Relative toxicity of nC24 agricultural mineral oil to Tetranychus urticae Koch (Acari: Tetranychidae) and Phytoseiulus persimilis Athias-Henriot (Acari: Phytoseiidae) and its possible relationship to egg ultrastructure

The relative toxicity (LC₅₀ values based on mg oil/cm2) is evaluated of aqueous nC24 agricultural mineral oil (AMO) emulsions to the egg, six-legged nymph (larva), eight-legged protonymph and adult stages of two-spotted mite (Tetranychus urticae) and its predator, Phytoseiulus persimilis, on French bean leaf discs, using a Potter spray tower to apply of the oil. The egg of P. persimilis was the least susceptible stage (LC₅₀ 444.84) and its LC₅₀ was significantly higher than all other stages tested of either P. persimilis or T. urticae. The LC₅₀ for adult female T. urticae (LC50 63.89) was significantly lower than the larva (LC₅₀ 93.86); however, there was no significant difference in response between the protonymph (LC₅₀ 70.44) and the larva, which were both higher than T. urticae eggs (LC₅₀ 17.55). LC₅₀s for P. persimilis larva (LC₅₀ 43.87), protonymph (LC₅₀ 41.55) and adult female (LC₅₀ 53.34) were similar. Scanning electron microscopy showed that the egg surface of T. urticae is usually well covered with fine silk that may trap more oil and increase AMO efficacy. Other possible differences in AMO efficacy between T. urticae and P. persimilis may be due to differences in egg size, egg incubation period, egg surface structure and the presence of vulnerable respiratory cones in T. urticae eggs. Dose of 0.2–0.3% (w/w) is considered to be the most appropriate for nC24 AMOs use against T. urticae in combination with P. persimilis in integrated pest management programs.