Toxicity and Synergistic Effect of Native Plants Extracts against the Cowpea Weevil (Callosobruchus maculatus Fab.) (Coleoptera: Bruchidae)

Callosobruchus maculatus causes loss of cowpea seeds and its weight after six months of unprotected storage. This study aimed to evaluate the toxicity and synergistic effects of plant extracts on the biological parameters of C. maculatus from Bobo Dioulasso in order to reduce its damage on cowpea stocks in Bobo Dioulasso city. Seven treatments including an untreated control and doses of three powdery extracts of Cassia nigricans, Zanthoxylum zanthoxyloides and Hyptis suaveolens, and their combinations were compared in a Fischer block with four completely randomized repetitions. One hundred healthy cowpea seeds were mixed with doses of each extract in Petri dishes then submitted as food substrates to ten pairs of C. maculatus for 15 days. The synergistic effect was evaluated following the formula of Chou and Talalay. Extracts of C. nigricans, Z. zanthoxyloides and H. suaveolens was toxic against the weevil. The extract of Z. zanthoxyloides was more toxic to adults (21.16 mg/g) followed by H. suaveolens (51.84 mg/g) and C. nigricans (52.09 mg/g). The infestation rates of batches treated with C. nigricans and Z. zanthoxyloides extracts did not exceed 1.00% compared to 2.00% for H. suaveolens at 20 mg/g. All combinations were synergistic and resulted in 100% mortality and less than 14.98 eggs/female in H. suaveolens coupled with C. nigricans. The application of Z. zanthoxyloides extract alone or the combination of H. suaveolens and C. nigricans extracts would be an alternative for the sustainable management of C. maculatus in cowpea stocks.