COMPARATIVE EVALUATION OF Cymbopogon citratus STAPF LEAF EXTRACT AS SEEDS PROTECTANT AGAINST Sitophilus zeamais MOTSCHULSKY (COLEOPTERA CURCULIONIDAE) ON STORED MAIZE (Zea mays L.)
ABSTRACT
A study was conducted to determine the efficacy of extracts of Cymbopogon citratus (Stapf) as post harvest protectants of maize seeds against Sitophilus zeamais Motsch, field to store insect pest of maize. For contact toxicity, 1 g each of acetone, chloroform, methanol and aqueous extracts was re-suspended in 5 ml of de-ionized water and used to impregnate filter papers, ten insects were introduced into the filter papers and observed daily for 28 days, the set up was replicated four times. Also, aqueous extracts of C. citratus from dose rates of 1.0 g, 2.0 g, 3.0 g, 4.0 g, 5.0 g and 6.0 g / 50 g of grains infested with 10 pairs of sexed S. zeamais for 28 days were tested for insecticidal properties and compared with experimental control and the synthetic conventional insecticide [Coopex (0.25 g)] against S. zeamais under laboratory conditions of temperature and relative humidity (28 ± 2oC; 75 ± 5% RH) and laid in a completely randomized design. Parameters assessed were effect of C. citratus extracts on weevil mortality (toxicity test), natality, repellency and longevity of S. zeamais treated with C. citratus extracts. Data obtained were analyszed using Analysis of Variance (ANOVA) and means were separated using New Duncan Multiple Range Test at 5% level of significance. Results showed that the insecticidal compounds found in C. citratus extract were flavonoids, phenolic, alkaloids, saponins, to which the insecticidal properties were attriburted. Also, results showed significant (p<0.05) increase in mortality of over 50% tested at 1g of the extract, and at higher treatment of 6 g, 90% mortality was recorded as the extract suppressed adult emergence compared to the experimental control. The results also revealed that the efficacy of C. citratus extract on the weevils were dose-dependent with higher doses providing greater protection of the maize grains. Progeny development was significantly (p<0.05) inhibited as fewer adults emerged from treated grains when compared with the control. The insecticidal potential of C. citratus extracts and its ready availability made it an attractive biopesticide in upgrading traditional post-harvest protection practices. The outcome of this study showed some promise in ensuring a steady supply of good quality maize grains, hence further investigation should be done to know the active ingredients that caused the actual mortality of the insect pest. The plant used in controlling this weevil is harmless and so, efforts should be intensified toward their cultivation, packaging and application on a large scale as botanical insecticides.