Influence of Cowpea Plant and Seed Characteristics and Packaging Material in Storage on Cowpea Weevil (Callosobruchus maculatus) Infestation

Abstract

Cowpea (Vigna unguiculata) is a self-pollinated popular legume. It is only propagated through seed. Cowpea is one of the popular African Indigenous Vegetables that can be consumed as vegetable and grains stew because it is rich in protein. Quality seed is a challenge facing African indigenous vegetable production in Kenya. Cowpea weevil is the main pest of cowpea seed causing damage during storage causing loss up to 100 % when the seed is stored without any form of treatment and improper packaging. This research was carried out to identify ways of minimizing seed damage through characterization of cowpea so as to identify superior lines, and to determine appropriate packaging material that can store cowpea seed, with minimum damage. Experiments were set up in October 2016 to August 2017 where plant characteristics were evaluated in field experiments. Upon harvesting the seed, characteristics were determined at the plant physiology laboratory in Jomo Kenyatta University of Agriculture and Technology. Morphological characteristics were assessed based on the cowpea descriptors of the International Board for Plant Genetic Resources. Seed biochemical composition including protein, tannin and amino acid were analyzed using the Association of Official Analytical Chemists methods. Amino acid composition was evaluated using Liquid Chromatography-Mass Spectroscopy at International Centre of Insect Physiology and Ecology for selected set of lines. An experiment to determine extent of damage among the lines due to cowpea weevil damage was carried out by storing the seed in cellulose bags for a period of three months in the laboratory in JKUAT. To assess effect of packaging material, a repeated experiment was carried out. A single cowpea variety was used to evaluate the efficacy of packaging materials; cellulose paper bags, gourds, grain storage bags, polyethylene bags and glass bottles. Completely Randomized Design method was used with each storage material being replicated thrice. Data was obtained on the number of seeds with holes, mean number of holes per seed, seed weight loss, damage score, and seed germination. Analysis of Variance was used to determine variation among cowpea lines for plant and seed characteristics, biochemical composition, seed damage and the variation in types of packaging materials for seed storage. Principal Component Analysis was used to classify cowpea lines. Correlation analysis was used to determine the relationship between plant and seed characteristics and pest infestation and relationship among seed damage parameters. From the evaluated characteristics, cowpea seed were classified based on skin characteristics. Significant differences (p≤0.05) on cowpea seed damage based on in plant and seed characteristics were observed. Cowpea lines had significant variation (p≤0.05) in protein level, tannin content and amino acid concentration. Both plant and seed characteristics had an influence on cowpea weevil infestation. Protein level and total amino acids were positively correlated (r= 0.698 and r=0.897 respectively) with cowpea weevil infestation while tannin content had a negative correlation (r=-0.666) with cowpea weevil infestation. There was significant variation (p≤0.05) among storage materials for the respective measured parameters for pest damage. The parameters used to evaluate damage were positively correlated except germination. Seed stored in glass bottles showed highest germination rate (92.7%); whereas, germination rates were less than 52% for the other seed packaging materials. Glass bottles were ranked the best packaging material (0.01% weight loss and no emergence holes), followed by polyethylene bags (7.5% weight loss and 5 emergence holes) and finally grain storage bags (7.5% weight loss and 5 emergence holes). Plant and seed characteristics among the cowpea varieties studied had an influence on the Callosobruchus maculatus infestation. Airtight packaging material effectively reduced weevil damage and preserved seed quality. Use of hermetic storage allows long term storage of cowpea and minimizes damage caused by the cowpea weevil. This information is important for farmers and breeders involved in cowpea improvement for weevil management, seed quality and sustainable production.