Effects of Hemizygia welwitschii leaf extract fractions on postharvest infestation of maize by Sitophilus zeamais Motsculsky (Coleoptera: Curculionidae)

As part of on-going efforts to use eco-friendly alternatives to chemical pesticides, leaf powder of Hemizygia welwitschii was sequentially extracted in hexane, acetone and methanol. Bioassays were carried out to establish the most active fraction(s) against Sitophilus zeamais in maize. Maize grains (50 g) were treated with concentrations within the range 2, 4, 6, and 10 g/kg of extract and Azadirachta indica seed oil (positive control) in the laboratory. The total number of progeny emerging from grains infested separately with S. zeamais eggs, larvae and pupae were recorded. Adult mortality counts were carried out 1, 3, 7 and 14 d post-exposure. Acetone extract was more toxic to the eggs, larvae and pupae than the other extracts, inhibiting progeny production by 90.90%, 88.10% and 100%, respectively, at the concentration 10 g/kg. For the same concentration, A. indica seed oil reduced progeny production by 100% for eggs, 96.08% for larvae and 70.93% for pupae. Hexane extract was more potent to the adult weevil than the other extracts, recording 100% mortality for the concentration 10 g/kg within 14 d. LC50 values were 0.78 (Hexane), 5.52 (acetone) and 1.69 g/kg (methanol). Extracts of H. welwitschii leaves had sufficient efficacy to be a component of storage pest management package for S. zeamais.