Abstract
The contact, repellent and fumigation effects of essential oils from Tagetes minuta L., Mentha longifolia L., Rosmarinus officinalis L., Helichrysum odoratissimum L. and Pelargonium graveolens L. were investigated against maize weevil, S. zeamais (Motschulsky) (Colepotera: Curculionidae). The effects of the oils were dose-dependent with gradual increases in insect mortality as the oil concentrations increased. For the contact bioassay, the oils of T. minuta and M. longifolia acted rapidly, causing 100% mortality within two days after treatment at 0.375 and 0.50 μL/g grain concentrations, respectively. The repellent effect of these two oils against S. zeamais was also well pronounced. A Percentage Repellency (PR) value of more than 90% was obtained for the two oils, which puts them in the highest repellency class V. The level of repellency caused by the essential oils of R. officinalis, H. odoratissimum and P. graveolens were 51.1%, 49.4% and 51.7%, respectively. However, most of the oils demonstrated very low fumigation activity against the weevil. For the fumigation bioassay, all the oil concentrations tested did not achieve insect mortality more than 12.5%, even six days after treatment, except for the oil of M. longifolia that exhibited over 70% mortality at 32 μL/L of air. The present study revealed that the essential oils of R. officinalis, H. odoratissimum and P. graveolens had weak contact and fumigation effects against S. zeamais.