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Preliminary Study of the Repellent Activities of Botanical Extracts against Cowpea Weevil, Callosobruchus maculatus (Fab.) on Cowpea, Vigna unguiculata (L.)

ABSTRACT

Callosobruchus maculatus (Fab.) is a field-to-store pest that causes quantitative and qualitative losses and is primarily managed using synthetic pesticides, which have led to health issues and even death. In view of the recent increased interest in developing biopesticides as an alternative to synthetic pesticides, this study therefore aimed at investigate the synergistic repellent activities of botanical extracts against C. maculatus on cowpea. The study involved harvesting, washing, drying, pulverizing, and soaking fresh plant leaves of Ocimum grattisimum, Vernonia amygdalina, and Gongronema latifolium in distilled water (1:10) and hexane (1:10) for 24 hours. Bioactive components were analyzed using Gas Chromatography-Flame Ionization Detector (GC-FID). A stock solution of 200mg/ml of each extract was prepared, reconstituted with distilled water, acetone, and Tween80 separately, and other solutions with lower concentrations (20, 50, 100, and 150 mg/ml) were produced. The repellency action of the reconstituted extracts on C. maculatus adult was assessed through the filter paper repellency method in the laboratory, with three replicates, three negative controls, and one standard (2.5 μl/ml Deltamethrin synthetic insecticide). Results indicated that Apigenin (31.36%) dominated the 17 compounds found in V. amygdalina (aqueous (AQ) extract); whereas Artemetin (80.94%) dominated the 15 compounds found in V. amygdalina (N-Hexane extract). O. grattisimum (AQ extract) had 11 compounds dominated by Sinapinic acid (72.04%), while Syringic acid (56.91%) dominated N-Hexane extract which contains 16 compounds. G. latifolium (AQ extract) contains 14 compounds dominated by Kaempferol (47.22%), while Luteolin (22.15%) dominated N-Hexane extract which contains 11 compounds. The study found that the differences in percentage repellence of all the various extracts were not statistically significant (p>0.05) for all their concentrations except for 50mg/ml concentration in Tween80 reconstituted hexane extract. The results indicated that the highest repellency of the pest across the various extracts over 96 hours was produced by the highest concentration of 200 mg/ml. For AQ extract, the percentage repellency ranges from 16.7%-100%, and from 11.1%-100% for acetone reconstituted hexane extract, while for  Tween80 reconstituted hexane extract it ranges from 6.7%-100%. The repellency effects of extracts are ranked in descending order at concentration ≥ 50mg/ml: aqueous extract, acetone reconstituted hexane extract, and Tween80 reconstituted hexane extract. Overall, taking the results holistically for concentration ≥ 20mg/ml, the repellent efficacy of the extracts followed this trend: acetone reconstituted hexane > aqueous extract > Tween80 reconstituted hexane extract. From the results, it is clear that the extracts can be used as natural repellent against C. maculatus adult as alternative to conventional repellents.

Keywords: Callosobruchus maculatus, Ocimum grattisimum, Vernonia amygdalina, Gongronema latifolium, Cowpea, Repellence, Bio-pesticides