Toxicity of powder and extracts of <i>Zanthoxylum zanthoxyloides</i> Lam (Rutaceae) root bark from Nigeria to three storage beetles

Abstract

The root of Zanthoxylum zanthoxyloides Lam is used as antibacterial toothbrush in southwestern Nigeria. The root bark was therefore screened as powder, aqueous and ethanolic extracts for toxicity to adult Callosobruchus maculatus F. (Coleoptera: Bruchidae), Sitophilus zeamais Motschulsky (Coleoptera: Curculionidae) and Tribolium castaneum (Herbst) (Coleoptera: Tenebrionidae) and the effects of the test extracts on oviposition and progeny development of C. maculatus in laboratory tests. A small scale field trial was also carried out to test the efficacy of test powder as a protectant of cowpea, Vigna unguiculata (L.) Walpers and maize, Zea mays L. grains against insect infestation. Results of the acute toxicity tests showed that all the formulations were toxic to the insects. The 48 h median lethal concentration (LC₅₀) values obtained for the test powder against C. maculatus, S. zeamais and T. castaneum are 0.05 g kg^-1, 0.01g kg^-1 and 0.04 g kg^-1, respectively. For the aqueous extracts the LC₅₀ values are 0.83 g L^-1, 0.34 g L^-1 and 0.38 g L^-1 against C. maculatus, S. zeamais and T. castaneum, respectively while the values are 0.02 g L^-1, 0.04 g L^-1 and 0.09 g L^-1, respectively for ethanolic extract, indicating higher toxicity against the test insects relative to the water-based extract. The ethanolic extract demonstrated residual property, the toxicity to C. maculatus remaining fairly constant over a total post-treatment time of 336 h. Cowpea grain treatment with test plant ethanolic extract resulted in reduction of the number of eggs laid from 93.30 ± 3.46 in the control to 21.00 ± 4.57 in grain treated with 0.10 g L^-1 extract without significant difference in the number of adult emergence from the treated grains. Field trials showed that cowpea and maize grains treated with test plant powder respectively were protected from insect infestation for 180 d. These results demonstrate the potentials of Z. zanthoxyloides for protecting cowpea and maize grains against storage insects.

Keywords: Zanthoxylum zanthoxyloides, Callosobruchus maculatus, Sitophilus zeamais, Tribolium castaneum, Toxicity