Demand-Driven Technologies for Sustainable Maize ... - IITA

285Evaluation of the insecticidal activity of the oils by ingestion. Maizegrains in the 25 ml fl asks were treated with the insecticide. Thereafter,the insects to be used for the tests were introduced into the fl asks andkept in the laboratory for 7 days. Data on the mortality rate of theinsects were recorded at 0, 24, 48, 72 and 96 hours of exposure tothe treated grains from which cumulative mortality was determined.Actelic, an organophosphorous synthetic insecticide commonly usedin Cameroon to control S. zeamais in stored maize, was used asreference insecticide for the control.Evaluation of the insecticidal activity of the oils by contact. Toxicity bycontact was assessed by direct application or the knock down effect ofthe oils. A mixture was put in a plastic fl ask and the insects were allowedto be in direct contact with the product for fi ve minutes. After this period,insects were removed and transferred on clean maize in fl asks. Insectmortality rate was noted after 48 and 96 hours of exposure to thetreated grains. Actelic was also used as the control in this experiment.In each of the two experiments, 20 adult weevils were introducedinto the plastic fl ask, which was covered with a piece of cloth and heldwith a rubber band. Five replicates were set up for each oil and controltratment. Insect mortality rate (in %) was calculated for each fl ask.Mortality was corrected for the natural mortality using the formulaproposed by Abbot (1925).Data analysisData collected on mortality rate were transformed by probit analysis forthe determination of LD 50(Finney 1971). The data were then subjectedto the analysis of variance (ANOVA), using SAS (SAS 1990). Meanmortality rate was used for the comparison of the treatments. Thecumulative mean mortality rates were adjusted using Abbot’s formula.Results and DiscussionFigures 1 and 2 represent the mortality rates after consumption ofS. zeamais with respect to the concentrations (5 and 10 %) of theessential oils and pirimiphos–methyl. The mortality rate of the insectsincreased with the exposure time and with increased concentration ofthe essential oils from the three plants and pirimiphos–methyl.At the 5% oil extract concentration, X. aethiopica and O. gratissimumhad higher mortality rates (63 and 53%) than the control (37%). Thiswas not the case with the essential oil from P. nigrum that had only 16%mortality (Fig. 1). The essential oil from X. aethiopica exhibited the bestinsecticidal activity by consumption test, with a mortality rate of 97.3%recorded after 96 hours of exposure at 10% concentration (Fig. 2).