Miticidal and Repellent Activity of thirty Essential Oils and their synergistic interaction with Vanillin against the Two Spotted Spider Mite, Tetranychus urticae

The two spotted spider mites (TSSM), Tetranychus urticae, are one of the most important and destructive agricultural arthropod pests. They show global distribution in temperate regions with a broad host range, and resistance to conventional synthetic pesticides has become a serious problem in controlling them. Botanicals have shown their potential to be considered as good alternatives for resistance management due to presumably different modes-of-action as well as low mammalian toxicity, and we are witnessing increasing volume of studies on botanical pesticides in the last decade. In the present study, contact toxicity and repellency of thirty commercial-grade essential oils via a two-choice test against the adults of TSSM were examined. Six oils including citronella, ginger, cinnamon, geranium, lemongrass, and cedarwood showed notable toxicity as well as repellent effect, and they were selected for further studies. Geranium oil was the most toxic and repellent oil (LD50 = 0.089 mg/cm2, RC50 = 0.659 mg/mL) among those tested, whereas ginger oil displayed the greatest increase in the repellent activity when mixed with vanillin. as showing significantly lowered RC50 values compared to its individual application (RC50 of 0.847 and 2.131 mg/mL, respectively). In order to understand the underlying mechanism of increased repellent activity of the mixture, GC-MS analyses through a SPME collection of volatile compounds were conducted. No apparent difference in the peak area was observed in the chemical analyses, suggesting the volatility of active constituent, Zingiberene, was not manipulated by the addition of vanillin. Other potential synergistic mechanisms are proposed.