Bioefficacy of Plectranthus kirbii Powder and Extracts on Stored Cowpea Pest Callosobruchus maculatus (Coleoptera: Chrysomelidae)

Abstract

Cowpea seed constitutes an important source of proteins for populations in Sub-Saharan Africa. But this food resource is heavily damaged by cowpea beetle Callosobruchus maculatus. The control of that pest is mainly carried out by using synthetic insecticides. Despite the efficacy of this method, it caused environmental and health problems. Therefore, the search for alternative methods is vivaciously needed. In this issue, the bio-efficacy of Plectranthus kirbii extracts was assessed on C. maculatus regarding adult mortality, suppression of population and grain damage as well as seed viability preservation and repellency. The leaf powder and aqueous extracts of the plant were tested at 2, 4, 8 and 16 g/kg on bruchid adult for toxicity and damage bioassays. Repellency test was carried out using the plant aqueous, methanolic and ethyl acetate extracts at 0.5, 1, 2 and 4 mg/cm2. The seed viability was evaluated using seeds preserved for three months at the single concentration of 16 g/kg of each plant extract. Significant mortality of cowpea beetle was induced by the plant aqueous extract and leaf powder. LC50 values decreased with the increasing exposure period, and aqueous extract and leaf powder recorded 33.42 and 9.48 g/kg respectively within 3 days whereas within 5 days, the same extracts in the same order recorded LC50 of 1.31 and 8.73 g/kg respectively. These extracts significantly reduced damage by suppressing almost completely the bruchid population growth. The non-infested grain preserved recorded high grain viability compared to the infested ones. The non-treated infested recorded the lowest germination rate (11.33%). The repellency rate ranged from 38.75% to 83.75%. Ethyl acetate and methanolic extracts were classified as the class III repellent product, while aqueous extract ranged as class IV in repellency. Considering these findings, the extracts of P. kirbii could favourably replace the synthetic insecticides used in the cowpea protection during storage.