Protectant Effect of Vegetable Oils against Cowpea Weevil (Callosobruchus maculatus) on Stored Cowpea [Vigna unguiculata (L.) Walp]

Abstract

Cowpea is one of the common edible annual herbaceous legume. Callosobruchus maculatus is one of the common post-harvest pest of cowpea.This work was aimed at determining the protectant effect of five vegetable oils; coconut oil, cotton seed oil, groundnut oil, palm oil and sesame oil for the protection of stored cowpea against cowpea weevil (Callosobruchus maculatus).The study was conducted in Gombe State University between October, 2018 and April, 2019. Black eye seeds were used for the experiment. A completely randomized design with five replicates per treatment was used.Black eyed seeds (susceptible variety) used for the experiment were subjected to different oil treatments namely: coconut, cotton, groundnut, palm and sesame oils applied at 0.4, 0.8 and 1.2 ml/100 g of cowpea equivalent to 4, 8 and 12 kg. Effect of the oils on weight lost and seed germination was also tested.Percentage adult mortality at 12 ml/Kg revealed that all the five oils killed 100% of the insects. Among the different edible oils evaluated, cotton seed oil was more effective killing 95% of the insect at 4 ml/kg and 100% at 8 ml/kg. The lowest mortality was recorded in the treatment with palm oil with 60 and 91.67% mortality at 4 and 8 ml/kg dose treatment respectively. Minimum percentage weight loss after 90 days of storage was observed in grains treated with cotton seed oil (1.95%) followed by sesame oil (2.00%) and groundnut oil (2.38%) at 12 ml/kg grains. Cotton seed oil, groundnut oil and sesame oil at doses between 8 and 12 ml/kg could be effective for protecting cowpea in storage against cowpea weevil for 90 days of storage.All the treatments indicated minimal effect on the germination of cowpea. The 8 ml/kg of cotton seed, groundnut and sesameoils investigated are promising bio friendly preservatives that could serve as an alternative formulations to synthetic chemical based insecticides for storage of cowpea.