Biological Activity of Organic Leaf Extracts against the Maize Weevil, Sitophilus zeamais Motsch. (Coleoptera: Curculionidae)

Laboratory studies were conducted in order to evaluate the bioactivity of methanolic leaf extracts of four plant species namely Euphorbia balsamifera Aiton, Lawsonia inermis L., Mitracarpus hirtus (L.) DC and Senna obtusifolia L. against Sitophilus zeamais Motsch. Ten 0 to 7 day old weevils were released in 20 g of disinfested sorghum grains treated with methanolic extracts of the botanicals in plastic bottles, at different concentrations of 2.5, 5 and 10% at 30oC and 70% R.H. Adult mortalities of the weevils at 24, 48 and 72 hours after treatment (HAT) were determined. Percentage oviposition deterrence (POD) and adult emergence as well as grain weight losses were also assessed. Results showed that adult mortality of the weevils ranged from 50.00 ± 4.08 to 100.00 ± 0.00% within 72 HAT. POD of the extracts varied between 58.51 ± 2.04 and 94.68 ± 2.68, while no adult emergence was recorded in grains treated with the botanical extracts. Highest (2.96 ± 0.40%) weight losses among the treated grains were observed in 2.5% of S. obtusifolia, while 0.85 ± 0.49% was the least in 10.0% of E. balsamifera. Findings of the study have revealed that the botanical extracts could serve as stored sorghum protectants against S. zeamais infestations.

Keywords: Adult emergence, Adult mortality, Botanicals, Grain damage, Oviposition deterrence, Sitophilus zeamais.