The Potentials of Bacillus thuringiensis Isolate and Its Extracellular Cuticle-Degrading Enzymes Activity in the Biological Control of Callosobruchus maculatus (Cowpea Weevil) in Stored Vigna unguiculata L. Walp (Cowpea)

Abstract

This study assessed the pesticidal effect of Bacillus thuringiensis isolate against Callosobruchus maculatus (cowpea weevil) infesting stored cowpea seeds. B. thuringiensis isolate was obtained through liquid fermentation using sucrose water as a substrate, then transferred onto talc powder for solid formulation. A six-month shelf-life study of the solid formulation was conducted using total microbial plate count. Insect mortality bioassays were performed by applying liquid and solid B. thuringiensis formulations to cowpea seeds containing first-generation adult C. maculatus, with mortality recorded over 12 and 7 days, respectively. Enzyme activity was assessed using specific enzyme substrates. Results showed a gradual decline in microbial count over time in the solid formulation. The bioassay revealed 100% mortality for the liquid medium and for the solid medium of the B. thuringiensis isolate. The B. thuringiensis isolate also significantly delayed the first-generation emergence of C. maculatus. Enzyme analysis indicated the production of cuticle-degrading enzymes—protease, lipase, and exochitinase with varying activity levels. This study concludes that B. thuringiensis isolate effectively controls C. maculatus, likely due to its cuticle-degrading enzymes, and has potential as a bio-control agent for stored cowpea pest management. However, higher concentrations are necessary in talc based formulations to maintain an adequate shelf life.