Effect Of Temperature On, And The Efficacy Of Eucalyptus Leaf Extracts Against Sitophilus Zeamais Motschulsky (Coleoptera: Curculionidae) In Stored Maize

ABSTRACT This study sought to determine the most effective concentration and favourable temperature at which Methanol and Diethyl ether extracts of Eucalyptus sp leaf are most effective in the control of Sitophilus zeamais in stored maize. Bioassay on efficacy of Methanol and Diethyl ether extract of Eucalyptus sp leaf was carried out with 4 treatment concentrations of 0.2%, 0.4%, 0.8% and 1.2% dissolved in acetone at 0.5 g/L of solvent. The extract was replicated three times and stored at temperatures of 20⁰C, 27⁰C and 31⁰C from 1 to 168 hours to determine effect of extract on mortality of S. zeamais. The efficacy of Methanol Eucalyptus sp leaf extract was evaluated over 5 month duration and percentage weight loss was recorded between the 3rd to the 5 th month as per the above treatment concentrations and temperatures. The results of the experiment indicated that Eucalyptus sp leaf extract was significantly effective (p < 0.001) in controlling Sitophilus zeamais. At different temperatures (20⁰C, 27⁰C and 31⁰C), the following mean mortality were recorded 76.67%, 66.67% and 23.33%, respectively by 168 hrs post treatment of maize with 1.2% Methanol extract of Eucalyptus sp leaf respectively. Results on efficacy of Eucalyptus sp leaf during 5 months storage showed that the extract effectively controlled S. zeamais for 3 months but with a steady decrease in mortality. Results on the immature stages indicated that the egg, larval and pupal stages were all susceptible to Eucalyptus sp leaf extract at all concentrations compared to acetone treated control and Azadirachtin reference control. The larval stage was most susceptible to treatment effects. The study also showed that Eucalyptus leaf extract had significant repellent effect with 100% repellence post exposure after 6hrs and 12hrs at all storage temperatures (20⁰C, 27⁰C and 31⁰C). The differences in grain weight loss were significantly lower with increasing concentration. At 1.2% of Eucalyptus sp leaf extract, percentage weight loss were 0.46% and 4.17% for the 3rd and 5th months respectively during storage at 31⁰C. Percentage mortality was directly proportional to the amount of Eucalyptus sp leaf extract used. The present study concluded that Eucalyptus sp leaf extract should be used to control S. zeamais especially with higher concentration and storage temperature of 27⁰C to significantly lower percentage weight loss of maize