ABSTRACT
Maize weevil, Sitophilus zeamais (Coleoptera: Curculionidae) is one of the major pests infesting stored grains worldwide. This research work was conducted in the laboratory to determine the phytochemicals present in the leaves of Eucalyptus citriodora and to evaluate the insecticidal activity of the oil extracts of the leaves investigated against S. zeamais Motschulsky, using adult mortality, oviposition, adult emergence, long term storage ability and seed viability as indices. The leaves of E. citriodora were extracted with alcohol at the 60 °C using soxhlets apparatus, while the method of Harborne (1973) was used to detect the various phytochemicals compounds present in the leaves. The leaf oil extracts were tested at the dosage rates of 0.2, 0.4, 0.6 and 0.8 mL/20 g of maize grains for each of the parameters evaluated, using three maize varieties, namely, sweet corn, popcorn and flint corn as the host. The results obtained from the quantitative phytochemical screening revealed the presence of saponins, tannins, flavonoids, phenols, quinones and alkaloids in the leaves. While the results obtained from the effects of the extract on +S. zeamais showed that mortality of adult S. zeamais in the three maize varieties was significantly different (P < 0.05) from that of the control grains. Mortality in the three maize varieties increased with increased oil extract dosage rates and the length of time of exposure to the oil extracts. The maize varieties at the dosage of 0.8 mL/20 g recorded 100 % weevil mortality by day 12 of exposure. The oil extracts drastically suppressed oviposition and adult emergence in the three maize varieties because no egg was laid and no adult emerged at the dosage rate of 0.8 ml/20 g in flint corn and popcorn. The extracts also totally prevented grain damage in the maize varieties at the dosage rate of 0.8 mL/20 g. There was significant difference (P < 0.05) between the treated and the untreated maize varieties in the germinability test, as germinability decreased with increased in the amount of dosages used. There was no significant difference as regards ability to germinate in the three varieties of maize at the dosage level of 0.6 and 0.8 mL/20g. The results obtained showed that extracts from E. citriodora are effective in controlling maize weevils and could serve as an alternatives to the widely used chemical insecticides.
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