Assessing the efficacy of wild basil (Ocimum americanum L. (Walp)) in the management of cowpea weevil (Callosobruchus maculatus)

Abstract

Cowpea (Vigna unguiculata (L.) Walp) belongs to the family Fabaceae and is one of the pulses that suffer postharvest losses greatly. The major insect that attacks stored cowpea seed in Kenya is the cowpea weevil (Callosobruchus maculatus). The insect can damage 100% of stored cowpea seeds causing weight loss of up to 60%. Although insecticides are widely available, they have high potential risks to users and also pollute the environment. In drier areas of Kenya where the pest is common, small-scale farmers use botanicals with varying levels of success. This study sought to assess the effectiveness of a locally available botanical, Ocimum americanum, in the management of the cowpea weevil. Different plant parts of O. americanum, the leaves, flowers and whole mature young plants were dried and ground into powder. From each plant part, 0.5, 1.0, 2.0, 4.0 and 8.0 g were used to determine their effectiveness against the weevil in stored cowpea seed. Two controls were used, that is, no protectant and Actellic super dust. For each treatment, 10 g of cowpea seeds were placed in plastic vials containing ten pairs (male and female) cowpea weevil adults. The experiments were laid out in a completely randomized design at the Kenya Agricultural and Livestock Research Organization (KALRO), Nairobi, Entomology Laboratory. Lowest post-harvest weight loss (3.0 g) of cowpea seed and highest mortality (92.5%) of cowpea weevil was recorded when 8 g of leaf powder were used. Since wild basil is a common weed in drier parts of Kenya such as Mbeere, Tharaka-Nithi, Kitui, Makueni and Mwingi, it can be utilized in these areas as a cheap control bioinsecticide for cowpea weevil.