Evidence for direct neural toxicity of a "Light" oil on the peripheral nerves of lightbrcown apple moth

The mode of action of petroleum oils on insects is usually assumed to be suffocation due to blocked spiracles. However, Citrus Postharvest Dip, a formulated C15 alkane used by Australian citrus packers to control surface pests, can also affect the neural activity of lightbrown apple moth (LBAM), Epiphyas postvittana Walker, (Lepidoptera: Tortricidae). The alkane penetrates deep into the tracheoles and absorbs onto nerve membranes, apparently causing direct nervous disruption. In electrophysiological experiments, Citrus Postharvest Dip in the ganglia induced a rapid onset of multiple nerve firing in peripheral nerves of LBAM larvae. Repetitive firing after exposure to the C15 alkane or surfactants used in the formulation showed that either of the components of Citrus Postharvest Dip can similarly affect the nerves. Nervous disruption by the oil is unlikely to be due to specific chemical binding. Assays with bovine acetylcholine esterase showed no specific inhibition of that enzyme using high oil doses (1%) and long incubation times (15 h). It is proposed that oils displace the protective lipids by their solvent action, affecting nerve activity by increasing membrane permeability to ion exchange. One of the major mechanisms of pyrethroid resistance in insects is reduced neuronal sensitivity. A role for alkanes in overcoming insecticide kdr-like resistance to pyrethroids is proposed.