and Development (AWARD) and became an awardee of the Global Innovation through Science and Technology (GIST Tech-I) competition in 2015 based on the lemongrass pesticide. She holds a Master’s degree in Biochemistry from the University of Ibadan.