Potentiality of Fractionated Extracts from Hemizygia welwitschii Rolfe-Ashby (Lamiaceae) Leaf to Protect Maize against Sitophilus zeamais Motschulsky (Coleoptera: Curculionidae) in Storage

Laboratory tests were conducted in order to assess the potential insecticidal effect of hexane, acetone and methanol fractionated extracts from Hemizygia welwitschii leaves against Sitophilus zeamais Motschulsky, important stored maize pest. The three extracts used individually were applied at four different concentrations (2, 4, 6 and 10 g/kg of maize) against the weevil. The efficacy of the treatment was evaluated by recording adult mortality after 1, 3, 7 and 14 days of infestation, and the F1 progeny emergence. Population increase, grain damage, and seeds germination were also assessed after three months of storage. The results showed that, among the three fractionated extracts, hexane extract was the most effective with 100% mortality recorded within 14 days after infestation at the concentration of 10 g/kg of maize, followed by acetone extract with 83.75% and methanol extract with 79.21% of mortality. The LC50 values decrease with the increase of exposure periods. The 14-day LC50 values were 0.78 g/kg, 1.58 g/kg and 3.10 g/kg respectively for hexane, acetone and methanol extract. The three extracts achieved significant inhibition of F1 progeny at all the concentrations. Among them, the hexane and methanol extracts induced complete inhibition of F1 progeny emergence at 10 g/kg of maize, acetone extract recorded 82.33% of inhibition. Significant reductions of insect population growth and percentage of seed damage were recorded after three months of storage on the maize treated with each extract at all the concentrations compared to negative control. Furthermore, no alive insects was recorded in maize seeds treated with the three extracts at the concentration of 10 g/kg. It is noticed that, percentage of gain damage were similar (0.15%) in maize treated with hexane and methanol extracts at 10 g/kg, while acetone extract recorded 0.76% of grain damage. In general, these extracts had no negative effect on the germination capacity of maize grains at the end of storage. Overall, the results obtained indicate that the use of these fractionated extracts could serve as an alternative to synthetic insecticides.