Miticidal and repellent activity of thirty essential oils and their synergistic interaction with vanillin against Tetranychus urticae Koch (Acari: Tetranychidae)

The two-spotted spider mites, Tetranychus urticae Koch (Acari: Tetranychidae), are one of the most important and destructive agricultural arthropod pests owing to their global distribution and resistance problem to synthetic pesticides. In the present study, miticidal activity and repellency of thirty commercial-grade essential oils via a contact bioassay and two-choice test against the adults of the mites were examined. Positive correlations between the miticidal activity and repellency were observed, as several oils exhibited notable toxicity and repellent activity comparable to a conventional pesticide, bifenthrin, based on their 95% confidence limits of LD50 values. Among the oils tested, geranium oil had the least LD50 (0.089 mg/cm2) and repellency concentration (RC50, 0.659 mg/mL), but it was not statistically different from other active oils including cedarwood, cinnamon, citronella, ginger, and lemongrass oils. About three-quarters of the oils showed increased repellency when they were mixed with vanillin, as the average repellency of 30 oils was significantly increased to 86.9 ± 2.1% from 62.1 ± 3.9% at 6.7 mg/mL application (p < 0.05). To understand the underlying mechanism of increased repellent activity of the mixtures, GC-MS analyses through an SPME collection of volatile compounds of ginger oil were conducted. No apparent difference in the aerial amounts of major constituents including (-)-zingiberene, the most abundant compound, was observed, suggesting the volatility of active components was not manipulated by the addition of vanillin.