Effectiveness of Sphaeranthus suaveolens (Forssk.) DC. powder in the control of bean bruchid (Acanthoscelides obtectus) (Coleoptera: Chrysomelidae) in stored common bean (Phaseolus vulgaris L.) seeds

Abstract

Storage insect pests have been reported to cause significant postharvest loss particularly in grains and legumes thereby threatening food security. The current study was conducted to assess the effectiveness of Sphaeranthus suaveolens (Forssk.) DC. powder in the management of bean bruchids in three bean varieties; Jesca, Lyamungu 90 and Selian 97 in northern Tanzania. The experiment was carried out using complete randomized design from six treatments; (a) four (3.75 g, 7.5 g, 11.25 g and 15 g) of S. suaveolens powder; (b) one positive control (0.25 g of actellic super dust) and (c) one negative control (no treatments). Afterwards, the number of live and dead insects, number of seeds with holes, number of holes per seed and seed weight loss were examined for a period of 90 days. Results showed that seed damage was significantly lower in seeds treated with actellic super dust (positive control) followed by S. suaveolens powder (11.25 g and 15 g), and the two treatments did not differ significantly across bean varieties. Seed weight loss on the other hand differed significantly across the treatments and bean varieties. Highest mean percentage of seed weight loss was observed under negative control treatment (9%) and Jesca bean variety (4%) while the lowest (1%) was recorded in seeds treated with positive control (0.25 g actellic super dust), 11.25 g and 15 g of S. suaveolens powder and in the Selian 97 bean variety (2%). Furthermore, the phytochemical screening and GC-MS analysis were conducted to determine the nature and identity of secondary metabolites present in the S. suaveolens powder. Results showed the presence of fourteen compounds which are p-cymene-2,5-diol; thymoquinone; geranyl acetate; 2,6-octadienal, 3,7-dimethyl-; eugenol; cubenol; guaiol; dihydroagarofurane; camphor; 4 H-pyran-4-one, 2,3-dihydro-3,5-dihydroxy-6-methyl-; methylparaben; caffeine and 1-tetradecene. On the other hand, the groups of secondary metabolites revealed include terpenes, alkaloids, flavonoids, saponins, glycosides, steroids and anthraquinones. These results support the insecticidal properties of S. suaveolens and its potential to significantly reduce postharvest loss caused by insect pests such as Acanthoscelides obtectus (Coleoptera: Chrysomelidae) in common bean.