Impact of physical characteristics of some mineral and plant oils on efficacy against selected pests

Mineral oils have been historically favoured over plant oils for insect pest control in horticultural crops because of their greater efficacy. Recently the increased pressure for environmentally sustainable pest management strategies has renewed interest in the use of plant oils and also in the reasons for differences in efficacy between plant and mineral oils. Efficacy of canola and mineral oils were compared for two modes of action: asphyxia in control of Saissetia oleae on olives and as an oviposition deterrent in control of Phyllocnistis citrella on lemons. On olives both canola and mineral oil treatments significantly reduced the number of black scale in comparison to the control but mineral oil reduced the number of black scale significantly more than canola oil. When oils were applied to lemons as a preventative spray, concentrations of canola oil above 0.5% significantly reduced the number of P. citrella mines per leaf compared to the control and there were no significant differences between any concentration above 0.5% canola oil and 0.5% mineral oil. Canola oil applied at a concentration of 0.5% was significantly less effective than mineral oil applied at the same concentration. Efficacy of canola oil was found to be lower than that of mineral oil in all experiments, but the higher efficacy of mineral oil was more pronounced in suffocation of S. oleae than in deterrence of P. citrella oviposition. Our results indicate that even though canola oil has very different molecular structures than mineral oils the resulting physical characteristics of canola oil, primarily high boiling point and viscosity, may contribute to their lower efficacy against arthropod pests. However, low phytotoxicity of canola oil indicates that the chemical structure of molecules contained in canola oil had much more influence on processes on the plant surface than the physical characteristics of the oil.