The toxicological effects of petroleum spray oils on insects – Evidence for an alternative mode of action and possible new control options

We tested the most widely held theory about the mode of action of petroleum spray oils (PSOs) on insects (i.e. anoxia). An nC24 petroleum oil was applied topically to cotton aphids ( Aphis gossypii) and cluster caterpillars ( Spodoptera litura), which then showed signs of mortality that are inconsistent with anoxia. The insects died soon after treatment, with most of the mortality occurring within the first 10 min. Toxicity symptoms included loss of locomotory ability, unusual abdominal contractions associated with spiracular fluttering, and ultimately dehydration and necrosis within 24 h. We therefore investigated the main mechanism(s) by which the nC24 petroleum oil interacts with the insects’ cells and organs, and ultimately kills the insects. The results suggest a mode of action that relates to the liphophylic properties of the oil. This includes rapid penetration through the insect cuticle followed by accumulation in the lipid-containing tissues, mainly those of the CNS, and finally penetration into the nerve cells themselves. In vitro tests with isolated insect cells further revealed that the oil penetrates the cytoplasm and induces 100% mortality of these cells within 2 min of application. No signs of oil accumulation within the tracheae were observed, so it is unlikely that anoxia is taking place at any stage of the intoxication process. Electrophysiological studies confirm that oil accumulation in the nerve ganglia has the direct effect of suppressing synaptic transmission in insect ganglia as well as in the neuromuscular junctions of vertebrates (toads and rats). These results demonstrate conclusively that at least some modern PSOs do not kill insects by anoxia, but by a range of cellular disruptions that lead to rapid insect death. The implications of our findings for the development of oil-based integrated pest management strategies are discussed.