Mechanism of action of soybean oil in arthropods: mortality results from tracheal infiltration and obstruction of gas exchange structures.

Oils are widely used in arthropod pest control, yet their modes of action remain poorly understood and often differ across studies. To investigate the mechanism of action of a commercial soybean oil formulation, we evaluated its physical and toxicological effects on three arthropod species across several developmental stages: adults and nymphs of Uroleucon sonchi (L.) (Hemiptera: Aphididae), adults and eggs of Tetranychus urticae Koch (Trombidiformes: Tetranychidae), and larvae and eggs of Plutella xylostella (L.) (Lepidoptera: Plutellidae). Adults, immature stages, and eggs were either topically treated or immersed in 2% soybean oil, and mortality was assessed alongside morphological and ultrastructural analyses using Scanning Electron Microscopy (SEM), Bright-field microscopy, Confocal Laser Scanning Microscopy (CLSM), Structured Illumination Microscopy (SIM), and Transmission Electron Microscopy (TEM). Immersion caused immediate mortality in U. sonchi and T. urticae, linked to oil penetration through spiracles and tracheal flooding, confirmed by fluorescent oil markers and bright-field, SIM, and CLSM imaging. In contrast, P. xylostella larvae exhibited partial and delayed mortality, likely due to spiracular morphology. Egg susceptibility varied by species and age. Egg hatch was completely inhibited in T. urticae, whereas only recently laid P. xylostella eggs were fully susceptible. SEM revealed extensive oil coverage and obstruction of respiratory structures, although TEM showed no damage to the cuticle. These findings indicate that soybean oil induces mortality by physically obstructing gas exchange structures rather than through chemical toxicity or cuticular disruption, and that spiracle architecture may modulate susceptibility. The implications for pest management programs are briefly discussed.