Effect of essential oils from leaves of Ageratum conyzoides, Lantana camara and Chromolaena odorata on the mortality of Sitophilus zeamais (Coleoptera, Curculionidae)

Ageratum conyzoides, Chromolaena odorata, and Lantana camara, are common weed species in Cameroon. Essential oil extracts from their leaves were tested for efficacy on the morality of the maize grain weevil, Sitophilus zeamais (Coleoptera, Curculionidae). Concentrations of the essential oils relative to the maize grains of 0.013, 0.025, 0.05 and 0.1% (v/w) were used for A. conyzoides and 0.063, 0.125, 0.25 and 0.50% (v/w) for C. odorata and L. camara. Twenty 7-day old adult weevils were fed on maize grains treated with the above concentrations of the essential oils in Petri dishes. Control dishes contained insects and maize grains without essential oils. The experiment was repeated three times. Dishes were incubated in the laboratory for 7 days at 26 degrees C and 75-85% relative humidity. Insect mortality was recorded every 24 h. Graphs of percentage mortality versus the duration of exposure were constructed and the LD(50) was computed for each oil. Significant insect mortality was obtained with all the essential oils used. The mortality of S. zeamais increased with the concentration of the essential oils of the three plants and the duration of exposure of the weevils on the treated substrates. The essential oil extract of Ageratum conyzoides was the most effective insecticide (LD(50)=0.09% in 24 h), followed by that of L. camara (LD(50)=0.16%) and C. odorata (LD(50)=6.78%). These results show that the essential oils of the leaves of some of these weed species may be exploited for insect control in stored products.